915 1st gear pops out with new synchros

[flatspin911]

I have rebuilt our 915/63 gearbox due to failure on 1st gear. New 1-2 slider, new syncros and dog teeth for 1 and 2.

When I put together the 1st gear syncho the diameter of synchro was less than 86.2 min spec, closer to 86 or a touch less. All new parts! So I figured its because the dog teeth have some coating on inside and as syncro wears into the dogs it will get larger.

Put box together. All good except the first pops out of gear now!
Checked linkage adjustment, motor mounts are good, the adjustment of shift forks was done correctly......
Hmmmm..... could it be because of installed synchro size smalled than spec and allowing the gear to jump out???

Has anyone experience putting all new parts in for 1st gear and synchro ring installed diameter less that 86mm?

What is advantage of using the 1st gear sinchro - which is unique. I am considering using 2nd gear synchro in its placed. When installed its within spec.....

Im double checking everything again before putting it back together.
Comments?

[Matt Monson]

Did you put new dogteeth on 1st? Were they made by Rauch and Speigle?

[Peter Zimmermann]

I would say that the last handful of 1st gear synchro rings that I've installed have been a tad bit undersized in their installed dimension. Is there any chance that you put the 1/2 slider backwards?

Are you positive that your gear lever (the part below the bend), in neutral, is vertical?

[Walt Fricke]

Peter - I thought that the asymetrical teeth on the later 915 1st gear slider were there to make engaging 1st easier - or something like that. If the slider were backward, wouldn't some problem show up in 2d? Or is the internal circumferential groove in the sliders located differently as well?

[Matt Monson]

The problem with the Rauch and Spiegle teeth is that they are not putting the same 9 degree backcut on the teeth that the factory did. The synchro ring is curved, not square, so when it gets pinched by the teeth that are perpendicular to the base of the tooth it gets pinched too tightly and is undersized. Until R&S changes their manufacturing specs this will continue to be a problem since Getrag teeth for 901 and 915 are now officially NLA.

[flatspin911]

Matt, your the MAN! Thanks for answering this question I have had for a while! All the R&S 1st gear teeth I have put in over the years have had this issue! I went around and ground the back of the teeth to relieve the synchro enough to get the 86.3mm dimension for synchro. Not with great precision but assume it will wear in some more and work ok!

Matt and all
Thanks for your ongoing support on these forums, great facility!

Yarko Matkiwsky
Response Engineering

[yelcab1]

Originally Posted by flatspin911

Matt, your the MAN! Thanks for answering this question I have had for a while! All the R&S 1st gear teeth I have put in over the years have had this issue! I went around and ground the back of the teeth to relieve the synchro enough to get the 86.3mm dimension for synchro. Not with great precision but assume it will wear in some more and work ok!

Matt and all
Thanks for your ongoing support on these forums, great facility!

Yarko Matkiwsky
Response Engineering

show a picture?

[Geary]

These are new replacement teeth. If you look at the inside tip of each tooth, you can barely see a taste of the proper 9-degree cut .. but it isn't enough to provide proper synchro dimension.

[Walt Fricke]

An experienced transmission builder once told me that no dog tooth manufacturer other than whoever made them for Porsche (Getrag?) got this right. He also thought you couldn't get the teeth that were made right - Porsche doesn't list this as a separate part, so to get theirs you had to buy a new gear set. And maybe Getrag didn't sell to retail outlets?

He had figured out a way of cutting or grinding the backcut. And wouldn't it be best if it were a radius?

Interesting that this seems to manifest itself as a popping out of gear problem only with 1st. Though it is good that the issue is not more widespread.

[Geary]

The 9* angle is most accurately cut after installation onto the gear (centered off the gear bore). This final cut was likely left unfinished on purpose by the German manufacturer.

The same is true of later model Borg Warner style, in which the synchro cone is delivered slightly oversize, precision grinding required after installation onto an idler gear.

Just a guess ..

[Matt Monson]

Walt,
You hear about it most on 1st but I see it fairly often on 2nd and 3rd gear as well.

A lot of car owners notice it on a freshly rebuilt 915 and complain to the builder. And since many builders are ignorant to the issue they just tell the car owner that they need to just drive it more and break it in more.

This sort of fixes the problem because the base of the synchro will start to wear in and loosen up. But at the same time the slider has seen extra wear and everything else is suffering premature wear. Just more to contribute to the misperception that the 915 is a crappy gearbox. Bullsh.. in and of course bullsh.. will come back out.

[Matt Monson]

Oh, and to be clear, the first gear pops out because the synchro is more softly spring than the other positions.

On 2 and 3 it manifests itself more as a crunch and poor engagement. It will go into gear and hold but because the synchro is too small it doesn't generate enough friction to properly alter the rotational speed of the idler gear.

[strikee]

So what is one supposed to do?

Walt,

You hear about it most on 1st but I see it fairly often on 2nd and 3rd gear as well.



A lot of car owners notice it on a freshly rebuilt 915 and complain to the builder. And since many builders are ignorant to the issue they just tell the car owner that they need to just drive it more and break it in more.



This sort of fixes the problem because the base of the synchro will start to wear in and loosen up. But at the same time the slider has seen extra wear and everything else is suffering premature wear. Just more to contribute to the misperception that the 915 is a crappy gearbox. Bullsh.. in and of course bullsh.. will come back out.

[Peter Zimmermann]

Originally Posted by strikee

So what is one supposed to do?

1. When installing new synchro rings never use an old slider. After assembling a gear, exercise (hold the gear in your hand while turning the ring back and forth) the new synchro ring for a while before doing the installed measurement. I have never had a "pop-out-of-1st" complaint on my rebuilds, but I also school my customers to be sure that #2 below is done...

2. With the trans installed in the car, and the coupler connected and ready to drive, put the driver's side seat all the way to its rear position. Lift the rubber boot away from the base of the shifter. Kneel outside the car, with BOTH doors open, and lean in with your head under the steering wheel at an angle so that you can clearly see the part of the shift lever below its bend. With the lever in neutral, gently move it forward and back to its stops - but DON'T engage a gear.

You will notice a few degrees of lever movement/play, between those stops. Approximate the center position of that play. Now, look past the lever and find something large, and vertical, OUTSIDE the car. A wall stud, a telephone pole, etc., all work well. Use that item for reference to compare your lever to - the lever, below its bend, should be vertical. If it's not, loosen the clamp bolt at the coupler, and move the lever until it is vertical - don't forget to re-set the lever to the middle of its play.

This adjustment is crucial for proper gear engagement, and overlooked so often it's amazing. If the lever is not vertical, it will favor one gear (say 1st or 2nd), but be unable to completely engage its opposing gear. We re-set shift lever vertical on customer cars, that were obviously set wrong at some point, from time-to-time, and got next day phone calls asking what we did, and why their car seemed to shift better. I used to say, "Oh, we made a little change at the coupler, thought you'd like it!"

This may not be the OP's problem, but it's worth looking at anyway.

[strikee]

Interesting Pete thanks. I imported my car from UK to Australia, I was advised they rebuilt the gearbox. Receipt says all new synchros and rings or something. I don't think they did the 1-2 slider. So I cant exactly take it back.

Ive put a new coupler on, and plan to do the cup and tunnel bushings at some stage.

It just doesn't feel right getting into first and getting into 2nd at speed. I have read countless threads and suspect im going on a wild goose chase to fix this problem. I suspect the box needs to come apart again and fixed properly.

Ill try the alignment trick as mentioned above.

Sorry to the original poster for hi-jacking thread.

Cheers
Ed

[MBEngineering]

HI Ed
where did your car come from in the UK?

as for the gearbox;

flatspin911
did you select all the gears when you where putting the box back together and check that they all selected OK??

regards mike

[Peter Zimmermann]

Originally Posted by strikee

Interesting Pete thanks. I imported my car from UK to Australia, I was advised they rebuilt the gearbox. Receipt says all new synchros and rings or something. I don't think they did the 1-2 slider. So I cant exactly take it back.

Without a new slider, the 1st/2nd shifting will eventually break-in (usually) and become smoother. The old slider will carve away friction material (which will reduce life-expectancy) on the two synchro rings, so be patient with it. Also, I don't think that you can get Kendall Hypoid where you're at, but you should change your trans oil after about 1K miles, and use a good quality GL-5 DINO 80w/90 oil.

Ive put a new coupler on, and plan to do the cup and tunnel bushings at some stage.

Good!

It just doesn't feel right getting into first and getting into 2nd at speed. I have read countless threads and suspect im going on a wild goose chase to fix this problem. I suspect the box needs to come apart again and fixed properly.

Getting into 1st "at speed?" Shame on you! Anything over about 5 mph is not acceptable! And what's this 2nd gear stuff? You are using the magic two-part shift, aren't you?

Ill try the alignment trick as mentioned above.

Let us know what you find!

Sorry to the original poster for hi-jacking thread.
Cheers
Ed

Don't think about taking it apart until you've gone about 5K miles, then see how it's behaving. Remember, an old slider with new synchros can really be a pain because the slider has to machine away friction material until it "fits" the new synchros. That usually plays hell with how long the synchros will last, but if the trans gets you 50K + miles, that might be OK.