108 mm fine spline output flange options for a 915
 

I am switching from a coarse spline to a fine spline differential in a 915. I'm using 930 CVs in a race application, so will need 108 mm fine spline output flanges. I understand that size is a rare bird and that there are options like shortening a set of G50 output flanges. Are there other options?

I think CMS might make and sell these as well.

Given it's probably considered a downgrade to keep the coarse spline hubs, but another option is sourcing coarse spline bevel gears to swap into your new diff. Matt M was able to get me a pair of fine spline bevel gears to install in a Guard LSD I sourced from an older car to go in my racecar 915. Price was reasonable as well.

sorry to bust in... I have an early 915 with a 8:31 R&P that i want to drop in my 86, obviously the axels won't work since the early tranny has course splines and flanges are too small to fit the axles, what's the cheapest way to get this done?
My mechanic quotes me $1200, (CMS I think) is this reasonable?
Thanks!

$1095

https://californiamotorsports.net/collections/porsche-915-transmission-parts/products/cms-911-930-conversion-axle-flanges

Ok, so $ 1200 installed it's not bad, thanks for the confirmation, this build has been a nightmare, I'm dropping a 3.6 in my 86, I had an early 915 with a Quaife LSD and 8:31 R&P that I want to use since I sold the original 915.

Thanks for, chiming in,
Merry Christmas!

Pretty sure there are cheaper options than $1095 for a pair of flanges.

Sure, if you can find them.......

Most cost effective would be early 930 output flanges. Hard to find...

http://forums.pelicanparts.com/porsche-911-technical-forum/172838-915-output-flanges-please-help-me-set-up-my-915-swap.html#post1414132

I'm not sure if this info will apply to my planned conversion, so let me ask here. I have a 915 magnesium case with the 4-bolt/2-pin flanges in my 1973 911T (with 3.2 long stroke, etc.). It's not the original transaxle, but in that same timeframe. I want to add LSD to it and 8/31 ring and pinion. Will I need to swap output flanges or other stuff? Or does the LSD+8/31 R/P drop directly into my case and use the same flanges?

You can buy 915 differentials from NPR or Guard with course or fine splines. Personally, if I had course spline output flanges and I were buying a differential, I would get a fine spline differential and get new flanges.

Also, when you install the new differential, you will need to get the ring and pinion adjusted (depth and backlash).

When changing only the differential, pinion depth is not affected and does not require setting. Only the backlash needs adjustment.

915
 

I think he wanted a LSD carrier swap, which requires R&P installation and set up.

I had a LSD/TBD installed in my 915 a few years ago and it was a somewhat pricey job, at least compared to a recent Truetrac installation in my old Toyota Tacoma 8.4” rear end!

You might only check/adjust backlash...

If you replace only the diff, check/adjust preload and backlash. Preload is measured with an in-lb torque wrench.

No differential carrier replacement, LSD of any type or open, require any pinion adjustments. The pinion is completely unaffected. Ring gear backlash and differential carrier preload are the only settings necessary.

Yep, my mistake.

Maybe it’s just me, but I wouldn’t install a replacement diff. into a 35-50 year old gearbox without first checking the pinion depth. Even if the box was previously rebuilt, with all the case bore issues and other long term problems these gearboxes are subject to, it just makes sense. I don’t know how many times people have reached out with a bad pattern on an install where the backlash and preload are within spec. It’s almost always a pinion depth problem. Assume nothing when you open up a gearbox for the first time.

Matt brings up a good point, even if it's not glaringly apparent in his response.

Get some gear marking compound and check the ring gear pattern as a final check. If the pattern is off, for any reason, you'll quickly know if you must go deeper. And give the pinion gear a good shake and feel for any looseness when the diff is out. Most experienced people can hear or feel a clunk of even a of thousandth or two and know if something is wrong. The cases wear much more than that when worn. And if you lack the requisite "feel," you can measure the play with a dial indicator. This is irrespective of the pinion depth, which should not change (unless someone else really butchered it up).

FWIW, after building a LOT of transmissions and differentials, I found that the factory shims should be your starting point and if you vary much from that point, you'd better double check your own work first and find out why you're off from what the factory did. JMHO

Good points, but I would go further and check it with paint before you ever take everything apart. That’s your first baseline “measurement “ before you even get out the special tools. Never assume the starting point is correct.

I certainly can’t argue with the logic of verifying there are no pre existing issues in the pinion setup before a carrier swap.

Thanks gents for the replies. Remember I have a 7/31 now, and want to change to a 8/31, in addition to adding a LSD. So I expect to have to make adjustments when installing a new pinion gear. I've set up diffs for American iron, and I don't expect that German diffs are from a different planet...

BTW, the current diff is quiet. I can't hear any R&P whines over the regular forward gear noise (and there's not much of that either). I expect to start by using whatever factory shims are already in there, test install the carrier, then check the pattern using Prussian blue, and check the lash at several points in the rotation. If the pattern looks good and there isn't excess lash, I'll call it good. And I will probably check the pattern of the current 7/31 R&P before I take it out, just to see what that is.

Other options I'm considering: (1) take the transaxle to a good shop in the Seattle area and let them deal with it. Any strong recommendations? (2) exchange for a complete transaxle that already has the 8/31 and LSD. I'm not particularly price sensitive (it IS a Porsche!), but I don't want the car being "garage art" for a couple months.

And here's some interesting trivia about Mercedes diffs if any of you are interested: To change the R&P in a MB rear end, the shop is supposed to use a special test stand in which the R&P is assembled and actually run with an electric motor to find the position where it runs quietest, rather than by measurements and pattern. Only a couple shops in the USA that I know of have/had those test stands. One of those is Stu Ritter Mercedes/Langston Motors in Denver.

Each pinion is marked with a pinion depth variance number, and it’s very likely they won’t be the same. These are measurements made to 1/100th of a mm and use a special tool (vw385 bar). If you don’t have access to one, it’s probably best to just give it to a shop. John Walker’s Workshop is close to you and very well respected here on the board.

Porsche ran them in like that at the factory before installing them, or putting them on the shelf for spares. That’s where the variance number comes from. They start at standard mounting depth and then move it until it’s quietest. That gets scribed on it for the installer to reference.

Bluing (or other marker paints) doesn't work for two reasons. First, on the bench you are getting a pattern from an unloaded ring-to-pinion set. The contact patch moves under load. Second is that your eyes and bluing cannot distinguish to the precision needed: +/- 0.04 mm. Get someone who has the factory tools to check for you -- goes quick, I do it for every transmission I rebuild.

I sort of disagree. If you mark it prior to tear down you have a decent idea of how it has been running. While your comment about it being under load is true, it’s good enough as a look see. If it initially looks good, the goal is at the end to have it look exactly the same.

Now what you do in the middle, with the tools and the actual set up, is absolutely required and I wouldn’t ever advise setting up anything by using just paint.

Then you'd better tell that to the people who actually MAKE ring and pinions. And I'm not talking about what you refer to as bluing, but actual gear marking compound.

https://www.youtube.com/watch?v=SUiOZOA4DoI

Watch the video at 1:19 and tell me what that is you see on that gear set.

There have been literally BILLIONS of gearsets manufactured using gear marking compound to check patterns.

This stuff isn't rocket science.

Can you show me their Porsche ring and pinions? ;)

Every ring and pinion I’ve ever had manufactured for Porsches (by Ricardo and Hoer) have always had standard mounting depth and backlash numbers on them, the way that Porsche has always done. http://forums.pelicanparts.com/uploa...1672883656.jpg
http://forums.pelicanparts.com/uploa...1672883656.jpg
The second one is a return I took back from a guy who didn’t have the tools and thought he could install it with just painting it. He failed.

You can't just tease us like that with the damaged goods in a bag!

But of course, installing a ring and pinion, any ring and pinion, requires a fundamental knowledge of repair procedures and skills. One or both were lacking if he damaged stuff. Unless of course, he confused a european 1 for a US 7. ;-)

And as one of my fellow mechanic friends used to say, "Some people can break a crowbar in a sandbox."

He didn’t damage anything. He gave up. I’m not going to embarrass anyone, but he’s a very well known Porsche engine builder. He just doesn’t do gearbox work. If you think Porsche stuff is the same as doing a Ford 9” rear end I’m going to disagree with you. I only sell wholesale. And these days I only sell to guys who I know have the tools. That avoids self proclaimed experts ruining good parts.