From: tcora@PICA.ARMY.MIL (Tom Coradeschi (FSE) <tcora>)
Newsgroups: rec.motorcycles
Subject: Re: Roasting Headnut's on a striped down bike.......
Date: 14 Dec 92 17:49:24 GMT
Organization: Electric Armts Div, US Army ARDEC, Picatinny Arsenal, NJ

leavitt@cs.umd.edu (Mr. Bill) writes:
>leavitt@cs.umd.edu (Mr. Bill) writes:
>>If you use heat, apply it more to the block around the stud and not the
>>studs themselves.  The heat will help loosen the bonding between the
>>studs and the block, but it also causes metal to expand.  If the stud
>>expands against the block too much, that'll make things worse.  Conversely,
>>heating the block will cause the stud hole to grow bigger, not smaller
>>as one might think.
>
>To which tcora@PICA.ARMY.MIL (Tom Coradeschi (FSE) <tcora>) replies:
>>Show me. 
>
>"Show" you?  Over a computer network?  It's tough, but I'll try:
>
>		Before:  o		 After:   O
>
>>And don't show me how the hole a small cross-section metal ring
>>expands, either. Show me how a block of metal with a small round hole in it
>>expands.
>
>Okay:		          _______	         _______
>		         |       |	        |       |
>		Before:  |   o   |	After:  |   O   |
>		         |_______|	        |_______|	8^)

Um. OK. You're typing a 2-dimensional picture, on a 2-D crt, which will
be read by people using 2-D crts.

Problem: typical cylinder heads are 3-dimensional. What happens to the
head, in thickness, as you heat it?

>If you want a written source, I don't have one for you.  My knowledge
>comes from what I've heard, the most authoritative source of which I'd
>cite as my brother-in-law, who has degrees in M.E. and Ch.E., and
>specializes in materials.  I also have my own experience, the most
>applicable of which is removing studs from Chevy small block intake
>and exhaust manifolds.  Heating just the studs didn't work much at all.
>Heating studs and manifolds evenly had a fair result.  Heating the
>manifolds primarily and the studs only marginally had the best results.

You are comparing a steel head/steel stud pair to an aluminum head/steel
stud pair. Apples and oranges.

>So Tom, give me some evidence to the contrary, either academically
>or from experience.  What, pray tell, are your qualifications to
>pontificate on this subject?

Sheesh. Getting touchy? I merely asked you to explain your assertion, as
an assertion without some backing information is, at best, suspect.

The couple in question is a steel stud/aluminum head pair. The objective is
to remove the stud without damaging the head. Wrenching has been tried, to
no useful effect, so the application of heat is being considered.

The basic question is what will happen to these bodies when the heat is
applied:

Bill is quite correct that the diameter of the hole will increase when
heat is applied. However, he missed the kicker, which is where I asked
that a _block_ of metal be considered. This same block will also increase
in thickness. Depending upon the engaged length of the threaded
connection, and given the dramatically different coefficient of thermal
expansion for the two materials in question (CTE=6.6* for steel vs 12.6 for
aluminum [all in microinch/inch/degF, cuz that's what's handy]), the
threaded lengths will change (or attempt to change) in a dramatically
different fashion.

[*"Mechanics of Materials", Higdon, Ohlsen, Stiles, et al, John Wiley &
Sons, New York, 1976, p. 674.]

It may be inconsequential. Or it may not. The differential stresses may
(or may not) be greater than the yield strength of the weaker mat'l (the
aluminum, most likely, unless some truly strange alloys are applied). It
is true that the materials are heated during operation. However, the mode
of heat appication is quite different (heating from within, rather than
from an external source), which means that the temperature gradient will
quite likely be different. The max temp may be quite a bit lower during
normal operation (particularly if you're using an oxy-acetlylene torch to
do the heating:-{).

And don't forget about what heat does to the mechanical properties of things
like aluminum. If you pick a casting alloy and look at the numbers, it's
pretty scary (more so than I remembered). Now I have no idea what kind of
alloys are used in making bike engines, but just to pick something from
the list*: (Numbers are for Yield Strength)

Alloy      75F    300F   400F   500F   600F
356-T6   27ksi   17ksi   9ksi   5ksi   3ksi (ksi=1000psi)

[*"Marks' Standard Handbook for Mechanical Engineers", Baumeister,
ed-in-chief, McGraw-Hill Book Co, New York, 1978,p. 6-80]

All in all, better to be a bit careful. Let that sucker cool before you
stress it.

Oh, my qualifications? Lessee.

Educational:

Bachelor of Science, Mechanical Engineering, New Jersey Institute of
Technology, Newark, NJ, 1985.

Master of Engineering, Mechanical Engineering, Stevens Institute of
Technology, Hoboken, NJ, 1991.

Experience:

Naaaah...

>>On a more positive note (and I've emailed the Mr. Nadzam on this), the
>>best results will likely be from heating and cooling the block. The
>
>"Best results" vary, the only real measure being what works.  The same can
>be said for what is "a more positive note" and what is, by Mr. Coradeschi's
>insinuation, a negative response.  I certainly saw none of this in my post.
>If you don't like the advise you see here, Mr. Coradeschi, then go hire a
>consultant with a Ph.D. in materials.  You get what you pay for, sometimes.

Sorry to have given the impression of negativity. Relax. Have a homebrew.

>Along this same thread, which expands more when heated, aluminum or iron?
>I would think that iron does, tho' I have sheer intuition working here.

See the above. Aluminum has a factor of ~2 greater CTE.
-- 
 
                tom coradeschi    <+>    tcora@pica.army.mil
                            Loud Pipes Suck
Newsgroups: rec.motorcycles