915 transmission: mixing ALU and MAG and 4 vs 5-speed query

[patrick911]

Hi everyone,

I am building a 1973 2.8RSR replica using a 1976 911S car as a basis, and a got a few questions related to the transmission.
The 1976 donor car came with a magnesium 915 4-speed transmission.
Q1: is it possible to turn this 4-speed transmission into a 5-speed (ie: is it just different internals?) or would I need to source a new (5-speed) case?

The RSR has a transmission oil pump (915.307.303.2R) that is pretty hard to find, but I see sometimes mated to a magnesium front/cover housing (original for the RSR i believe: 915.301.301.0R) and sometimes with a aluminium housing (915.301.301.6R).

Q2: If you have a mag case, would it be possible to use a alu front/cover housing, or would the different material properties cause issues?

thanks,
Patrick

[gled49]

Easy swap to 5 speed, u just need to pick your ratios and swap 5th gear spacers for the gear set. By 77 the factory had mixed Mag and Alu parts. Pivot cover, side cover and shift housing. Sometimes 1 or 2, sometimes all 3. There is a guy selling all the RSR trans oil pump parts u need, in for sale. I just built a RS Mag cover. There are different drives, covers etc. pictures won’t upload I’ll try again later. G

[gled49]

The 76 trans is the last with non helper spring clutch like the RSR’s. Also mainshaft and 3-4 are reinforced parts, 1-2 reinforcement was added in 77, also easy to upgrade and u can’t see it.

[Jon B]

Patrick,

As Gordon said, but be aware that the RSR setup currently in the classifieds is missing the spray nozzle for the pinion gear.
And on 911 transmissions, that is the front cover or housing, not the tail housing... ;-)

Jon B.
Vista, CA

[gled49]

More pictures. The only difference between RS and RSR is pump cover and sometimes drive gear.

[patrick911]

Thanks guys, that's exactly what i was after!

[gled49]

The main shaft driven oil pump was a little fragile because of engine RPM speeds. Later final drive speed, side cover oil pumps were more durable but didn’t include a spray bar. Original fork drive gears, driven off the main shaft spiral roll pin would break and the RSR’s received an updated pin drive gear, requiring machining a slot in the main shaft and the use of a collared nut. The slot can be seen in the gear cluster shot if u blow the pix up. Here is a broken gear, seen this a few times.

[patrick911]

Hi G,
think I'm a bit confused now, but then again, I'm not too technical so please humor me.
The main shaft seems to have the opening that interconnects to the gear of the trans pump (the gear with the pin sticking out).
But I don't get the broken gear you show above? Did they turn the male/female order around? And is that what cause the broken one as depicted above?



Reason for asking is that I always see the two transmission pump gears, but only one of them has a pin, the other doesn't have anything, and I also wouldn't know why, other than sitting in a bearing, it would need anything.



Are you saying the replaced the left one with the right variant, but then had to machine a slot in the main shaft?

[Jon B]

But I don't get the broken gear you show above? Did they turn the male/female order around? And is that what cause the broken one as depicted above?
...Are you saying the replaced the left one with the right variant, but then had to machine a slot in the main shaft?

Patrick, as Gordon explained...

Original fork drive gears, driven off the main shaft spiral roll pin would break and the RSR’s received an updated pin drive gear, requiring machining a slot in the main shaft and the use of a collared nut.
The slot can be seen in the gear cluster shot if u blow the pix up.

The original RS pump gear is the one you've circled in green. It was driven by the spiral pin that secured the castellated nut on the main shaft.
The improved RSR version, which you've indicated in red, included its own drive pin, so that the pump shaft could be closed around it. This version required the main shaft to be slotted, to drive the pinned pump shaft.

Gordon, thanks for taking & posting these photos.

Jon B.
Vista, CA

[patrick911]

Thanks Gordon, thanks Jon!
All clear now. English is not my first language so i struggle with certain technical terms