From: jws@billy.mlb.semi.harris.com (James W. Swonger)
Subject: TH350 Transmission Rebuilding (or, what I did on my winter vacation)
Date: Mon, 4 Jan 1993 19:12:32 GMT
Distribution: usa
Organization: Harris Semiconductor, Melbourne FL

 The transmission on my Chevy van was getting pretty doggy. Had to wait a 
couple of minutes in the morning before it would go into drive, noticeable
slip on the 2-3 upshift, and so on. Whilst changing fluid-n-filter over break
I found a bunch of chunks in the pan. Emboldened by net.assertions that the
job wasn't so big that you'd need, say, a Trained Expert, I Went For It.
Sent my wife uptown for a rebuild kit from a local rebuilder, went and 
rented a cherry picker hoist, and waded into it.

 The rebuild kit I got included the basics - front and rear seals, gaskets,
a bunch of rubber and 4 sets of clutch rings. No steels, bands or bushings
were included. The kit set me back less than $70. Not a $40 problem, but
way under $400. 

 There were no instructions. Fortunately I had an older shop manual for
another of my vehicles which had some good detail (legible pictures, even)
on the internals and procedures. Without said book I would probably have
been stumped a few times.

 It took me most of a day to remove the engine and transmission, a day to 
do the rebuild and a day to re-install. Vans are not the most straightforward
vehicles to swap engines in and out of; the hoist I rented was actually too 
tall to fit under the upper firewall frame and I had to jack the front end of
the van up an extra foot before I could even grab the block. Things had to be
removed (like the motor mounts, and the entire face of the van, for starts)
which would not in a normal vehicle.

 Before taking out the engine/trans pair, I drained the pan and reattached it
to protect the innards, removed the fill tube and stuffed the hole with paper 
towel, etc. Removing the tail support brace, I found that the rubber tail
mount had turned into jelly some time ago and was not actually doing
anything aside from making me eat a U-joint every 20K miles and making some
thumping noises under hard acceleration.

 The rebuilding of the transmission was pretty easy - a lot faster than, say,
an engine. Not as easy as brakes all 'round. That pretty well brackets the 
skill level involved. With the engine detached from the transmission (the 
torque converter left on the flex plate to drool ATF on the floor), the trans
was ready for dissection. 

 First step was to lay out a bunch of clean towels. This was to (1) sop up
the ATF which remained in the trans and (2) catch any small parts which 
might harbor thoughts of escape. I set the trans down on the towels and 
started taking things apart. 

 I began with the pump face. This is the iron core that you see looking into
the face of the trans. It's held in by a ring of bolts. When you remove all
of these bolts, the pump is still held tightly by a light interference fit. 
Two of the bolt holes are threaded. Automotive bolts of the next higher 
size fit the threads. The Book recommends using a slide hammer to pop the
pump off, using these holes. I found that putting a bolt in each and using a
claw hammer to pry it up worked well (not having a slide hammer with a
properly threaded end handy). I used a couple of small bits of hardwood
(1/2x1/2x2") as the fulcrum, to avoid damage to the case.

 Setting aside the pump assembly, I then went down through the guts stage by
stage. Most everything is held in by snap rings. I had a pair of snap ring
pliers which were just good enough to gap the ring out so I could get a
small screwdriver under it. The rings could be worked out with a circular
sweep of the screwdriver. The trans guts came out, chunk after chunk. I
set them out in a row, as they were removed, to keep the order straight. 
I bagged collections of related small parts.

 The only part which was hard to remove was the L-R clutch piston. This is the
rearmost part, so you're about 2' down a hole trying to get a snap ring off a
spring-loaded piston. I had to make my own spring compressor for this, to
unload the ring. I used a piece of 5/16" threaded rod, a wing nut and washer,
a bar of 1/2"x1" steel drilled to slip the 5/16 rod through and cut long 
enough to straddle the open end where the tail housing was removed, and a piece
of 1/2x1/2" steel drilled and tapped for 5/16", cut to straddle the return
spring ring but fit inside the piston center, about 4" long. Two hardwood 
pieces were used, as before, to spread the load. By cranking down the wing
nut the return spring set was compressed and the snap ring freed enough to
remove. With this done, the piston needed some wiggling to get it to come out.
There's the "Park" pawl to get out of the way and an odd spring thing-mo-Bob
to keep track of, but that's it and you're down to about 10 pounds of empty
beer can.

 Do not overtighten your Handy Home-made Spring Compressor; you can warp the 
return spring retainer plate :( ... I had to spend some time straightening
it and getting the springs back to the right length. Just enough to unload the
snap ring is all that's necessary.

 There are four clutch packs, one band, and two planetary gear sets in a
TH350. The band just sort of sits there. The first clutch pack sits there 
loose, right behind the pump face. Then come the two assembled clutch/
planetary sets, then the last clutch pack right in front of the L-R piston.
The two assembled sets have pistons inside them which have to be removed
if you want to redo the seals; for these I used C-clamps on either side,
a bit tricky but workable. 

 Only one of the clutch packs showed any wear to speak of, but that set was
down to paper-thin on the faced clutch rings. This was the intermediate set,
the one that takes care of the 2-3 shift that was not doing so well. I
didn't see any obvious damage to the steels fortunately. I replaced all of
the clutch packs anyway; I didn't figure on being back that way again.

 Each of the pistons has three rubber seals; two on the outer face and one
inner. The replacements in the kit were visibly larger - the seal lip on
the old ones was quite worn down. I replaced the seals (remember the right
direction as you swap them) and has a bit of trouble getting the larger,
new seals past the edges of the piston cavity, even with the chamfer on the
steps. The Book suggested .020" music wire as an aid to getting the lip of
the seal down into the cavity; I used my feeler gauge set instead. With some
careful poking at the inner and outer seals I managed to seat the piston down,
then recompressed the return spring, snapped the keeper ring back on, and 
loaded the clutch packs. The new clutches were impregnated with ATF and I
honed the steels lightly with 180-grit carbide paper in a cross-hatch
pattern, since they looked shiny but not warped, cracked or pitted. The
pressure plate for each assemble went on last and got its own snap ring.
Nice, simple mechanisms really.

 The pump assembly has a piston in it as well which acts on the front clutch
pack. Same deal as the others. There are also a bunch of sealing rings,
steel interlocking types, to replace. I disassembled the pump to have a look
since there was a gasket for it. The old gasket required scraping and got
all over, so time was spent picking out bits of paper. Nothing untoward
in the pump, so I reassembled it after cleaning out a bit of gray sludge.

 The valve body was the last thing I messed with. I took off the iron body
and again had to scrape and pick gasket material. There are four little steel
balls to keep track of. My book had a picture showing where they went, but I
made tiny scribe marks by each of their holes anyhow. The only valves I
removed were the vacuum modulator and detent valves; the others were staked
in behind plates so I left them alone. I got a bunch more goo out of the valve
body.

 Everything got coated with ATF during assembly, to help the seals seal and
the moving parts move.

 Reassembly was easier than disassembly since I now understood how things went
together. I figure the whole thing would take 1/2 day next time, what with
eliminating false starts etc. Keeping things in natural order helped. I had
the assembled units assembled, so they dropped right in, in a stack. Tighten
her up and hang her back on the engine (playing with the flywheel to get the
torque converter and the oil pump input to line up), and call it a night. 

 I kind of regret not replacing the bushings while I had it apart. However, 
only the tail bushing showed any wear to speak of, and it looks to be
pullable by just taking off the tail housing with the trans still in the
vehicle, so I'm not too upset. I figure another 100K miles anyhow.

 Installation was tedious but easier than removel; all of the rusted nuts &
bolts were now oiled and everything was out of the way. The only cardiac
arhythmia came when I fired it up (or tried). I couldn't get the transmission
to go into Park and the starter was locked out. I was, as you might surmise,
bumming. I disconnected the linkage so I could get the lever into the "P"
position and ratcheted the detent lever over to where I thought Park ought to
be; I figured maybe the pawl was jammed or something. I started the van up
and idled for a bit (having already taken it down off the blocks). I noticed
that I was in Low. "Aha", sez I - I had put the shifter linkage on upside-
down, so P was L1 and so on. Swapped it 'round and everything was looking good.

 The transmission is a hell of a lot better now. Nice and firm, no waiting for
drive in the morning and no clunks when it engages either. Almost like having a new van. Except for the rust. And the interior. And the roof leaks. And the 
power steering and master cylinder leaks. And the ...
-- 
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#Irresponsible rantings of the author alone. Any resemblance to persons  #
#living or dead then yer bummin. May cause drowsiness. Alcohol may inten-#
#sify this effect. Pay no attention to the man behind the curtain. Billy!#
Newsgroups: rec.autos.tech