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Can anyone identify this 915 syncro?

Is there any way to identify what brand/manufacture this syncro ring is?

Iīm going through the 915 transmission for my -81 SC, issue has been baulky shift into 2nd and sometimes 1st (not really grinding but more like resisting going in to gear), and also slightly "sticky" 3rd gear when shifting out of gear (bushings in linkage are all new and properly adjusted).

These symptoms Iīve experienced in several old 915 transmissions. However, thing is that this tranny has been "restored" by a firm here in Sweden (prev owner, about 10 years ago) where 1,2 and 3rd syncros were replaced along with 2nd gear dog teeth and sleeve for 3-4 gear.







My trouble is, to me these parts doesnīt look all that bad? 2nd dog teeth, although slightly blunted, still seem pretty good and measures within spec and sleeve isnīt that worn. 1st gear dog is worn and will be replaced but it would be interesting to know whats causing the baulky shift?
I can see that during the last "rebuild" the syncros were ordered separately from another firm which now seems to be out of business so I cannot tell what part brand they are. Is there anyway to tell if the syncros alone are the cause of issues?

Thanks
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Jakob H

1981 911 3.0
Old 05-11-2018, 01:29 AM
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Old 05-11-2018, 01:30 AM
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Any shiney areas on the syncro surface would be suspect. Scratch the entire surface with your fingernail, looking for slick areas instead of the normal rough surface.
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Old 05-11-2018, 03:17 AM
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Thanks for quick reply. There are only minor slick areas on these syncros as seen on first pic, however 4th gear is much much worse for wear but despite this gear slips in super easy. Thats why Iīm thinking maybe there is a difference in manufacturer/quality but could still be within spec? R&S or porsche or similar experience?

Iīve done a few moly-transmissions before (not porsche) and they usually shift much better despite being almost worn out syncros...
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Jakob H

1981 911 3.0
Old 05-11-2018, 04:44 AM
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There was no R&S ten years ago... The only option was German OEM.
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Old 05-11-2018, 05:42 AM
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Thanks, that is very interesting info! So perhaps it could be the sleeve being worn out, despite having pretty sharp teeth. I havenīt measured wear inside of the sleeve (and donīt know how to) but I guess the ramp angle or dia could be wrong compared to new syncro rings (prev owner also said it shifted like this which is why he sent it out for resto in the first place).


Another question while I have this vast 915-experience at hand.. I also have a lose pinion bearing race to deal with. Do any of you have experience with the oversize bearing? I have not decided weather to machine the case for this bearing (+ adding the one piece retainer) or to go with the full figure of 8-steel insert.

Will it hold up for standing starts in track/race condition with new press fit bearing and one-piece retainer or is it just a short time solution?
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Old 05-14-2018, 03:11 AM
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Jakob H

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Old 05-14-2018, 03:24 AM
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I would check the 2nd gear dog teeth. I bet they don't have 9 degree backcut on them. German OEM replacement teeth have never come with the backcut. You have to add it.

We have seen the sleeves that CMS and others insert for a repair to be quite reliable and durable. While the WEVO insert is a great piece, it's really overkill, IMO.

Regards,

Matt
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Old 05-14-2018, 06:17 AM
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HI Jakob

I have , or can get O/S outer bearing race, at 80.10mm, 80.15mm, 80.20mm, 80.25mm any thing around the 80.25mm depending on where the bearing housing is out, may need machine the case out to make it round again, have you some kit to check the size of the casing, a bore gauge to check the size of the case bearing hole

I have a chart to take the bearing hole size readings;



give me a shout if you need any help,

regards mike
Old 05-16-2018, 12:47 AM
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Matt, I am not familiar with backcut on dog teeth. Where, under the teeth? I expect the same could be said for first gear dogs?

Mike, that could be interesting. I can probably measure the hole for ovality but for off center measurement I will have to ask a machine shop (which I will have to do anyway to cut the hole).
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Old 05-16-2018, 04:55 AM
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Reading the original post, my understanding is that the 1st and 2nd synchros were replaced but their shift sleeve (slider) was not. The sleeve should have been replaced with the synchros, they wear together.
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Old 05-16-2018, 05:11 AM
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I did a rebuild 3 years ago (thanks JW and the others here). I recall being told to replace sleeve if replacing synchros.

Once you've got it open, I would go ahead and do all 3 parts. The effort of opening/closing outweighs the $300 for the pair of synchros and the sleeve. You can see pics of my parts here:
Can't Shift Into First

Also, I recommend getting the OEM synchros, not the R&S ones. I had to return my R&S due to manufacture defects and it cost me precious driving time. Here's my end-thread recap of what I learned:
Can't Shift Into First
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Old 05-16-2018, 05:55 AM
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Quote:
Originally Posted by Jakesta View Post
Matt, I am not familiar with backcut on dog teeth. Where, under the teeth? I expect the same could be said for first gear dogs?
The outer edge of the synchro is curved. The backcut is on the backside of the teeth to accomodate the curve in the synchro. Without it, the synchro is pinched too tightly.

Quote:
Originally Posted by kevingross View Post
Reading the original post, my understanding is that the 1st and 2nd synchros were replaced but their shift sleeve (slider) was not. The sleeve should have been replaced with the synchros, they wear together.
While I agree that this also likely a problem, I wouldn't chase it until both 1 and 2 are confirmed to be backcut and there's a loaded synchro in the thing that measures to spec. Then one would be able to "test fit" it to the shift sleeve and see how well it "pop" in and out of the sleeve. While much of my experience is in race boxes, in general, we see the synchros get replaced two times for every shift sleeve replacement. YMMV.
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Old 05-16-2018, 08:04 AM
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Quote:
Originally Posted by OsoMoore View Post
I did a rebuild 3 years ago
Have read your thread and a couple of others

Thanks Matt, I will check for backcut. I did measured the assembled syncro and they are close to 86.3mm on both gears which seems to be within spec. I do suspect shift sleeve is worn and it will be replaced this time but on the bench anyway it pops in and out of gear with relative ease.
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Old 05-16-2018, 10:58 PM
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If it slides in and out too easily, it's worn out. There should be a bit of resistance as you push it over the hump, and then the synchro drops into the divot on the other side. The wear you are concerned about is off the top of the hump.
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Old 05-17-2018, 06:06 AM
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ZF diff

Have some guru-questions regarding the diff I planned to use, its a zf LSD supposedly from a 911 race car. This is how it looked when I opened the it up..
Seems there is very few clutch disks? only one friction plate per side, however i got several more plates supplied with the diff. If I should put one more "stack" of plate-disc on either side this would then double the locking percentage? Wouldnīt this increase total thickness so that preload would now be extreme?

Regarding the ramps.. it seems the ramps angles would mean this way there is increased lockup under acceleration and less lockup on coast? Or am I thinking backwards?

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Old 06-01-2018, 03:08 AM
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That’s not a ZF diff. That’s a GKN lsd from a 993 G50 gearbox. While you can assemble it as a 4 plate lsd, you cannot install it in a 915.
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Old 06-01-2018, 05:10 AM
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Wtf! Thought I had this covered.. will have to look for another diff now $$
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Last edited by Jakesta; 06-01-2018 at 05:46 AM..
Old 06-01-2018, 05:37 AM
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