Transmission Countershaft Bearing Shim

[Dat73z]

Hello, 

 

I am currently mating a 280z 5 speed bellhousing to a SR20 transmission. All of the machining has been completed and shifting is smooth when assembled. 

 

I was taking measurements on the countershaft front bearing (larger 62mm) against the bellhousing and came up with ~4.85-4.9mm. 

 

Following the chapter in the service manual (http://zhome.com/ZCMnL/tech/240SX5spd/Mt.pdf) MT30 (page 30 of 30 in PDF) I am slightly beyond the maximum allowable clearance listed in the table (4.71mm...which requires no shim). So I am somewhere on the order of 0.15-0.25mm out from the maximum clearance. 

 

Test fitting the front cover on the bellhousing with no gasket just -barely- touches the outer race of the countershaft bearing first and leaves a hair gap around the cover where the cover doesn't sit flush against the bellhousing. 

 

Does anyone have any insights on the following? 

 

Am I correct to assume I can run no countershaft bearing shim here and the front cover gasket will take up the additional room?

 

I am assuming that since the outer race remains stationary and countershaft/inner race area in the front cover has been relieved/machined for movement this wouldn't cause any issues? 

 

Am I missing something major here (like needing to press the countershaft bearing in or something)?  

 

Thank you! 

[NewZed]

It might help you to read the early 280Z and 280ZX transmission chapters.  I think that they have a different procedure but for the same purpose.  I remember thinking that the purpose of the shim was to apply some preload to the bearing.  The Z and ZX procedure seemed to make more sense.  Hard to tell what the 240SX procedure means by "allowable clearance 0 - 0.16mm".  They mixed the directions in their table, I think, and are using the word clearance instead of tolerance.  Confusing.

 

 

 

[NewZed]

If I recall right the shim contacts the bearing and the A dimension is how far the bearing sticks out.  You're not shimming the cover, creating a gap, your shimming the bearing race to get a certain force.  So the shim is meant to apply force to the bearing race, to keep it in place after expansion of the case.  So, in your situation, you'd have a little extra force on the bearing race because your bearing is sticking out a bit far.  Maybe give it a few more taps and see if it drives in farther.  Might solve the dilemma.  Otherwise, I think you'd actually want a thicker gasket.  But that might affect your other dimensions.

 

I've read that the racers often run a thinner shim to get more force on the counter bearing, to keep it in place when the transmission is really hot and expanded.  Food for thought.

 

That's how I remember things from back when I put one together.  I could be completely wrong in my thinking.  I also have a book on rebuilding transmissions and seem to remember some good info there too.

 

From 82 ZX...

 

 

 

[Dat73z]

NewZed, thanks for the thoughtful reply and detailed info. Yes, dimension A is slightly beyond the FSM chart (which I assume takes into account the thickness of the front cover gasket) so likely I'd be getting too much preload on the bearing just from the front cover with no shim. I'm going to attempt to tap the countershaft bearing inner race with an appropriately sized socket to ensure it is fully seated against the counter gear and re-measure just to be sure. 

[Dat73z]

NewZed- good point on the "Allowable Clearance." I was scratching my head on what that was in reference to (e.g. is a gap of 0.16mm between the countershaft bearing and the front cover acceptable to account for thermal expansion or conversely if the cover fits flush to the bellhousing and there is no gap between the countershaft bearing and the cover is acceptable...it would have been helpful to have those measurements called out in the drawing unless I've missed it in the 30 page body of text).  

[NewZed]

46 minutes ago, NewZed said:

 

I've read that the racers often run a thinner shim to get more force on the counter bearing, to keep it in place when the transmission is really hot and expanded.  Food for thought.

 

 

I meant thicker shim above, not thinner.  Sounds like you're on it anyway.

 

Good luck. 

[Dat73z]

Update- got it all sorted. Should have checked this first: took everything apart again and tapped the inner counter bearing race with a socket. Reassembled and tapped around counter bearing with a rubber mallet. Remeasured around and found ~4.3-4.35 dimension A which indicates a 0.2-0.3 shim in the table. The shim from the SR20 transmission cover was a 0.3 so will be reusing that. Cover now sits flat against the bellhousing with the shim in place and no gasket. Thanks again for the help!

Edited August 30, 2018 by Dat73z