What is purpose of aluminum carrier plate on 915 transmission?

[gssereik]

Does anybody know the purpose and/or applicability of the aluminum transmission carrier plate (911-375-311-00) installed between the transmission case and the steel engine mount carrier at the nose of the 915 transmission on 1974 through 1977 911’s? See picture below showing this plate labeled as #2.

The reason I ask is because I am wondering if my car should have this plate installed or not…. Sometime prior to 1994, a previous owner of my 83 targa pulled the original aluminum cased 915 transmission (which happened to have a limited slip differential - oh well) and swapped in a 1976 magnesium cased 915 transmission. They included this aluminum plate. But, apparently, my 1983 911sc would NOT have had this plate installed when it had its original transmission.

Thanks, George

[75 911s]

It is effectively a shim used on some (but not all 74-77 911s) to adjust transmission height due to inconsistencies or tolerance differences in the body.

https://forums.pelicanparts.com/porsche-911-technical-forum/717835-wont-shift-into-reverse.html

Part number 911 375 311 00

[gssereik]

Interesting! I am wondering if I might try adjusting or even removing this aluminum plate as my 83 targa technically should not even have it. But it does have a 76 tranny installed, so who knows. I rebuilt the gearshift linkage with all new bushings and installed new engine/tranny mounts recently. But I get a “clunk” when engaging first gear unless I move the gearshift really slowly. And I get some grind if I move quickly into third too, which I tend to think is a synchro issue. Anyway, I am trying my best to adjust the linkage and tranny in every possible way before pulling the tranny. And that is what got me thinking about what to do with this aluminum spacer. Thanks, George

[PeteKz]

Your clunk and grind are inside the transmission. The key issue is whether the transmission shift rod is centered in the hole in the end of the tunnel, and then, that it aligns easily to the tunnel rod and coupling, without up or down pressure. If it does, leave what you have alone.

[gled49]

The ALU spacer was used on Mag gearboxes only. It also has a role in electrolysis. Center the shift rod in the hole. The clunk in first is probably early Syncros, updated starting in 77. 3rd is syncro and shift sleeve.

[gssereik]

Thank you all for your responses. I understand this area of my 911sc much better now.. I will have a close look at alignment but I think likely the aluminum spacer needs to remain in place and my clunk and grind issues will be internal to the tranny.
George (gssereik)

[E Sully]

I have a thread about this back in 2014. No one seemed to know why, but it was cheap so I bought one. It was in one way or another used up until the SC's, but still used with the Turbo. Earlier than my '73 it was 2 separate plates. gled49 might be on to something as they did switch to aluminum around the time it was discontinued.

[75 911s]

Not saying electrolysis is wrong, but I don't think that's it for a few reasons.

1. I don't think early mag 915s that came on longhoods (Example 1973) had the aluminum plate. I don't know for sure.
2. I don't think all midyears had them (74-77) But I don't know for sure.
3.Turbo has it in the parts manual but the N/A car has it omitted starting with 1978.So it was used for an aluminum transaxle then.
4. The spacer takes up space, ergo, it raises the shift rod up in the body hole by that amount. It has that function.

[Tyson Schmidt]

Originally Posted by 75 911s

Not saying electrolysis is wrong, but I don't think that's it for a few reasons.

1. I don't think early mag 915s that came on longhoods (Example 1973) had the aluminum plate. I don't know for sure.
2. I don't think all midyears had them (74-77) But I don't know for sure.
3.Turbo has it in the parts manual but the N/A car has it omitted starting with 1978.So it was used for an aluminum transaxle then.
4. The spacer takes up space, ergo, it raises the shift rod up in the body hole by that amount. It has that function.

My ‘72 has the spacer, and the vast majority of mag gearboxes I’ve seen over the years have had one. I had always assumed it was a shim to correct shift shaft height in the tunnel.

It may be to prevent corrosion, much like the aluminum washers used for engine case perimeter nuts and valve cover nuts. I’ve seen that mentioned in Porsche literature somewhere about that being the purpose of the aluminum washers.

[75 911s]

Thanks Tyson, good data point. So my next questions are:
1. What are the variances in the steel trans crossmember mounts if any between 69-83?
2. Are mag 915 trans dimensionally different than the aluminum ones in terms of height at the area that mounts to the crossmember?