Method for the dimension checking of the timing system of an engine

A method and an associated apparatus for the dimension checking of the timing system of an internal combustion engine, in particular for checking the clearance existing between the cams of the camshaft and the relevant valves in the cylinder head, by dimensional measurements of the cylinder head and the camshaft separately, and processing, in a storing, processing and display unit, the dimensional measurements for evaluating how the camshaft actually positions itself with respect to the cylinder head on which it is mounted, in the course of the normal running of the engine.

TECHNICAL FIELD 
The invention relates to a method for the dimension checking of the timing 
system of an internal combustion engine comprising at least a camshaft 
with cams and main journals, and a cylinder head with valves and seats for 
housing the main journals and defining the position of the mentioned 
camshaft in a longitudinal direction, the method including the steps of 
detecting and processing values relating to diametral dimensions of the 
main journals of the camshaft and radial dimensions of the cams, detecting 
and processing values relating to diametral dimensions of the seats of the 
cylinder head, and the transversal arrangement of the valves with respect 
to the formerly mentioned longitudinal direction, and calculating--on the 
basis of the detected values and in the course of the camshaft rotation--a 
clearance value between each cam and its associated valve. The invention 
also relates to an apparatus for checking the timing system of an internal 
combustion engine according to the formerly mentioned method. 
BACKGROUND ART 
There are known internal combustion engines with mechanical tappet 
comprising elements commonly called "bucket type tappets" positioned 
between the cylinder head valves and the associated cams of the camshaft 
and having the purpose of remaining in contact with the valves and 
cooperating with the cams lobes in the course of the camshaft rotation. In 
order to ensure a correct performance in the valve opening and closure 
phases, it is necessary that the clearance existing between the base 
circle of each cam and the related bucket type tappet be determined in an 
accurate way. 
In fact, if on the one hand no clearance, or an extremely limited amount of 
clearance, would not allow the proper closure of the valves, on the other 
hand an excessive clearance would detrimentally affect the performance and 
the life of the engine and, among other things, increase noise. 
In order to attain, for each single cam/bucket type tappet coupling the 
required amount of clearance, generally there is foreseen the 
insertion--in a suitable bucket type tappet recess, at the cam or valve 
side--of a specifically thick adjustment shim so that the clearance 
between the base circle of the cam and the bucket type tappet (or the 
shim) be of the desired value. According to a variant, that does not 
involve the insertion of shims, there can be foreseen, for each cam/valve 
coupling, the selection of an appropriate bucket type tappet among a 
series of bucket type tappets that have different predetermined 
thicknesses. The English abstract of Japanese patent application 
JP-A-57013205 shows a device for calculating the gaps existing between the 
top faces of the valve lifters 3 and the base circles of the relevant cams 
of a camshaft when the latter is assembled to the cylinder head. The 
thickness of the shims to be inserted are chosen on the basis of the 
values of the calculated gaps and of the desired clearances. Two 
measurements are taken to calculate each gap. A first measurement is taken 
on the cylinder head, substantially corresponding to the distance between 
the top surface of each bucket type valve lifter and a camshaft bearing 
surface 9 of the cylinder head. A second measurement is taken on the 
camshaft, corresponding to the distance between the surfaces of the base 
circle of the cam and a bearing journal of the camshaft. 
The calculations for determining the thickness of the adjustment shims, or 
that of the bucket type tappets, are troublesome due to various reasons 
among which the radial clearance existing between the main journals of the 
camshaft and the cylindrical seats of the cylinder head in which these 
journals are seated. This clearance is limited, but necessary for 
guaranteeing a correct rotation of the camshaft and allowing an 
appropriate lubrication. Devices like the one shown in the English 
abstract of JP-A-57013205 do not take into account such radial clearance 
in the calculation of the gaps. 
A checking method presently used for determining the thicknesses foresees 
the use of apparatuses that check the dimensions of the cylinder head and 
those of the camshaft separately (as shown in the above cited English 
abstract) and the processing of the results thus obtained by supposing 
that, in the course of the running of the engine and the rotating of the 
camshaft, the main journals of the latter--urged by the thrust of the 
valve springs alternatively compressed by the various cams--are in 
constant contact with the associated cylindrical seats, at diametrally 
opposite positions with respect to the valves. This assumption is an 
approximation that depends, among other things, on the number and the 
angular position of the cams on the camshaft and does not guarantee highly 
reliable results. 
In order to improve the method reliability, the results can be compensated 
in an empiric way, on the basis of statistics on errors detected in the 
course of subsequent checkings, for example when the selected shims (or 
the bucket type tappets) have been inserted and the camshaft is mounted in 
the cylinder head. In any case, this is not a really practical method of 
operating, since there is the need to collect an enormous amount of data 
and process them in an appropriate way, hence involves a considerable 
amount of time, high costs and not always achieves satisfying results. 
DISCLOSURE OF THE INVENTION 
Object of the present invention is to provide a method for determining the 
thickness of plates, or shims, to be inserted in the bucket type tappets, 
or the thickness of the bucket type tappets, that is particularly accurate 
and reliable and enables to overcome the disadvantages of the known 
methods. 
A further object is to provide a checking apparatus that enables to 
implement this method in a simple and effective way. These objects are 
achieved by a method and a checking apparatus according to the present 
invention. 
A method and a checking apparatus according to the invention provide the 
main result of determining, in an extremely accurate and reliable way, the 
thickness of the individual shims, or of the bucket type tappets, coupled 
to the valves, consequently attaining an extremely high accuracy in 
implementing the desired clearance between the base circle of each cam and 
the associated valve. 
A further advantage, that the method according to the present invention 
provides, is the application flexibility, in other terms the possibility 
of attaining particularly reliable results, no matter what the shape of 
the camshaft and that of the associated cylinder head--that undergo the 
checking--be.

BEST MODE FOR CARRYING OUT THE INVENTION 
The figures numbered 1, 2 and 3 illustrate checking stations that are part 
of an apparatus for implementing the method according to the invention. 
More particularly, FIGS. 1, 2 and 3 show--in an extremely schematic and 
incomplete way--devices for the dimension checking of elements of the 
timing system of an engine while leaving out of account, for the sake of 
simplicity, some known structural details of the checking systems. 
The embodiment referred to in FIGS. 1, 2 and 3 regards elements of the 
timing system of an internal combustion engine with four cylinders and 
four valves per cylinder and two substantially identical camshafts 25 
(only one has been schematically illustrated in the drawings) comprising 
eight cams 26, 27, 28, 29, 30, 31, 32 and 33 and four cylindrical portions 
or main journals 34, 35, 36 and 37. The cams 26-33 are angularly arranged 
by pairs in four directions spaced at 90.degree. apart, and a main journal 
is placed between each pair of cams. 
In FIG. 1, that refers to a first station 1 for checking a cylinder head 2 
of an internal combustion engine, there are shown just some fundamental 
elements of the cylinder head 2, more specifically, a central body with 
valves 3, 3.sup.I, 3.sup.II, 3.sup.III, 3.sup.IV, 3.sup.V, 3.sup.VI, 
3.sup.VII housed in associated openings 4 and caps 5 coupled to the main 
body in a dismantable way, by means of screws (not shown in the figure). 
The internal surfaces of the caps 5 define, with corresponding surfaces of 
the central body of the cylinder head 2, substantially cylindrical seats 
6, 6.sup.I, 6.sup.II, 6.sup.III for housing the main journals 34-37 of the 
camshaft 25 and supporting and referring the position of the camshaft 25 
in the cylinder head 2 in a longitudinal direction. An end of each of the 
valves 3-3.sup.VII --that can displace in reciprocally parallel 
transversal directions--contacts a bucket type tappet 9, 9.sup.I, 
9.sup.II, 9.sup.III, 9.sup.IV, 9.sup.V, 9.sup.VI, 9.sup.VII that has a 
recess 10, 10.sup.I, 10.sup.II, 10.sup.III, 10.sup.IV, 10.sup.V, 
10.sup.VI, 10.sup.VII for housing an appropriately thick adjustment shim. 
FIG. 1 depicts, as an example only, just two adjustment shims 11 and 
11.sup.I. Obviously, as will become apparent from the following 
description, in the course of the checking operation referred to in FIG. 
1, shims 11 and 11.sup.I are not inserted in recesses 10 and 10.sup.I. 
Compression springs 12 are housed in openings 4 and urge valves 
3-3.sup.VII towards the exterior of the cylinder head body 2. The first 
checking station 1 comprises a structure 13 for supporting and referring 
cylinder head 2 and first detecting means with first gauging heads 14. 
Each of the gauging heads 14--of a known type--comprises a casing, coupled 
to structure 13, and a pair of arms movable with respect to the casing, 
including associated feelers 15 for contacting diametrically opposite 
points of the seats 6, 6.sup.I, 6.sup.II and 6.sup.III at associated 
transversal, measurement cross-sections. Moreover, the gauging heads 14 
comprise known transducer means (not shown in the figures) connected to 
the movable arms and the casing for sending to a storing, processing and 
display unit 16 signals responsive to the deviations from the nominal 
values of the distances of the lower and upper generating lines 17 and 44 
of the seats 6, 6.sup.I, 6.sup.II, 6.sup.III, in other terms, the 
arrangement of these generating lines 17 and 44 with respect to the 
reference structure 13. Moreover, the first detecting means comprise 
second gauging heads 18 of known type too, including casings coupled to 
the structure 13, and movable arms with feelers 19 for cooperating with 
the bottom surfaces of the recesses 10-10.sup.VII of the bucket type 
tappets 9-9.sup.VII (FIG. 1 does not show the feelers 19 arranged in the 
two recesses 10 and 10.sup.I, but instead--as an example and as 
hereinbefore previously described--the adjustment shims 11 and 11.sup.I). 
In gauging heads 18 there are transducer means (of a known type and not 
illustrated in the figures) for detecting displacements of the movable arm 
and providing the storing, processing and display unit 16 with signals 
responsive to deviations from the nominal values of the arrangement of 
those surfaces, in other terms the associated transversal positions with 
respect to the reference structure 13. 
FIG. 2 shows a second checking station 20 comprising a second support and 
reference structure 21, with elements for supporting camshaft 25, 
comprising a live center 22 and a dead center 23, that define a 
longitudinal geometrical axis. 
A motor 24 is coupled to live center 22 and drives the rotation of camshaft 
25 about the formerly mentioned longitudinal geometrical axis. 
Second detecting means comprise third gauging heads 38, of a known type, 
with casings fixed to the support structure 21 and arms--movable with 
respect to the casing--including feelers 39 for cooperating with the 
surface of the main journals 34-37 at diametrically, reciprocally opposite 
points at transversal cross-sections of measurement. There are transducers 
(not illustrated in the drawings) connected with the movable arms for 
sending to the storing, processing and display unit 16 signals responsive 
to the deviations from the nominal values of the distances of the lower 
and upper generating lines 40 and 41 of the main journals 34-37, in other 
terms, the associated transversal positions with respect to the support 
structure 21. The second detecting means also comprise fourth gauging 
heads 42, of a known type too, with casings fixed to the support structure 
21 and movable arms with feeler elements 43 for cooperating with the 
surface of the cams 26-33. The transducers (not shown in the drawings) are 
connected with the movable arms for sending to the storing, processing and 
display unit 16 signals responsive to the deviations from the nominal 
values of the radial dimensions of the base circles of the cams 26-33. 
In the diagram shown in FIG. 4, the logic blocks identify the different 
phases of a checking method according to the invention and hereinafter 
described, and more specifically: 
block 45: positioning of the cylinder head 2 in the first checking station 
1; 
block 46: checking the dimensions of the cylinder head 2; 
block 47: processing the detected dimensions relating to cylinder head 2, 
defining a first reference axis and calculating the dimension values with 
respect to such axis; 
block 48: positioning of the camshaft 25 in the second checking station 20; 
block 49: positioning of the camshaft 25 in a predetermined angular 
position; 
block 50: detecting the dimensions of the main journals 34-37 and the 
radial dimensions of the cams 26-33; 
block 51: processing the detected dimensions correlated with camshaft 25, 
defining a second reference axis and calculating the dimension values with 
respect to such axis; 
block 52: checking the angular positions in which camshaft 25 has undergone 
measurements in the second station 20, and comparison with a pre-set 
number of predetermined angular positions (more specifically, four); 
block 53: selecting for camshaft 25 an angular position in cylinder head 2 
among a certain number of predetermined positions (specifically, four); 
block 54: calculating the possibility of displacement of each journal 34-37 
in its associated seat 6-6.sup.III ; 
block 55: attributing initial deviation values to the seat/journal pairs; 
block 56: calculating the elastic energy stored by camshaft 25 at 
predetermined deviation values; 
block 57: checking relating to the calculated elastic energy; 
block 58: modifying the deviation values correlated to the seat/journal 
pairs; 
block 59: calculating the gap between a pair of cams in phase and their 
associated valves, and determining the thicknesses of the associated 
adjustment shims; 
block 60: verifying the number of checkings that have been performed; 
block 61: ending of the procedure. 
According to the invented method, the first steps to be performed are 
parallel checkings on cylinder head 2 and camshaft 25 at checking stations 
1 and 20, respectively, as hereinafter described. Generally, before these 
checkings take place, there is a calibrating phase in which identical 
checkings are performed on master pieces (with nominal reference 
dimensions) mounted in the two checking stations 1 and 20. 
At the first checking station 1 (block 45), the cylinder head 2 is 
positioned and referred on structure 13 and feelers 15 and 19 contact 
pairs of points on the internal surfaces of seats 6-6.sup.III and the 
bottom surfaces of the recesses 10-10.sup.VII of the bucket type tappets 
9-9.sup.VII, respectively. The first and the second gauging heads 14 and 
18 send to unit 16 signals responsive to the arrangement of the lower and 
upper generating lines 17 and 44 of seats 6-6.sup.III and, respectively, 
of the bottom surfaces of the recesses 10-10.sup.III (block 46). These 
latter signals are indicative of the transversal arrangement of the 
associated valves 3-3.sup.VII. 
The signals representative of the arrangement of the lower and upper 
generating lines 17 and 44 of the two end seats 6, 6.sup.III are processed 
by the storing, processing and display unit 16 for defining the position 
of the cross-section centers of the end seats 6 and 6.sup.III, in other 
terms, the position of the axes of these seats, at the transversal 
cross-sections of measurement, and determining a first longitudinal, 
reference axis passing through those centers (block 47). Hence, further 
processings are carried out for referring the detected dimensions to the 
previously mentioned first reference axis, and obtaining the distance 
values of the lower and upper generating lines 17 and 44 of all the seats 
6-6.sup.III from the first longitudinal, reference axis, and the distances 
of the bottom surfaces of the recesses 10-10.sup.VII of the bucket type 
tappets 9-9.sup.VII from the first longitudinal, reference axis (block 
47). 
In the second checking station 20, camshaft 25 is positioned between the 
live center 22 and the dead center 23 (block 48) and rotated by motor 24 
about its longitudinal geometrical axis until there is reached an angular 
position whereby a pair of cams 32 and 33, in phase, have their eccentric 
portion, or lobe, in a position that is diametrically opposite to the 
feelers 43 of an associated pair of fourth gauging heads 42 (block 49). 
This pair of gauging heads 42, with its feelers 43 thus contacting the 
surface of the base circles of the cams 32 and 33, sends to the storing, 
processing and display unit 16 signals responsive to the radial dimensions 
of the base circles. 
At this angular position, the feelers 39 of the third gauging heads 38 
contact the main journals 34-37 and the gauging heads 38 send to unit 16 
associated signals responsive to the arrangement of the lower and upper 
generating lines 40 and 41 of these journals (block 50). The processings 
of these signals by unit 16 comprise the checking of the position of the 
cross-section centers of the end journals 34 and 37, in other terms the 
position of the axes of these journals at the associated cross-sections of 
measurement, and the definition of a second longitudinal, reference axis, 
passing through the formerly mentioned cross-section centers (block 51). 
Further simple processings enable to refer to the previously mentioned 
second longitudinal axis the arrangement of both the lower and upper 
generating lines 40 and 41 of all journals 34-37 and the arrangements of 
the base circles of the pair of cams 32 and 33 (block 51). 
The steps described with reference to blocks 49, 50 and 51 are repeated 
again (block 52) at other three different angular positions of camshaft 
25, each time by rotating camshaft 25 until there is reached an angular 
position at which a different pair of cams in phase 26, 27, 28, 29 and 30, 
31 have their lobes in positions diametrally opposite to the feelers 43 of 
the associated fourth gauging heads 42 (block 49). At each position, there 
is defined a second longitudinal, reference axis and the arrangements 
(detected each time--block 50) of the generating lines of the main 
journals 34-37 and of the base circles of one of the pairs of cams in 
phase 26, 27, 28, 29 and 30, 31, respectively, are referred to this second 
axis (block 51). The four sequences of values processed at the different 
angular positions are memorized each time in unit 16 (block 51). 
At each of the four angular positions taken by camshaft 25 (block 53), the 
values relating to the dimensions of seats 6-6.sup.III of the cylinder 
head 2 and the journals 34-37 of camshaft 25, that are referred to the 
first and second reference axis (blocks 47 and 51), respectively, are 
processed in unit 16 as hereinafter described, simulating an assembly of 
the camshaft 25 in the cylinder head 2 wherein these axes overlap so as to 
form a common single reference axis, and evaluating the possible 
reciprocal positions among journals 34-37 and seats 6-6.sup.III. 
At each seat/main journal pair (for example, pair 6/34), there are defined, 
respectively, an upper maximum deviation Y.sup.34.sub.Smax and a lower 
maximum deviation Y.sup.34.sub.Imax between the cross-section centers of 
journal 34 and its associated seat 6, by calculating the difference 
between the distances of the upper and lower generating lines 41, 44 and 
40, 17, respectively (block 54). 
The maximum deviation values Y.sup.j.sub.Smax and Y.sup.j.sub.Imax (j=34, . 
. . , 37) thus defined for each seat/main journal pair (6/34)--and at a 
specific angular position taken by camshaft 25 with respect to cylinder 
head 2--delimit a range wherein there is comprised a deviation value 
Y.sup.j (j=34, . . . , 37) among the cross-section centers of journal (34) 
and those of seat (6) of that pair, that represents the actual transversal 
position of journal (34) in seat (6); in other terms, the position in a 
transversal direction parallel to the direction of displacement of valves 
3-3.sup.III. 
In order to calculate the formerly mentioned deviation Y.sup.j of journals 
34-37 (blocks 55-59), for each of the four predetermined angular positions 
taken by camshaft 25 in the cylinder head 2, it is assumed that owing to 
the thrust of some of the springs 12 associated with valves 3-3.sup.VII, 
the upper generating line 41 of one of the journals 34-37 contacts the 
upper generating line 44 of the corresponding seat 6-6.sup.III. 
Consequently, the deviation value Y.sup.j of that journal coincides with 
that of the associated upper maximum deviation Y.sup.j.sub.Smax. 
More specifically, assuming that camshaft 25 is mounted in the cylinder 
head 2 angularly positioned, as shown in FIG. 2, the lobes of the cams 32, 
33 of one of the four pairs are angularly positioned in such a way so as 
to contact the bucket type tappets 9.sup.VI -9.sup.VII of the associated 
valves 3.sup.VI -3.sup.VII and apply a thrust for opening these valves. 
Under this condition, springs 12--associated with valves 3.sup.VI 
-3.sup.VII --apply a force to cams 32, 33 and to the journal 37 positioned 
therebetween, that tends to oppose to the opening of the valves 3.sup.VI 
-3.sup.VII and is sufficient for urging journal 37 to contact the 
associated seat 6.sup.III at the associated upper generating lines 41 and 
44. Hence, it will be Y.sup.37 =Y.sup.37.sub.Smax. 
The processing in unit 16 for calculating the deviations Y.sup.j (block 55) 
includes the attributing to the deviations that refer to the other 
seat/journal pairs of initial values comprised within the associated 
ranges delimited as already described (block 54), in particular, with 
reference to the previous example, (block 54) to deviations Y.sup.34, 
Y.sup.35, and Y.sup.16. For each value attributed to the Y.sup.j 
deviations, there corresponds a position taken by camshaft 25 in cylinder 
head 2, as schematically shown in FIG. 3 by way of a broken line 70, with 
segments representing the portions comprised between the cross-section 
centers of the main journals 34-37. In substance, the deformations that 
camshaft 25 undergoes when it is mounted in the cylinder head 2 and takes 
different angular positions are concentrated--as schematically indicated 
by broken line 70--in the areas for supporting camshaft 25 in the cylinder 
head 2, in other terms the seats/journals pairs. 
In order to calculate the values of the deviations Y.sup.j that best 
approximate the arrangement of camshaft 25 when mounted in cylinder head 
2, it is assumed that camshaft 25 tends to position itself in such a way 
as to minimize the total amount of deformations that it undergoes, i.e. 
the condition in which the stored elastic energy has a minimum value. 
The processings performed in unit 16 consist in evaluating (block 56) the 
elastic deformation energy of camshaft 25 at certain values Y.sup.j, and 
modifying the values Y.sup.j (block 58) until there is reached a 
combination that corresponds to a total minimum value of this elastic 
energy (block 57). This condition of minimum elastic energy represents a 
unique balance configuration for camshaft 25. 
The calculating of the elastic deformation energy of camshaft 25 and the 
determining of the combination of Y.sup.j values that make it minimum is 
achieved in a known way, hereinafter only cursorily described. 
With reference to a cartesian axis x, parallel to the first longitudinal 
reference axis, the elastic deformation energy of camshaft 25 can be 
expressed according to the following mathematical formula: 
EQU E=k.intg.(d.sup.2 Y/dx.sup.2).sup.2 dx (1) 
Where: 
the integral is extended to all the length I of camshaft 25 
the proportionality constant k depends on the shape of camshaft 25 and on 
the elasticity modulus of the material used for its manufacture and 
d.sup.2 Y/dx.sup.2 is the curvature of the line representing the elastic 
deformation of camshaft 25. 
As a possible simplification, the trend of the elastic deformation line, 
along which the neutral axis of the camshaft 25 positions itself, can be 
approximated by the broken line 70 obtained by considering the 
deformations of the camshaft 25 concentrated at points corresponding to 
the main journals 34-37 and, in particular, to the intermediate journals 
35 and 36 where the curvature assumes more significant values. Moreover, 
it is possible to express the curvature at each intermediate journal 35, 
36 with respect to the adjacent journals, as a function of the deviations 
Y.sup.j of the journal taken into consideration and the deviations 
Y.sup.j-1 and Y.sup.j+1 of the adjacent journals, like [Y.sup.j -Y.sup.j-1 
+Y.sup.j+1)/2]. 
On the basis of such approximations, a simplified mathematical expression 
representing the elastic energy is as follows: 
EQU E.congruent.k.SIGMA..sup.j [Y.sup.j (Y.sup.j -(Y.sup.j+1)/2].sup.2(2) 
where j is only referred to the intermediate journals. Thus, the elastic 
energy stored by camshaft 25 mounted in cylinder head 2 can be formulated 
as: 
EQU E.congruent.k{[Y.sup.35 -(Y.sup.34 +Y.sup.36)/2].sup.2 +[Y.sup.36 
-(Y.sup.35 +Y.sup.37)/2].sup.2 } (3) 
Assuming that camshaft 25 is mounted in the cylinder head 2 and arranged 
according to the angular position shown in FIG. 2, in other terms with the 
lobes of cams 26 and 27 at diametrically opposite positions with respect 
to the associated valves 3, 3.sup.I, it is also assumed, as previously 
mentioned, that the position of journal 37 is defined by the deviation 
value Y.sup.37 =Y.sup.37.sub.Smax, while initial values, comprised within 
the associated variability ranges, are attributed to the deviations of the 
other journals (Y.sup.34,Y.sup.35 and Y.sup.36). 
Subsequent processings in unit 16 include the calculation of the elastic 
energy variation, according to the mathematical formula (3), as the values 
Y.sup.34, Y.sup.35 and Y.sup.36 vary, and the identifying of the specific 
tern of values that make the E value minimum. These processings involve, 
for example, the calculation of the partial derivatives of the formula (3) 
with respect to the deviations of each of the three journals, the 
increment (or decrement) of the value of one of the three deviations 
Y.sup.34, Y.sup.35 or Y.sup.36 on the basis of the comparison between the 
calculated derivatives and the associated variability ranges, a subsequent 
further calculation of the derivatives, a subsequent new increment (or 
decrement) of a deviation value, and the repetition of these steps for 
minimizing the E value. 
The procedure ends when it is no longer possible to proceed, in other 
terms, for example, when all the deviations have reached a value that is 
at the limits of their associated variability ranges, or when the elastic 
energy becomes null, or after a certain number of repetitions (for example 
100), ensuring the required accuracy. 
In this way there are determined the values of the deviations Y.sup.j for 
the single journals 34-37 that minimize the elastic energy, and among 
these the deviation value Y.sup.34 correlated to the journal 34 that is 
positioned between the cams 26 and 27 that, according to the specific 
angular position, have lobes oppositely arranged with respect to valves 
3-3.sup.I. Then, for each of the two cams 26 and 27 there is calculated 
the gap G.sup.26 (and G.sup.27) that, in the absence of adjustment shims 
11 (and 11.sup.I), exists between the surface of the cam 26 (and 27), in 
correspondence with its associated base circle, and the bottom of the 
recess 10 (and 10.sup.I) of the bucket type tappet 9 (and 9.sup.I) of the 
associated valve 3 (and 3.sup.I) as follows: 
EQU G.sup.26 =A.sup.10 +Y.sup.34 -B.sup.26 
where A.sup.10 is the distance of recess 10 from the first longitudinal 
reference axis (block 47), and B.sup.26 is the distance of the surface of 
cam 26, in correspondence with its base circle, from the second 
longitudinal reference axis (block 51). The thickness values S.sup.11 of 
the adjustment shims 11 (and 11.sup.I) to be inserted in each of the two 
recesses 10 (and 10.sup.I) associated with cams 26 and 27 are obtained by 
subtracting from the calculated gap value G.sup.26 (and G.sup.27) the 
value of the nominal clearance G.sup.nom that it is desired be maintained 
between the adjustment shims 11 (and 11.sup.I) and the base circles of the 
cams 26 and 27. 
EQU S.sup.11 =G.sup.26 -G.sup.nom 
The procedure described (with reference to blocks 54-59) is repeated (block 
60) for the other three angular positions chosen by camshaft 25, and at 
each repetition there are calculated the distances and the thickness 
values of the shims intended to cooperate with one of the remaining pairs 
of cams in phase (28, 29, 30 31, 32 33), and, hence, the checking 
procedure ends (block 61). 
Therefore, the herein described method makes it possible to foresee how the 
clearance among the main journals 24-37 and associated seats 6-6.sup.III 
will be distributed over each journal-seat pair and at each angular 
position taken by camshaft 25. 
This possibility is extremely important for correctly calculating the 
clearance between each cam of the camshaft 25 and its associated valve 
3-3.sup.VII, as this calculation takes into account the transversal 
positioning of the main journals 34-37 in the seats of the cylinder head 
2. 
As described at the beginning of the description, the known methods for 
adjusting the clearance between the cams of a camshaft and the associated 
valves regard various procedures (for example, the insertion of shims 
between the end of each valve and the bucket type tappet, or the selection 
of appropriately thick bucket type tappets). Needless to say, this 
invention can also apply to similar methods, as in such cases too it is 
necessary to correctly calculate the distance between the base circles of 
the cams and the associated bucket type tappets (or other elements 
intended to displace with the valves), for defining the shims to be 
inserted (or, in any case, to be modified) appropriate for obtaining the 
required clearance values. 
The method according to the invention has been described with specific 
reference to a timing system of an engine with four valves per cylinder. 
However, the method is particularly flexible and hence easily applicable 
to any timing system, regardless of the camshaft 25 configuration (number 
and axial and angular position of the cams on camshaft 25, etc.) and 
cylinder head 2 (number and axial and angular position of the valves 
3-3.sup.VII, etc.) undergoing the checking. Obviously, as the timing 
system configuration varies, there are changes, for example, in the number 
and the arrangement of the feelers, and/or the number of the angular 
positions taken by the camshaft in which dimensions are detected and 
processed, but the processings that are performed do not substantially 
change. 
Furthermore, the method herein described avoids using empiric and not too 
accurate procedures for calculating the thickness of the shims 
(11-11.sup.I) to be inserted in recesses 10-10.sup.VII of the bucket type 
tappets 9-9.sup.VII of valves 3-3.sup.VII and, consequently, it permits to 
reduce the number of out-of-tolerance parts and the processing time. 
There can be foreseen variants with respect to what has been described 
hereinbefore, without departing from the objects and the scope of the 
invention. 
For example, the detectings described with reference to the first checking 
station 1 can also be performed at two distinct checking stations. A first 
checking station, including gauging heads with pairs of feelers, measures 
the inside diameter of the seats 6-6.sup.III of the cylinder head 2, with 
the caps 5 still to be removed further to the machining of seats 
6-6.sup.III by a suitable machine tool, while a second checking station 
with gauging heads and feelers detects the arrangement of the recesses 
10-10.sup.VII of bucket type tappets 9-9.sup.VII, and the arrangement of 
the lower generating lines 17 of seats 6-6.sup.III subsequently to the 
mounting of the cylinder head in the associated cylinder block and the 
removal of caps 5. It is possible to immediately determine, by processing 
in a simple way the diameter values and the values relating to the 
arrangement of the lower generating lines 17 of the seats 6-6.sup.III, the 
arrangement of the upper generating lines 44 of the seats. According to 
this embodiment of the invention, it is possible to measure more easily 
the arrangement of the recesses 10-10.sup.VII of the bucket type tappets 
9-9.sup.VII and keep into account, upon calculating the shims 11, 
11.sup.I, the deformations that the cylinder head 2 undergoes when it is 
mounted in the cylinder block. 
The checking stations can include checking means that differ from those 
described (in a very schematic way), and comprise, for example, optical 
type heads, or heads of another type. 
The storing, processing and display unit 16 can be connected to each 
checking station, or not be directly connected to the checking stations. 
Under this second circumstance, the data detected by each checking station 
are stored in a magnetic support that accompanies each element (cylinder 
head 2 and camshaft 25) along all the production line and contains the 
results of all the measurements taken. The data are thereafter entered--by 
means of a scanner--in the storing, processing and display unit 16, that 
performs the necessary processings. 
In order to obtain the values of the deviations Y.sup.j that best 
approximate the arrangement of camshaft 25 once it is mounted in cylinder 
head 2, it is possible to follow a different principle with respect to the 
one herein described, based on the checking of a number of mechanical 
conditions in the coupling camshaft/cylinder head, but that in any case 
allows to foresee, at every angular position taken by the camshaft, the 
distribution of the clearance existing between the main journals 34-37 and 
the seats 6-6.sup.III over every single journal 34-37.