I'm into the 915 rebuild too, have question?

[mackskibum]

Just broke into the 915 this morning- have it mostly apart. I have a question- what is the end play supposed to be on the driveshaft flanges? I have .040 on the cover side, and .015 on the opposite.

Anybody have the specs for this, and how are they adjusted?

Thanks gang!

[Grady Clay]

Mack,

The axial play of the transmission axle flanges is determined by the differential spider gears. It isn’t a closely held tolerance and it is OK over a fairly wide range. It goes to zero when the differential gears are loaded. The differential gears push the flange toward the outboard.

At this stage you should remove the old axle flange seals and check the pre-load on the differential tapered roller bearings, the installed position (“E” = “R” + “r”) of the pinion and the backlash in at least four places 90° apart.

Best,
Grady

[mackskibum]

Thanks Grady- that makes sense. I haven't removes the flanges yet- I will probably tomorrow. Suprisingly the trans looks to be in pretty good shape- I caught it early enough- no catastrophic failure.

[Grady Clay]

Mack,

Good for you. The time to go into a 915 is before there is a major problem. Big Deals are expensive and very difficult to totally recover from. I agree with the Pelican who, on another thread, waxed poetic about the beauty of Porsche transmissions. The 915/930 are possibly the zenith. From what I have seen the G50/G64 are pretty close.



When checking these settings during disassembly, the best order of operation is:

1) Remove the differential first. Remove the side cover O-ring and the two differential seals.

2) Measure the installed position “E” of the pinion using P258 or VW385. This is normally within spec based on “r” etched on the pinion. This way you are measuring a totally undisturbed pinion.

3) Remove the nose piece and install the pinion blocking tool. If you are concerned about the pinion position or the 4-point pinion bearing, repeat 1) with the pinion loaded away from the differential. If there is more than about 0.01 mm difference, something is amiss.

4) Measure the backlash at four positions using P259. This is normally within the spec etched on the ring gear.

5) Remove the intermediate casting.

6) Now is a good time to install P260a and measure the individual gear backlash and end play of the free gears.

7) Remove the pinion, input shaft and all the gears. Be careful to not wrinkle the pinion shims. Don’t disassemble the stacks yet.

8) Reinstall the differential, side cover with 0.40 mm clearance and measure the bearing drag torque. This is often out of spec on the low side. It should still have some preload. The specs depend on the type of differential bearings: SKF = 26-37 in-lbs; FAG = 30-56 in-lbs. If you measure zero preload, clean the differential bearings and races with brake-clean and reinstall. With the differential axis vertical, see if there is any axial movement of the differential when you lift with the upper (cover side) flange. If in doubt about what you are feeling, set the side cover clearance larger (1.0 mm) and see how it feels. If there isn’t preload (ie is clearance) you should feel/hear a noticeable click. If any movement, seriously look at the pinion to ring contact pattern.

9) Visually inspect the contact pattern of the ring gear and the pinion gear. Look at both the leading (engine power accelerating the 911) direction and the trailing (allowing engine compression to slow the 911) direction.

10) If in doubt or concerned about the pattern, reinstall just the complete pinion with shims and torque the pinion bearing cover back in place. Reinstall the intermediate casting. With the ring & pinion oil free, a dry marker works well to see the pattern. Do only a couple of teeth at a time leading and a couple at a time trailing. You will feel like you are wearing out the differential side cover nuts having them off & on so many times. Take your time. This is tedious but will give you some useful information. Take pictures each time.

The question arises – Do you have to do this when you rebuild a 915?

I wouldn’t necessarily do this on a virgin, low mileage 915 that apparently just needed syncros and related pieces. If you have high mileage, unknown number of prior rebuilds by persons unknown, or some symptoms (whine, debris in the oil, etc.) then YES. It is senseless to go to all the effort and expense to get the transmission out, apart and repair things and do a half-ass incomplete job.

I’m loathe to risk a nice pinion and ring gear when a careful adjustment can extend its life or prevent a major catastrophe. This procedure is necessary when changing the 4-point pinion bearing – the most important and highest stress bearing in the transmission.

Where do you find the P-tools? Every major city must have several sets. Between Dealers, independents, retired mechanics, collectors and DIYers there is plenty of opportunity to find access. Almost everything except the P258/VW385 can be “of local manufacture” or a commercial tool.


For the next few months I’m going to help with several 915 rebuilds and refine and document the procedures. If I’m not clear about something, mis-state something or omit something, please jump on me and let’s get it right. I won’t claim to be perfect but the procedure must be. Please add all the input and images possible. I’ll edit what I produce and all else into a concise but detailed treatise on the 915. Links are particularly useful. We will give Wayne the 915 chapter in his forthcoming transmission book, Hehe.

Best,
Grady

PS, I sure like it when Pelicans post their real first names. There is a social aspect to all this.
G.
(gradyskibum68)

[mackskibum]

Grady, thanks for taking the time to explain the procedure. I had hoped I could get away without disturbing the settings by replacing the shims into their previous position without going through the measurement process. Based on your reply, it looks like I really should bite the bullet and properly set this up. I may defer to a local expert for this, as well as the shift fork adjustments- there's a good local Pcar independent that has no problem with me going through the disassembly, cleanup and reassembly, while he does the "critical" settings.

So far it looks like just 1-2 slider, 1,2 dogs, syncro parts. Everything else is in amazingly good shape for 108K mi.

BTW, my first name is Dave, the Mack part if my userid actually belongs to Mack Trucks- I belong to their affiliated ski club. I have been using that name on various BB since about 1998- I never registered with my full name- not all boards are as friendly and civilized as this one. I will add to my signature, however.

Thanks again!

[rcecale]

Originally posted by mackskibum
...not all boards are as friendly and civilized as this one. I will add to my signature, however.

Thanks again!

You haven't made it over to OT yet, have you?

Seriously, though. You're fortunate to have Grady in your corner. He's probably forgotten more about these cars than most of us will ever know. You're in good hands!

Randy

[afterburn 549]

great INFO !! make a book ,I will buy it !!!!

[Determined]

Subscribe, Need this information.

[Josh D]

Originally Posted by Determined

Subscribe, Need this information.

Thread is over a decade old. Maybe just start a new one with your specific questions?