After getting the 65 Coupe's transmission issues squared away (mostly), I started in on the Mosport 66 Coupe since it also has speedo issues. Short history on the car - bought in '89, major mechanical overhaul in the early 90's to include going through the '68 vintage Muncie. Coupe has about 6K miles on it since then. Speedo always read low (sounds just like the 65 Coupe problem) so I had dropped the existing 19 tooth gear for an 18 and eventually a 17 tooth driven gear and all was fine till late last year when the speedo died. So, it went over 5K miles with a functioning speedo. Since I found out the 66/67 box in the 65 Cpe had wrong drive gear in it I figured I'd check to see if that might be the story here. Pulled the speedo plug and found the 17T gear was chewed up mostly on about a third of the teeth. Then I saw the drive gear was not centered.

That was enough reason to pull the trans out but there was another nagging problem with this trans that surfaced at the 5K mile mark. While it shifted smoothly with no grinding, no popping out of gear, no whine or anything 'between' gears, it did start to have this odd growl as I was letting out the clutch in first gear. Mentioned this to Don at the local trans shop a couple weeks ago when he was working on the 65's box and he said that might indicate a problem with the pilot bushing. Pulled the trans out this morning and check out the pic of the disintegrated GM roller pilot bearing in the back of the crankshaft and the pic of the 'pieces' once I had it out. I put a new GM bearing alongside for comparison.


Not sure why the roller bearing gave up like it did but doubt misalignment caused it because it's a stock 403 bellhousing and there were no issues with shifting. The input shaft tip did get a bit scarred up but should be fine with a new bushing. In any case, I will be going back to the old style bronze pilot bushing on this one. Also, since someone added an early 63-65 tailhousing with the driver side speedo I'll be changing that to a later version with the passenger side speedo, just personal preference.
Mike T.

 

Now that's living proof that needle bearing pilot bushing will disintergrate. I hope the damage on the main shaft tip doesn't eat up the new bushing. Dennis

 

Dennis, the tip of the input shaft did take a little damage but other than that 'ring depression' the rest is in good shape and will work alright with a stock bronze bushing with a little wetsanding. That input shaft was new when we had the trans rebuilt in the early 90's, 6K miles ago.

Mike T.

 

That's a perfect example of why I would NEVER use a roller pilot bearing. You're lucky you don't have to replace the input shaft. Go back to what GM originally used and you should be fine.

Jim

 

Jim, this car was the first one I installed one of those GM roller bearings and since I don't put many miles on my cars (6K in 18 years on this 66 Coupe, 500 miles on the 65 Coupe in 26 years, etc), I rarely ever take a drivetrain apart afterwards but this development sure is an eye-opener. Hell, I still have two more of the GM roller bearings stashed for future projects but after seeing what happened here, I'm going to let them stay in the drawer a while longer. The writeup's I read back then made the GM versions seem like the best thing since sliced bread (okay, not too many probably remember when bread was not already sliced in the package...but I digress) since they used something like 17 rollers and the other version offered by Mr Gasket (I think) only had something like 12 rollers. I fully understand that something else might have caused this failure but I'm at a loss to figure out what that might be since the old bronze bushing we removed was in good shape, the block is the same and the GM 403 bellhousing is also the same as before. We bought the car in '89 and that trans was already in there. I don't mind playing detective but after a while it's not worthwhile to ponder what could have caused this other than my belief that the grease eventually got displaced and the result shows in the pics.
A new solid bushing is going in tomorrow.
Mike T.

 

FINALLY!!!!!!!!! I get to say "I told you so!!!!
The question of pilot bushing vs bearing has come up numerous times here as well as on other Chevy forums. And I've expressed MANY times about the possibility of a bearing coming apart, shredding and causing damage to the input shaft. Now, you guys who swear by the bearing can see what can-------------and does-----------happen with a failed pilot bearing. Sometimes they last 5000mi, sometimes they last 100,000mi. But it seems that eventually they fail and the failure can, and sometimes does, damage the input shaft to the extent that it needs to be replaced. Not so with a bushing.
Below is an example of an input shaft damaged by a failed pilot bearing. The shaft needed to be replaced.
You wanna keep using a pilot bearing instead of a bushing? Fine, but remember I told you so!


Tom Parsons

 

In Tom We Trust! Dennis

 

Thanks, will put the roller bearing back and get a good bronze bushing. in the summer I put about 800-1000 miles week, twice a month on my 64 vette. don't remember what is in there now but will be putting a rebuild trans in next week and had a roller pilot to put in, but not now, thanks. Frank.

 

About 19 years ago I installed a Doug Nash 4+1 five speed with a Centerforce clutch and pressure plate. They wanted you to use the roller bearing style TOB. I had a slight chatter when I was done and the bell had been checked for center, new FW and all.

I had to pull the trans to investigate and when I did I found the bearing in pieces. The car had about 100 miles on it at the time. I didn't force the trans in with the bolts, or anything like that.

My guess is that with a 100 lb. transmission, you don't have to bump the bearing very hard to deform it or damage it in some way. I think the majority of early failures comes from installation of either the bearing itself, or the transmission.

I've never used one since and never will again.

 

John, not sure why my GM roller bearing gave up the ghost but can say that it's less likely, in my case, that it was caused by the trans installation because we had the engine on the hoist but sitting on the deck and loaded up the trans at that point in order to stab them into the car as one unit. I didn't dial in the bellhousing back then so maybe, as has been mentioned, it is off enough to cause a problem with a roller bearing setup. Going to the roller type bearing instead of the old tried-n-true bronze bushing was just one of those attempts at making it better than it was, even if the expected gain was miniscule.
I dug up the old GM part number for their Bronze Bushing from an old post by Tom Parsons and ordered up three of them from the local Chevy Parts Dept. They still can get them but they are sitting on a dusty shelf back in Willow Run, Michigan. That also happens to be the parts storage location for the seldom seen 17-tooth Purple Driven Speedo Gear.
Mike T.

 

that makes two of us.

 

Well, I ordered up all the parts I need to put the 66 Coupe back on the road and that included a new Autogear Tailhousing from D&L in Long Island, new clutch disc, new driveshaft yoke (old one had some deteriorated splines due to corrosion) and a new GM Bronze Pilot Bushing. Just picked up the bushings from my local Chevy Parts Dept and while it's non-magnetic as I expected, it does seem like GM (or whoever manufacturers these items for GM) got a little creative with slotting the I.D. of the bushing. Don't understand the engineering behind this modification from the old style but no doubt it will work just fine.

Mike T.

 

I had the reverse experience. After eating 6 bronze bushings in 10 years, the GM roller
bearing for diesels (recommended by my dealer's parts people) has lasted 11 years
and is still rolling.