915 rebuild: ? re backlash expert help please

[robmog]

or at least someone who stayed at a holiday inn express!
i'm using zimmermans excellent wiki but can't get the backlash in spec.
my first setup (prior rebuild?)had 3.0 and 3.0 diff shims , there was 0 preload on the diff.
i put in a 3.2 and a 3.4 shim, preload on my new skf bearings was too high.
i tried 3.0 and 3.2, pre load was too low and backlash was 0.30mm (spec is 0.15).

i bought 3.1 and 3.3 shims, preload was dead on at 26in/lbs but backlash was
still high at 0.25 mm.

any ideas what i can try next?

help much appreciated.

[KeoRS]

I believe Peter Zimmermann is a frequent poster. Bump for you my friend. Hope you get some tips soon. I'm in the process of finishing my 915 clean-up (http://forums.pelicanparts.com/cleaning-detailing-polishing/550748-best-cleaner-bar-none.html) and will be going through his walkthrough like you are!

[Jon B]

Bob,

Try 3.2 and 3.2 next. You can also create 3.15 or 3.25 sizes using a .25mm shim in combinations. I'm assuming this is a mag case for your '72, so you can increase the preload if necessary. It takes time to get spot-on, unless you're lucky.

[Jon B]

You can also surface grind larger shims to smaller sizes, which might be quicker than ordering sizes you don't have. It doesn't sound like you'll need those 3.4 and 3.3 shims for this transmission, so they're available to re-size.

[LUFTMAN]

Contact gary faibanks he will help you

[Trackrash]

I'm no expert, but I have done this a couple of times. If you have the factory manual there is some information on setting up your R&P that is helpful.

You are half way there. You know what your total shims are. Now all you have to do is adjust that total side to side to get your back lash set. Lots of fun right?

I would start by switching the shims you have in there now from side to side and re-measuring your backlash. If you are lucky it will be OK. If not you should be able to calculate the final thicknesses needed and use the shims you have.

[Peter Zimmermann]

Bob, as Jon B. wrote, you need .25mm shims so that you can make minimal changes and sneak up on the spec. Be sure that you don't accidentally change the total shim thickness when you start changing shim sizes! Also, when you get it close remember to make a measurement in 3 or 4 places.

I assume that your 3.3 shim is in "S1" (ring gear side), and your 3.1 shim in "S2," correct? That means no matter what you do you want to end up with a total thickness of 6.4mm. Because your allowed tolerance is .12mm to .18mm, you don't have far to go. You want to reduce the thickness of S2 in order to allow the ring gear to get closer to the pinion head (reduce back lash).

If you go from 3.1 to 3.05 (S2) you will need a 2.8 shim and an .25 shim. To maintain your 6.4 total you will need to go up to 3.35 on S1; a 3.1 shim and a .25 shim. 3.05 + 3.35 = 6.4.

If you have 3.3 in S2, and 3.1 in S1 (doubtful), you have to make some different calculations, maybe start by flipping the two shims to see where you are. Just be extremely careful to not lose all of your back lash while tightening the side cover; you don't want to damage the gears. Also, always be sure that the bevel on your shims faces toward the diff.


You might want to look through this thread...

http://forums.pelicanparts.com/porsche-911-technical-forum/469037-915-rebuild-part-ii-time-assembly.html?highlight=differential+back+lash+915

[Jon B]

Pete,

I assumed his S1/S2 was "3.1 and 3.3", having written it in that order. S1 is always listed first, as in 1 before 2, and as printed in the manual, but maybe he's referring to them as you say. Bob?

Other things to remember- when measuring backlash, the pinion nut must be torqued, and it's best to support the shaft(s) with the midcase. When measuring bearing drag, the pinion must be removed.

[Jon B]

Originally Posted by robmog

...i put in a 3.2 and a 3.4 shim... i tried 3.0 and 3.2... i bought 3.1 and 3.3 shims...

Bob, which are your S1 shims, which are your S2 shims?

[robmog]

Doh! moment.I thought I had it all worked out but with all this assemble disassembly BS I got mixed up. Currently my S1 is 3.3 S2 is 3.1.
I have 3.0 3.1 3.2 3.3 and 3.4 shims available and those suckers are pricey!
I've established 6.4 is the correct total thickness. Should I try for the correct ratio where S2 > S1?
And how important is the 0.2 mm difference?
With my shims I can pair 3.0+3.4 and 3.1+3.3, I only have one 3.2 so unless I buy another that's out. I could buy the 0.25 shims to increase my options but that will take time. I had to wait two weeks for the 3.3 shim to come from Germany! They are not all available here.
Thanks to all for the advice.

[Jon B]

Bob,

OK, so Pete assumed correctly! It's best to refer to these in S1/S2 order, to avoid confusion. The book exercise S2>S1 is only for initial set-ups from scratch, otherwise it's irrelevant.

With the only five shims you currently have, you might as well try S1/3.4 and S2/3.0 next, and tell us the results. Make sure your puller didn't burr or distort the shims before installing them.

[robmog]

Will do.

[Peter Zimmermann]

Originally Posted by Werkstatt

Bob,

OK, so Pete assumed correctly! It's best to refer to these in S1/S2 order, to avoid confusion. The book exercise S2>S1 is only for initial set-ups from scratch, otherwise it's irrelevant.

With the only five shims you currently have, you might as well try S1/3.4 and S2/3.0 next, and tell us the results. Make sure your puller didn't burr or distort the shims before installing them.

Good suggestion, Jon! It's all guesswork anyway, and this combo might get Bob where he needs to be.

[robmog]

i'd rather be lucky than smart.
as a first step i reversed S1 and S2 ie 3.3 is now on ring gear side (S1)
bingo! back lash is perfect at 0.15mm.
i can move on!
one last related question, i notice that the ring gear and pinion have hash marks.
2 on the ring 1 on the pinion.
this suggests indexing to me, is this important to line up?
thanks for all your help

[Jon B]

Originally Posted by robmog

...as a first step i reversed S1 and S2 ie 3.3 is now on ring gear side (S1)...

So you did have 3.1 on S1, as I first assumed. I think. Very confusing, the results don't really add up, but I'm glad you've arrived at a happy conclusion! The hash marks in question have no relevance to what you're doing, don't worry about them.

[robmog]

yes, originally 3.1 on S1, now 3.3 on S1.
by switching, ring gear got closer to pinion and backlash got smaller.
thanks again jon.