108 mm fine spline output flange options for a 915

[Matt Monson]

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.

[LJ851]

Originally Posted by Matt Monson

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.

[PeteKz]

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.

[PeteKz]

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.

[Matt Monson]

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.

[kevingross]

Originally Posted by dannobee

Get some gear marking compound and check the ring gear pattern as a final check.

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.

[Matt Monson]

Originally Posted by kevingross

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.

[dannobee]

Originally Posted by kevingross

Bluing (or other marker paints) doesn't work for two reasons...

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.

[Matt Monson]

Originally Posted by dannobee

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.

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.

[dannobee]

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."

[Matt Monson]

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.