Newbie with 915

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Re: Newbie with 915

Post by Clatter » Sat Dec 23, 2017 5:24 pm

Had some more good fun with this thing today..

The 'piece of wire' trick worked great for locking the slop out of the diff. 8)

Built a mainshaft holder/lock out of a 4-speed 915 R hub I got off ebay for $20,
And some scraps..

Seems to work well for locking the shafts together like this:

Got my back-lash measurement,
And it's .011".
(Can't tell by the blurry pic, hardly, sorry)
But it's .011".. All day long.
That translates to .279mm.
Zimmerman didn't seem happy with his .25mm measurement - .0098".
He wanted to get down to .19mm - .0075"..

So it looks like I'll be doing the diff bearings and shim thing after all..

Unless .28mm or so is acceptable actually..?

The 'pre-load by feel' sure 'feels' tight.
Snugging down the side cover nuts is like "Dag! This thing is together wrong!" feeling as you keep tightening and tightening them,
All the while increasing the turning torque on the diff..

Any other assembly I woulda put together, that bound up like that,
I'd have stopped, and seen what was put together wrong. :wink:

Pelican sells the diff bearings for $26.75ea. for FAG, and $39.75 for SKF.
(And $155ea. for Porsche brand.. :roll: )
They are probably SKF or FAG anyways, right?

Any preference? Real-life experience favor one over the other?

Pelican doesn't seem to sell diff shims for the GT3 yet..
Can I get your source for those thin shims, please Mr. Guard, sir?

The diff shim adventure does seem to be rather worthwhile at this point..

Thanks again for all of your help.
I do feel like this is actually working like it's supposed to so far! :D :D :D
What's that Noise??

Beginners' how-to Type 4 build thread ---> viewtopic.php?f=1&t=145853

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Re: Newbie with 915

Post by Clatter » Sat Dec 23, 2017 8:49 pm

Anybody have any ideas where to get part #14 here? Pelican doesn't have it.
Think the 911 has the same part?

It's described as "Delrin Bearing Sleeve".

Here's mine, all mangled and cracked.

Just seems a shame to use it when the shaft is plated, and the case is all clean.. :?
What's that Noise??

Beginners' how-to Type 4 build thread ---> viewtopic.php?f=1&t=145853

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Re: Newbie with 915

Post by Pablo2 » Sun Dec 24, 2017 9:24 am


Although I've given my 2 cents, I couldn't possibly improve on Zimmerman's tutorial .. but a few comments:

Your point of measurement for backlash should probably be more toward the ring gear's outer circumference. Actual backlash could be slightly more than what you're measuring. I concur with Peter's target figure, although this number could be higher with more worn R&P sets. Tightening up the backlash slightly is common with Porsche trans rebuilds.

All preload determination (especially "by feel") of course needs to be done with pinion REMOVED. Otherwise, you can't freely spin the diff.

If the preload feels scary to you, something's amiss. As I understand it, you've done nothing at this point that's changed the previous preload. If the feel when spinning the diff is "notchy", the preload is too high. Dinged up diff shims are your clue that someone's previously been playing with diff settings.

New bearings, as precise as they are, do not automatically receive the same shims (even if your backlash measured correctly). The old shims MAY only be a starting point after changing bearings.

The "official Porsche" bearings are probably just FAG bearings. However, one must ALWAYS be certain the inner chamfer (on the inner race) matches the chamfer on the diff .. although existence of diff shims between diff body and bearings normally negates this concern.

GT3 shims may only be available in the U.S. through the dealer. I used to import them from Germany and sell full kits as accessory to our LSDs. Obviously, I no longer do. I only put forth that info as an "update" for rebuilders who may be reading this thread, as they're so much easier and more precise. However, most shops continue using old stock shims.

Needless to say, you can't reinstall that bushing. I'm betting there are higher quality replacements for those sold today, but I don't know. You can contact me directly after the holidays, and I'll ask my local rebuilder. Pelican's OK, but certainly not the only source for parts.

BTW, thanks for sharing your GREAT photos and your step-by-step rebuild. This forum (like most) is lacking meaty threads such as this one.
aka Pablo, gears, Geary
9.36 @ 146 in '86

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Re: Newbie with 915

Post by Clatter » Sun Dec 24, 2017 12:15 pm

Thanks again, as always, Paul.
Nobody ever did a 915 build here, and when you started your detailed answering, I just ran with it.
Figured it's good to have this all here for posterity..
A lot of time my words aren't exactly thought through before I post, though.. :wink:

Spent an evening digging for that bushing,
And it's amazing how many sources there are for Porsche parts and info.
You can even go to the Manufacturer's website and download the parts catalog! :shock: :shock: :shock:
Coming from VW world, especially doing a type 3, I just automatically assume everything is NLA! :!:
Found the bushing at the Sierra Madre Collection - a place with a VERY expensive-sounding name. :lol:

Double-checked my distance from center, where the R&P backlash gets measured;
76.5mm, or 3.0118" inches.

Got some SKF diff bearings on the way from Pelican, along with a gallon of Swepco 201 and some throwout shaft bushings.

Having reservations about installing them diff bearings.
I'm kinda skeered, TBPO. :oops:
They already feel plenty tight from all of my years' experience doing other things like this.

Here's a diatribe/tangent that will be long and nonsensical, but I'll put it up here anyways:

A few years' experience as a machinist haunts me in another way;
The theory of 'work hardening' a.k.a. breaking in.
Those diff bearings get 'pounded in', 'bedded in', through use.
I see the rollers getting made smaller and harder, and the races made thinner.
The wear parts of the bearing get compacted, they don't wear away.
Pressure is at work on them, not abrasion.

They might have less pre-load on them now,
But they are worn in perfectly to each other, and shaped in exactly the correct way to work in harmony.
They are hard in exactly the right spots where they are subject to the most pressure during use.

They are likely to last a good while longer, as long as everything goes back together with no changes.
Any changes in geometry here, or dirt intrusion, obviously will cause rapid wear,
If it all gets cleaned and goes exactly back together..?

I'm really skeptical of what I might accomplish by installing new diff bearings and tightening the preload.
Some things like this I did in the past didn't end so well in the long run.

Pre-load will just open up as new parts wear in.
Changing the backlash might make the R&P 'sing' on the highway.

Because the rest of the parts of this trans were in pretty good shape,
I'm ready to believe the story about low mileage and the dentist.
I know for sure the 'fire' part of the story is true. :wink:
Paid a guy ($1000(!) :shock: ) to go in there and look at everything (and mangle my shim),
And he said it all looked like a low-mileage box that was never opened
(for whatever that is worth).

One of my main goals was to know what was going on in there, and to de-mystify these internals;
To check things out, and see with my own eyes what was in there.

Should there be any issues with this trans, I'm all set to go back in there and continue along.
The tools are all made up and most everything to build these is right here.
If there is any balky shifting, or the trans sings, or..? We're all set to dig right back in.
Things like case blasting and fastener plating and throwout bushings won't need to ever be done again.
I'll be able to save up for a nice dial inch-pound wrench and a complete set of bearings, and a billet bearing retainer and billet side cover..
(OK, maybe I'm lying about the side cover :D ).
It would, however, be great timing, if you were setting up diff pre-load anyways, to get a billet side cover, right?

This car is going to be driven occasionally, on the street, by an old man.
It will go to shows, and cars n' coffee, maybe to work on Friday.
The motor is a feeble pushrod 4-banger.
It's not some 3.2 Carrera being driven daily.

This car has an experimental/home-made shift linkage that I don't even know will work.

I'm not shop or professional mechanic with a reputation to maintain.
A come-back just means more fun!
Especially now that everything is clean, and I'm all tooled up..

I have the next week off for the holidays, which is rare.
The stuff is all sitting here ready to go..


I'm Pussing Out, and throwing it back together.
The new diff bearings will be sitting here on the shelf with my new trans tools.

I know I might regret it later,
But there are just WAY too many other fish to fry with this car right now...

Thanks for listening,
Send me a bill. :D :oops:
What's that Noise??

Beginners' how-to Type 4 build thread ---> viewtopic.php?f=1&t=145853

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Re: Newbie with 915

Post by Pablo2 » Sun Dec 24, 2017 2:43 pm

Honestly, I concur with your reasoning. I'm glad you were able to get in there and look around and verify everything looks good. Your shift sleeves WERE ready for the dremel clean-up work you did, so the box should shift even sweeter now. If you purchased new synchro rings, at least install these, and call it a day.

Good luck ..
aka Pablo, gears, Geary
9.36 @ 146 in '86

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Re: Newbie with 915

Post by Clatter » Wed Dec 27, 2017 11:34 am

So, we'll wrap this one up here (for now :wink: )..

First of all, I can't thank you enough for holding my hand through this, Paul.
It would have been an entirely different experience without your input.
That you took the time to type all of this up, with pictures and all, is very much appreciated.

The decision to leave the diff bearings in place came down to the 'feel' measurement you describe.
Loaded the diff housing, bearings oiled, into the case (no shafts).
With the cover loose, but just tight enough to align the bearings, the turning feel was, er, felt.

Snugged down the cover by hand, evenly criss-cross, until there was some turning resistance felt.
This was about the point where the nuts were hard to turn with just my fingers,
And there was definitely an increase in resistance; pre-load was just starting to happen.

.008" all the way around.. Not the .012" you say is spec for these new, but still some pre-load, no doubt.

Loaded up some 201 in an old syringe, to fill the synchros and rollers - everything being 100% clean and dry at that point.

Most important tool handy nearby - This thread,
Plus Zimmerman's tutorial,
The Porsche parts diagram,
And a couple of Pelican threads.

My 'holder tool' is handy for a final clean of the mainshaft,

Then again for the big tightening..

Used the jig I bought at Pelican to set up the forks like peter Zimmerman says to.

Mock up to check adjustment In Real Life.

Got all the way to assembling 5/R when I realized I had read the torque wrong on the big mainshaft collared nut.
It only got 101 ft. lbs instead of the 110-130 like it was supposed to have.. :oops:
I agonized over a while, got mad, denied it, stages of grief thing..
Since it's a 'single use' deal, it gets ruined taking it off.
101 is real close to the bottom of the proper spec..
I couldn't stand it,
And set myself back about 1/2 a day ripping it all back apart,
and torqueing the nut just that much more.
It got staked back, kinda, not as good as it shoulda been..
It all got cleaned and chased, and some blue Lock-Tite, and some extra torque, like 145,
But.. It probably will be liking to back off now.
Probably woulda stayed better at 101 with a proper stake!
These get hammered free, and are known for being loose in regular use.
In hindsight, I shoulda put a tack-weld onto it.

So, there's my excuse for tearing it apart again one day..
A newbie blunder no doubt, and I'm putting it up here for others to learn.
No, waiting until after the holidays to finish this job just wasn't an option.
My life doesn't have room to leave this apart for another couple of months right now..

My struggles even started to mangle my new shim.. :oops:

But there it goes, all back together; I'm calling it a win even if it's not perfect.
Sometimes a first try at things goes like that.

Got my bench back!

Have the tools and the know-how now to do this right some day..

We'll get some miles on this thing once the car gets built for it.
Might be another year or two, but it's happening.

Should there be any issues, it will be a lot easier next time, no doubt.
It'll give me some time to cash-flow the parts it really wants..

Thanks again for taking the time to share.
This is a LOT better trans than it would have been without the help.
What's that Noise??

Beginners' how-to Type 4 build thread ---> viewtopic.php?f=1&t=145853

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