Building a 915, diff is misaligned.
 

Gentleman,

Im working on building a 915 that we recently acquired and I've run into an issue. When installing the diff, the ring and pinion
seem to meet way before the inner bearing has fully seated in the inner race. The coverplate won't fully close, things don't fit.


Notes on the transmission:

• Standard 915 open diff
• had original bearings with a spacer ring installed for each bearing, as such:

Inner Bearing - spacer - dddddddiiiiiifffffffff - spacer - Outer Bearing



Possible solution 1: move the spacer ring from the outer bearing to the inner as such:

Inner Bearing - spacer - spacer - dddddddiiiiiifffffffff - Outer Bearing

This config would allow the inner bearing to seat properly before the R&P come into contact, BUT Im concerned that the
output flanges would not properly seat or the inner flange would seat too deeply into the case.


Possible solution 2: machine the ring or the diff by ~4mm (the width of the spacer ring)

This solution would keep the original bearing/spacer setup and not misalign the diff in the housing, but Im concerned that
its a "brute force" option that could weaken either the ring or the diff, maybe even causing the diff bolts to not fully seat.


Possible solution 3: the R&P are mismatched and I'll need to find a matching set

Ring #s: 10.75 / 213 / 0.15 - 31 teeth
Pinion #s: 11.75 / 213 / N33 - 8 teeth

Which of the numbers on the diff need to match?


Possible solution 4: The diff is incorrect. Is there a difference between 7:31 and 8:31 diffs? Could it be a 901 diff?

???


Thanks in advance,
Mat

http://forums.pelicanparts.com/uploa...1389116880.jpg
http://forums.pelicanparts.com/uploa...1389116888.jpg
http://forums.pelicanparts.com/uploa...1389116898.jpg
http://forums.pelicanparts.com/uploa...1389116904.jpg
http://forums.pelicanparts.com/uploa...1389116912.jpg

You need a spacer between the ring gear and the differential.

I need to move the ring further away from the pinion, not closer.

This is not a do it yourself project. The ring and pinion I believe, are made as sets and are machined to work together. You should not mix them. The depth of the pinion into the ring and the backlash are very critical settings. The proper gauges to set them cost several thousand dollars. You need to send the transaxle to someone who knows what they are doing or you will end up with a box full of junk in a few thousand miles. My son, John Flesburg, could advise you or could do it. He has the necessary tools, the question is does he have the time to do it?

Totally agreed. Im actually more looking for information as to why these parts are not playing well together. Are they mismatched? What do the numbers mean?

The good news is your R&P is a matched set (the "213" match number). The bad is news is something is seriously out of whack and daflesburg is right. You don't move the ring closer or nearer to the pinion. You put it in the correct location in the housing with spacers and the correct measurement tools. Then you move the pinion in or out to the correct engagement depth (the other numbers on your R&P), again using spacers and the correct ($expensive$) measuring tools. My hunch is this box started as a pile of parts and there are missing pieces (spacer and/or shims) on the pinion shaft causing it to sit too far into the differential housing.

My 915 Tutorial should help;

Welcome to Red Line Porsche Wiki - Porsche Wiki

Scroll left side of page, click on "915" under "page tags." The site will take you to the 9-part Tutorial.

"213" is your pair number, which indicates a matched set.

You may have an 8:31 ring gear mounted to a 7:31 differential.

Hmm yeah, I was *just* thinking that. I'll add it to the original post.

Is there a way to tell the diff's apart? Should I look for a part number and try and run it?

Great, thanks Peter!

As a courtesy to people who have already responded, you shouldn't update your original post with new thoughts or information, it can make their answers look foolish.

The 8:31 differential flange is offset 2.0mm further from the pinion, as the ring gear is larger in this direction. The fact that your diff has no provision for a speedo ring indicates it's pre-'76, and very likely a 7:31 version. Maybe Matt Monson can post his graphic with diff dimensions.

Great suggestion! Thank you, Jon!

Jon, correct me if I am mistaken, but the relatively rare 8:31 r/p sets from the mid 70's that used the mechanical speedo drive I believe had the same ring gear flange thickness as a 7:31.

My original comment was based on a guess that he has a fine spline open diff from an SC or 3.2 and his 1975 date stamped ring and pinion is too skinny.

We make a spacer ring that goes between the diff and back of the ring gear that allows one to put a 7:31 r/p set into a late box with the diff flange out farther towards the sidecover.

Or maybe I am completely misunderstanding the issue here but the pictures suggest that the diff needs to be moved more towards the side cover and with fewer shims under the bearing.

I've had the flu and my computer is turned off. I can post some drawings with measurements tomorrow when I am back in the office.

In the meantime, original poster, would you post the part number and prodate off your differential along with whether it is course or fine spline. Is should be on the edge of the ring and will be an 11 digit number and the prodate will have a format like the ones you posted for your ring and pinion ( 10/75 and 11/75 respectively). As others have said your r/p is a matched set.

Matt M,

Dates on the R&P indicate they're 1976 model year parts, for transmissions with electronic speedo sensors. The 8:31 ratio began in '75 m.y. on US models, with ROW being 7:31. I'm not aware of a '75 m.y.-only 8:31 offset, could be, but these are not '75 m.y. gears. The orignal quote above suggests to me a ring gear and diff mismatch. He stated the diff had "a spacer ring for each bearing", no speedo disc or doubled-up spacer. I suspect he cannot close the coverplate because the diff offset is incorrect for the ring gear, based on the info provided (so far).

(Sorry to hear you have the flu)

Heres the original ad for the trans: http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/748712-fs-75-76-915-w-mech-speedo-cooler-8-31-sf-bay-pickup.html

From the ad:

We took the course spline diff... hrm.

Pics of the part numbers and the flanges:

http://forums.pelicanparts.com/uploa...1389156360.jpg
http://forums.pelicanparts.com/uploa...1389199290.jpg
http://forums.pelicanparts.com/uploa...1389156369.jpg

Yes, the first 6 months of 8:31 production were of unique dimension, with mechanical speedo, and with a unique differential (offset 2.5mm).

You need this unique offset differential, and you need the expertise to determine diff shims "from scratch" (even though pinion depth is already set).

Hi Gears, thanks for the info! So a unique diff is needed in order to use this trans? How would an aftermarket LSD work for this, do they make these special offsets particular for this trans or ?

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

Regards,

Matt Monson
Guard Transmission llc.

This is an odd transmission, with a '76 serial number but a '75 code. The case has provision for electronic speedo sensor, but is a mechanical speedo drive. Maybe built in '76 for an earlier car, or maybe '76 ZF's didn't have sensor discs.

Assuming Paul's correct about 2.5mm offset, I don't see a problem using 2.0mm offset differential and adjusting bearing shims for 0.5mm (0.020") difference.

I should clarify earlier comment about 8:31 ring gear, the larger pinion head is also a factor in different offset.

Just a comment. Not sure who installed those lock plates...

http://forums.pelicanparts.com/uploa...1389199290.jpg

...but I would much prefer to see them locked down like this:

http://forums.pelicanparts.com/uploa...1389203737.jpg

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

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?

http://i205.photobucket.com/albums/b...1DiffTypes.jpg

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

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:


http://forums.pelicanparts.com/uploa...1389206179.jpg

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.

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...

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...

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.

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.

http://forums.pelicanparts.com/uploa...1389217927.jpg

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.

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.

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.

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

http://i205.photobucket.com/albums/b...1DiffTypes.jpg

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?

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.

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.

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.

Good point, we plan to use course spline 914 output flanges as Haycait used in his build:
http://forums.pelicanparts.com/porsche-911-technical-forum/446271-help-please-putting-915-into-swb-axle-set-up.html

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

http://forums.pelicanparts.com/porsche-911-technical-forum/548265-83-3-0l-engine-trans-swap-85-3-2l-engine-trans-axle-options.html?highlight=915+axle+flanges

See post #12...

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.

http://forums.pelicanparts.com/uploa...1390883250.jpg

http://forums.pelicanparts.com/uploa...1390883240.jpg

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.