four speed 915 mainshaft same as 5 speed?

[Christien]

As the title says, is the mainshaft for a 5 speed the same as that for a 4 speed? I need to repair or replace my mainshaft from my 1972 4 speed 915, and I'd rather replace, but 4 speeds are hard to come by. Can I just replace it with a 5 speed mainshaft? Here's a pic, if that helps:






Thanks!

[Paulporsche]

Haven't got a clue, Chris, but I'm glad to see you're still at it!

I was beginning to wonder how it was going.

[lin7310948]

i have two four speeds in good condition. one came from a 73 911 and one from a 72 911. if interested email me at larry_ingram@bellsouth.com

[Christien]

Thanks - email sent!

[Christien]

Hey Paul, yep, still at it. I put it off after disassembly for a week or 2 because work was really busy, but the nice weather has lit a fire under my ass to get it done! I'm actually now kind of pissed off that I left it, because I could be driving right now, if my tranny weren't still in a million pieces on my workbench...

[dentist90]

Now there's a title for a Porsche repair manual... "A Million Little Pieces"

[MBEngineering]

HI Christien
the shafts are the same with matching 1st gear's, 4 or 5 speed, just the ratios for 2nd, 3rd, 4th and no 5th, only 2 spacers in the 4 speed box which are different to the 5 speed.

regards mike

[Christien]

Ok, so the shafts are interchangeable - what about differences in years? Will, say, a 915 shaft from an SC or Carrera work? I know anything earlier than 72 (901) won't.

[MBEngineering]

HI Christien
Yes the SC/3.2 shaft will work , BUT you will have to check the input shaft oil seal is actually running on the shaft, as in the photo!!




check with your old shaft as to the oil seal position/running line on your's and check on your re-placement one, also check in the gearbox for the correct position, a later shaft is a good idea and will avoid this,




as you can see from the photos the lower shaft is damaged in the gear stops, due to the fact the shaft was build with oil on the assembled parts and the nut was not torqued up correctly, thus letting the gears and components move on the shaft to the extent the shaft is damaged under acceleration and over run,



the later SC/3.2 shaft has a collar on it to stop the damaged parts falling into the box, BUT can still be damaged if assembled with oil and in-correct torque,



as you can see from the top edge of the collar it is rubbed/damaged/hammered by the 2nd gear to a point of a lip forming on the sleeve/collar in the above photo,



the good thing about the later shaft is this collar can be replaced, for approx $30.
I hope this helped with your gear box work and I did not waffle on to long.

regards mike

[ChrisBennet]

Wow Mike, great info!
-Chris

[Christien]

Yeah, that's awesome info! Thanks so much! Definitely no waffling on too long! I'm going to re-read that a few times to make sure I understand it properly. There's a ton of good info in there that I've never heard before. Thanks!

[Christien]

Ok, I've reread that a few times, and had a close look at my shaft (hee hee!).

First, my oil seal area is definitely the shorter kind:

I'm not sure what you meant by "check the input shaft oil seal is actually running on the shaft" - will a newer shaft, with the longer oil seal section, still work in my transmission?

Next, I did notice some scoring where you mentioned. How bad does this look?


Finally, I found the number 3139 inscribed (by hand) on the shaft. Does this mean anything?


Thanks!

[MBEngineering]

HI Christien
When you put the input shaft into the gearbox , you need to check that the oil seal is running on the shaft where you have checked the measurement, as the oil seals run in different positions in different year box's , so swapping different year shafts from one box to a different box could put the oil seal off the section the oil seal should run on.

The scoring on the shaft is not a prob' , if the lugs where damaged on the right of your photo it would be a prob' , if you look on the photo I posted and look at the 2 shafts , one has good lugs and the other is bashed to smithereens ones and lost it's lugs.

The No' on the shaft is the matching/mating No' of the gear set (shaft/gear for 1st), if you look at the rest of the gear set's in your box you will find , should find, thay have matching No' also.

regards mike

[MBEngineering]

HI Christien
just why do you need the change your input shaft??

"I need to repair or replace my mainshaft"????

regards mike

[Christien]

This is why:



When I pulled the transmission and clutch off the engine, I discovered that the pilot bearing in my flywheel had completely disintegrated. In my other thread about the entire rebuild, I posted a pic of the end of the mainshaft, and John Walker said it was "toast". The suggestion came up to machine the end of it and attach a sleeve, but I figured replacing it might be the better option.

Here's the other thread. Pic and JW's analysis are on the first page:
Christien's 915 rebuild

[Christien]

So the parts arrived today, and I slid the new pilot bearing on the old shaft. It jiggles a tiny bit - too small to accurately measure, but I'd say the inner diameter of the pilot bearing is about 1/4 to 1/2 a mm larger than the outer diameter of the shaft. I'm guessing that's too much play, so it looks like I'm definitely in the hunt for a new shaft.

[MBEngineering]

HI Christien
you will require a shaft and matching gear, with matching No's, if you change the shaft for a later type with a collar on, you will/may have to change the dog teeth, sincro band and sincro slider from your old type on to the shaft or to change to the later type like this;



or swap the dog teeth from your old shaft on to the later gear.

regards mike

[Christien]

Yikes! Is the fraction of a mm play on the new pilot bearing too much, in your opinion? How much heat gets in there when then tranny heats up? Enough to expand the shaft any measurable degree?

(I'm grasping at straws, aren't I?)

[MBEngineering]

HI Christien

(I'm grasping at straws, aren't I?)

no just confused?? the end of your input shaft is cream crackered and requires re placing with a S/H one with it's matching 1st gear in good condition , if you get a shaft the same year (ish) then it will be a straight swap , if you go for a later shaft with the collar on you will have to sort out 1st gear sincro' as it should have the dog teeth same as the photo I last posted, which are the later type, to what you have on your old one.

regards mike

[Christien]

I think I get it - the problem is either 1. sourcing the same year shaft or 2. easily finding a later year shaft, then getting all the assorted 1st gear stuff, which will greatly increase both cost and time of this rebuild. I was hoping (need, really) to get the car back on the road this weekend, and waiting for more stuff to arrive from Pelican (at least a week to Canada, even with express shipping) is too long. There's a Porsche dismantler north of Toronto, but he's notorious for screwing up the details of part #s (i.e. sold me an MFI fuel pump for my carbureted car because he didn't check part #s), so I'd have to physically go there to make sure the new shaft is a match, and it's about a 2 hour drive from here.

So basically I'm trying to find a way to make the old shaft work.