Building a 915, diff is misaligned.

[Jon B]

Originally Posted by Matt Monson

Sounds like I nailed it. And yes, we have an LSD that could be made to work with what you have there.

Matt, please explain. You said he needed a spacer between the ring gear and differential.

[MichiganMike]

Thanks Matt for the info. Glad to know there is an option out there for this trans, as it will want an LSD at some point in its life!

The first photo above from Peter shows "3 2.5", would this indicate the 2.5mm offset or no? Wondering still if this is the original diff or not. Would there be a way to tell if this diff is in fact the special diff needed for this trans?

[Matt Monson]

Here's the chart that was promised.

What I suspect happened is when they snatched the LSD out of that gearbox they just mated it to whatever midyear 915 course open diff they had laying around.

Regards,

Matt

[Matt Monson]

Originally Posted by Werkstatt

Matt, please explain. You said he needed a spacer between the ring gear and differential.

I'm going to guess that if he measures the distances as shown in the picture, his differential will measure 101.4 like a midyear.

To make an early ring and pinion (which I am pretty sure this one will measure out at even though it's not 7:31) fit onto a mid or late diff you need to push the differential farther away from the center line. I've seen guys do it using shims, but then what can happens is you are actually only moving the bearing outwards and then the bearing itself can overhang the end of the flange of the differential.

So we came up with this:




It measures approximately 2mm and makes the majority of the correction for you before you start messing around with the rest of the set up. This is how most of our customers install a 7:31 ring and pinion into their SC or 3.2l Carrera racers.

[Jon B]

Originally Posted by Matt Monson

I'm going to guess that if he measures the distances as shown in the picture, his differential will measure 101.4 like a midyear.

Matt, if so, then he's missing the speedo disc or appropriate sustitute spacer. He indicated there was only one spacer under each bearing. There's no offset difference between a midyear and late 915 diff, only a thicker mounting flange which has no effect on offset. And it wouldn't explain why he cannot close the case coverplate...

Originally Posted by Matt Monson

To make an early ring and pinion (which I am pretty sure this one will measure out at even though it's not 7:31) fit onto a mid or late diff you need to push the differential farther away from the center line. I've seen guys do it using shims, but then what can happens is you are actually only moving the bearing outwards and then the bearing itself can overhang the end of the flange of the differential.

Paul says the early 8:31 had a 2.5mm offset, even greater than 2.0mm, and not equal to a 7:31. I'm skeptical that an 8:31 with that large pinion head could have no offset difference to a 7:31 set, have not seen this nor does it agree with Paul's comments...

[Matt Monson]

Jon,
The OP will need to correct me if I am wrong but I am guessing he got this gearbox as we are seeing it and whomever replaced the LSD with the open diff in question didn't know that there would be a speedo tone ring missing.

I would like some measurements off the diff before making any more guesses. The casting number he showed closely matches both the early and the mid year part numbers as published in PET but the 05 at the end doesn't correspond to an actual part number.

I've got an NOS 8:31 here that I can measure the height of the ring gear itself. Then the OP can measure his and we can quit guessing and start dealing with actuals.

We can get him to where he needs to go but we still don't have 100% of the technical facts. To get there that ring gear is going to need to come off the diff and some numbers need to be measured.

[Jon B]

Matt,

My understanding is he bought the transmission with ZF removed, and seller provided a standard diff for him to install himself, which he's now attempting to do.

If his differential has pinhole for the speedo disc and measures 28mm, as shown below, then it's a 915 '76-'86 diff with 2.0mm offset per your diagrams. I suspect it isn't, based on what we've been told so far, and will measure 22mm. Just to be certain, do remove the ring gear and confirm other measurements as well.

[Geary]

It's been a very long time since I measured, but I believe the diff required has a 103.9 dimension, rather than 101.4. It's the odd-ball ring gear that that requires the 2.5mm difference.

This is all divorced from the 7:31 into 8:31 install. This is a unique 6 month 8:31, differing from earlier 7:31 dimension or later 8:31 dimension. It's the only diff of this era that isn't on that dimension sheet.

[Jon B]

Originally Posted by Gears

It's been a very long time since I measured, but I believe the diff required has a 103.9 dimension, rather than 101.4. It's the odd-ball ring gear that that requires the 2.5mm difference.

Paul, you're adding 2.5mm to a speedo disc diff dimension (101.4) that already has a 2.0mm offset, meaning a 4.5mm total offset from the early 915 diff. Is that what you mean to do?

The easiest way to understand offset on Matt's diagram is with 127.500 dimension on early 915 diff, 129.500 dimension on later two 915 diffs, a difference of 2.00mm. Otherwise you'll have to factor in a 6mm speedo disc on the later diff dimensions.

[Geary]

I'm actually on a ski trip, Jon .. 500 miles from any sketches or notes that I have on this particular oddball diff, so until I can get to them, the cobwebs win. I just remember a 2.5mm difference in mounting flange position.

[MichiganMat]

I measured the diff last night, it appears to similar to the diff marked 914 in the below chart:

[MichiganMike]

Sounds like we have the right diff then. Are we missing a piece for the mechanical speedo? Is there some special piece that belongs in there for it?

[Matt Monson]

The mechanical speedo is driven off the tail end of the pinion. Not related to diff set up.

It's starting to sound like Jon is right and I was talking backwards. But we still need to figure out if you've got the weird width ring gear or if it is just a standard 8:31 width.

If it is standard your solution is to just get a regular 915 course spline Midyear diff.

[juanbenae]

mat, i still have the diff removed for the LSD install from my 78 if interested & it could help. <80k miles when removed.

[MichiganMat]

Originally Posted by car 311

mat, i still have the diff removed for the LSD install from my 78 if interested & it could help. <80k miles when removed.

Badass! I may take you up on that. I'll take a few more measurements tonight and get back to you.

[Matt Monson]

Just be aware that that SC diff will be fine spline. It will pose a stub axle conundrum while potentially fixing your set up problems.

Also there are several different diameter cv axles. If you change stubs you need to make sure they are right for your cv.

[MichiganMike]

Good point, we plan to use course spline 914 output flanges as Haycait used in his build:
HELP please - putting a 915 into a SWB - axle set-up?

We will be aware of this if we change the diff. Thanks for the heads up Matt M.

[Peter Zimmermann]

83 3.0L Engine & Trans Swap to 85 3.2L Engine & Trans - Axle Options

See post #12...

[MichiganMat]

I went over to car311's place this weekend and confirmed my suspicions about the diff we have.
His diff has the mounts for the speedo wheel and has the right offset, looks like the right fit.

He gave me a great deal, looks like we can move forward.

[MichiganMat]

Does anyone know where I can purchase spacer rings to properly align/space the carrier bearings?

With a missing speedo ring on the diff I need to either acquire a speedo ring and/or find spacers.