Synchro question

[Guest 51c8Joe]

I've posted this question on the General Discussion board and received one reply. Maybe I'll get more reponses here. The 2nd gear synchro on my 35 Pontiac 6 is "slow". A quick shift results in a heavy crunch. Not wanting to pull the trans for a rebuild, is there anything I can do to lessen the impact when shifting? On newer transmissions users suggest drain and refill with GM Synchromesh Transmission fluid. This supposedly has a friction modifier that gives extra grab to worn cups. Any and all comments would be appreciated.

Thanks,

Joe

[Guest]

I had a 1992 Lotus/Subaru Impulse that I had about 100,000 miles on, and 2nd gear just started to get notchy. It would shift ok when warmed up but cold or if I tried a super fast gear change it would grind just a little. I really didn't feel like doing a transaxle at the time so I drained and refilled it with synthetic. After that it was smooth as silk:) and got me through another couple of years before I sold it. What a great little car. 35MPG and was the only car I ever had that could redline the engine on the rev limiter (7,200 rpm) in 5th gear overdrive at 137mph.:cool: on the straight and level.

D.

Edited April 25, 2012 by helfen (see edit history)

[1937-44]

You might want to check on the weight of the GM synchromesh transmission fluid. I toyed with the idea of putting it in one of my old cars to help protect the yellow metal in the transmission. Upon shaking the bottle it seems to be a very light weight compared to the gear lube most of these old cars require. I opted to stick with gear lube.

[Rusty_OToole]

Shift slower and less often. A car like that with its high torque motor should not need to shift very often. Let the motor do its work, you might be surprised. In those days drivers did not expect to shift gears all the time like today.

Of course you must make sure you have the correct lube in the trans.

[Guest 51c8Joe]

Thanks for the input, I'm still researching the GMSMFM. I read on a site somewhere that they made two weights, as I recall, an 85/90 and an 85/145. Still trying to track that down. Supposedly the Friction Modifier makes a big difference in Honda and Lexus transmissions, but I'm concerned that it might be too thin to do me any good. I have a thread started on the Bob Is The Oil Guy forum and should get some more input before I make a decision.

Joe

[1937-44]

Hi Joe,

If the GM synchromesh transmission fluid is that heavy I would think it would be worth a try. The yellow metal safe transmission fluid I found at a local parts store felt like a 10 or 15 weight. Hope you can find out the weight and give us an update. Carl

[Owen_Dyneto]

If your transmission has any yellow metal parts, best to stay away from any of the EP gear oils. You can still buy SAE 140 gear oil (mineral oil + defoamer) as GL-1 gear oil. Readily available though you'll probably have to order it and it will probably only come in a 5 gallon pail.

[Guest fossiltin62]

I called it "syncro-smash". I had the similar problem shifting into high until the transmission warmed up, and reasoned thus: When the transmission cools down after use, the warm oil runs off those parts that are above the oil fill level, including the syncronizers on the mainshaft, and, since there is some wear on the syncronizer where it rides on the "cone" it backs up on, the "groves" on the tapered side of the ring won't hold enough oil, and it won't readily "float" so it can do it's job until it warms up. (Then the oil thins, and the ring expands a bit). So, after thinking it over, I added an agent with a property called "cling" which won't as readily run off those parts out of the oil when at rest. I used STP. There are several others. Believe it or not, it works.

[Guest 51c8Joe]

Thanks all for the input. I'm still doing research, but it seems that most "friction modified" fluids work to reduce the coefficent of friction, while I think for the worn synchros to work I need an increase in friction, not a decrease. As I understand synchros, as the gear slides into position the synchro shoulder contacts the synchro cup and aligns the gearset for a smooth mesh. If that's right, worn synchros don't lock up quickly enough to align the gears, and they grind into contact. Problem is how to balance friction needs on the synchro with lubrication needs of the gearsets and bearings. Maybe I'm better off shifting slow and double clutching until I can pull the tranny and fix it right. Your comments and suggestions are always welcome. Thanks again,

Joe

[Bhigdog]

Thanks all for the input. I'm still doing research, but it seems that most "friction modified" fluids work to reduce the coefficent of friction, while I think for the worn synchros to work I need an increase in friction, not a decrease. As I understand synchros, as the gear slides into position the synchro shoulder contacts the synchro cup and aligns the gearset for a smooth mesh. If that's right, worn synchros don't lock up quickly enough to align the gears, and they grind into contact. Problem is how to balance friction needs on the synchro with lubrication needs of the gearsets and bearings. Maybe I'm better off shifting slow and double clutching until I can pull the tranny and fix it right. Your comments and suggestions are always welcome. Thanks again,

Joe

BINGO!................Bob

[Guest Don-WI]

I've found that if you get some 85-140 or 140 gear lube that is regular mineral oil and not a synthetic it will shift much better. Don