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Username Post: 47 Cabriolet transmission/shifting        (Topic#347255)
Forum Newbie
Posts: 11

Reg: 04-20-13
09-20-17 08:09 PM - Post#2708317    

216 CI, standard three speed, has vacuum assisted clutch. We have adjusted and played with the vacuum assist but the shifting continues to drive us nuts. Cold the car seems to shift normally but as it warms up and finally gets to operating temperature it is impossible to consistently shift from first to second or second to third without "grinding the gears". Double clutching, shifting molasses slowly does not seem to help. Advice please.

Honored Member
Posts: 27580
Loc: San Francisco, CA
Reg: 11-26-02
09-20-17 11:05 PM - Post#2708331    
    In response to Bandit60

The vacuum shift is well deserving of a place on the list of GM's worst mistakes. However, causing the gears to grind is not one of its usual sins. The most frequent causes of gears grinding are worn synchro rings or the clutch not disengaging cleanly or the pilot bushing dragging on the trans input shaft.

The clutch is easy enough to check. With the car at a standstill and the engine idling in neutral, push in the clutch pedal, wait about 10 seconds, and then shift to 1st or reverse gear. The trans should go into gear without grinding. If it doesn't, either the clutch isn't disengaging fully or the pilot bushing is dragging. The symptoms are similar for both.

The synchro rings are harder to check without disassembly, but if the trans shifts into 2nd or 3rd better when cold (and the trans lube is thicker), that's a pretty good clue. The good news is that replacement rings are once again available in the aftermarket, and they work quite well. I recently helped a friend install a pair in his 50 trans, and it now shifts like new. The same rings fit all the way back to 1937.

Do the clutch check first though, because it's easier.


Those who choose an automatic transmission want transportation. Those who choose a manual transmission want to drive.

Forum Newbie
Posts: 11

Reg: 04-20-13
11-01-17 02:28 PM - Post#2712973    
    In response to raycow

Thank You very much. We've been herding cats so it took a while to get back to you. Your advice will be invaluable And I really appreciate it. Now to test it out but from what you described it is the synchro rings.

Ray P W 
Posts: 362

Reg: 09-30-15
11-03-17 08:11 AM - Post#2713153    
    In response to Bandit60


A more viscous gear oil is Lubriplate SPO288. I use it in my '36 Chevy PU and now it shifts the way it is supposed to for the first time in the 50+ years I have owned it. The specified oil for my application is SAE viscosity 160, a viscosity that is not available in auto parts store gear oil any longer and hasn't been for at least half a century.

The Lubriplate product is a gear oil, unlike the "600W" oil that is sold by antique car parts vendors. That stuff is a steam cylinder oil, not a gear oil.

Ray W

Edited by Ray P W on 11-03-17 08:12 AM. Reason for edit: No reason given.

Forum Newbie
Posts: 11

Reg: 04-20-13
11-27-17 09:25 PM - Post#2716025    
    In response to Ray P W

Thank you. Been herding cats and trip to visit family.

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