930 Gearbox failure

[paganrj]

Hi!!
Looking for some advise on what can be wrong with my '86 930 gearbox.

Last summer overhauled my gearbox replacing the dogteeth's, sliders, synchros, brake bands and stop blocks. No bearings were replaced as they were all in good condition. It is a track only car so it does not get a lot of miles but gets punishment.
Last December, while on track, all of the sudden it didn't want to go in 4th gear. After checking everything externally (linkages, coupling etc.) I decided to open the gearbox again. The attached pictures show what I found after taking 4th gear apart. Not sure how that can happen but the anchor block carved into the dogteeth preventing the synchro ring from compressing and the slider form engaging 4th. This time I only replaced the 4th gear synchro, dogteeth and anchor block. Put everything back together couple of weeks ago and it was functioning normally. Last weekend the same situation happened again, it does not want to go into 4th. I have not check anything yet but judging by the symptoms I might be the same failure.

I will like to know if anyone has experience this kind of failure and what can be causing it. What else needs to be checked or replaced? I know there are a lot of experts in this forum so any advise will be appreciated.
Thanks, Rey

[MikeD930]

I'm not an expert but am wondering what brand are the syncro hub and stop? Did you replace the syncro ring? Did you reinstall everything in correct orientation?

[full quack]

What I don't get are the big dents/gouges in the inner gear hub face/shoulder. I can see 3, maybe 4 of them in picture #3. They are not evenly spaced, and there could be more that are not apparent in the photos.

I completely went through my tranny about 60K miles ago, and do not recall seeing any such feature on the gear.

Did you by chance damage that gear shoulder when you pressed on the syncro ring???? Or possibly is that where you ground off the old syncro ring, and slightly over did it, causing the damage??

Here might lie your issue...hopefully some tranny guru will chime in to better help.

Mark

[paganrj]

Originally Posted by MikeD94

I'm not an expert but am wondering what brand are the syncro hub and stop? Did you replace the syncro ring? Did you reinstall everything in correct orientation?

The parts were purchased from EBS Racing. All synchros were replaced the first time. Only the 4th synchro, dogteeth, stop and brake bands were replaced the second time. Everything was installed the same orientation as it was found and verified with the shop manual.

[paganrj]

Originally Posted by full quack

What I don't get are the big dents/gouges in the inner gear hub face/shoulder. I can see 3, maybe 4 of them in picture #3. They are not evenly spaced, and there could be more that are not apparent in the photos.

I completely went through my tranny about 60K miles ago, and do not recall seeing any such feature on the gear.

Did you by chance damage that gear shoulder when you pressed on the syncro ring???? Or possibly is that where you ground off the old syncro ring, and slightly over did it, causing the damage??

Here might lie your issue...hopefully some tranny guru will chime in to better help.

Mark

Mark,
I used a chisel to remove the dogteeth, the gouges on the gear shoulders were there. All gears have them, I believe this is for better lubrication of the shoulder face.

[full quack]

Yes, I looked through the pictures I took during my tranny overhaul, and sure enough, those notches are on my gears as well. So obviously that is not your issue!
Sure hope you figure it out, and let us all know the cause....curious monkeys want to know.

Mark

[MikeD930]

The 4th gear assembly looks correct. Did you replace the synchro ring? If everything is new then I suspect the synchro hub/teeth to be of poor quality (in the area of where the stop/anchor block nest into). Or maybe some things (i.e. synchro ring) was out of spec even if new. I'd hate to see this happen again not knowing the cause.

Oh, I forgot to ask...did you apply high heat to the synchro hub/teeth before installing onto the gear? I'm not sure if this would cause the change of temper of the material making it "softer".

[paganrj]

Originally Posted by MikeD94

The 4th gear assembly looks correct. Did you replace the synchro ring? If everything is new then I suspect the synchro hub/teeth to be of poor quality (in the area of where the stop/anchor block nest into). Or maybe some things (i.e. synchro ring) was out of spec even if new. I'd hate to see this happen again not knowing the cause.

Oh, I forgot to ask...did you apply high heat to the synchro hub/teeth before installing onto the gear? I'm not sure if this would cause the change of temper of the material making it "softer".

The 4th synchro has been replaced twice as well as dogteeth, stop block and brake bands. I have used two different suppliers trying to rule out poor quality. Obviously I don't know if both use the same source for their parts.
Before the installation the gears were in a freezer for a few days and the dogteeth's were heated to about 300F right before installation in a press

I took the engine out last night, will post pictures again when the gearbox is apart.
R

[paganrj]

Gearbox apart as of last night. The 4th gear failure is identical to the one last December. the anchor block did not went into the dog teeth as far as it did last time.

What can be happening here?
1- Dog teeth material not to original specs (softer)
2- Too much friction between the synchro ring and the anchor block
3- Combination of both
4- ???

[MikeD930]

Rey...that's awful having to go thru this again!!! Do you still have the original syncro hub? Maybe have both original and EBS/aftermarket parts tested for hardness using Rockwell scale in the area of the hub and teeth.

[paganrj]

Thanks for the suggestion Mike. I do have all the old parts removed, will try to do a hardness test. Since you are rebuilding yours this might be useful info before you purchase your replacement parts.

Noticed last night that there is about .010" difference between the old synchro ring and the new one. The picture shows the old synchro on the left, the new on the right. The lip that fit on the dog teeth is also ticker on the new synchro. Not sure if that will be contributing to the failure but it is certainly a difference.




Below the condition of the old dog teeth after many years of use (and track). Not a sign of stop block impact on it in the area the other two failed. Also there is difference in color among them which might be an indication of heat treatment or alloy used.

[Matt Monson]

EDIT: Nevermind. Angle of the photo deceived me.

[MikeD930]

In my opinion it looks like the problem were contributed by out of tolerance synchro parts (too much gap between anchor block and synchro ring) and possibly too soft synchro hub. I would think that the genuine synchro hub would break rather than being gouged.

[paganrj]

Originally Posted by MikeD94

In my opinion it looks like the problem were contributed by out of tolerance synchro parts (too much gap between anchor block and synchro ring) and possibly too soft synchro hub. I would think that the genuine synchro hub would break rather than being gouged.

Well, if that is the case we need to be very careful where we purchase our gearbox spare parts. Like I mentioned before, I used two different suppliers (EBS and PP) with the same results. I guess they both get supplies from the same source, Raush & Spiegel.... Thanks to Matt Monson I should be getting OEM quality parts very soon.

By no means I am an expert in gearboxes but one reason I still believe there is something else going on is the amount of force required to make the stop block carve into the dog teeth is large. Considering that the slider and the floating 4th gear are already spinning (not at the same speed obviously) it shouldn't require too much force just to synchronize the floating 4th to the slider rotational speed before the shift engages.

[MikeD930]

I did not know that Matt sells synchro parts.

I agree that there should not be too much force acting on the sycnchros. You may want to check the inside diameter of the 4th gear (also check other gears) after the synchro hub is installed. It may need to be slightly honed.

[paganrj]

Here are pictures of difference between the original synchro ring



and the Raush & Spiegel = .013" difference!



Also note that due to the difference in thickness the initial contact with the slider (shiny area) was made on the original syncrho, left (about mid section of the ring) vs. the R&S ring right (top portion of the ring)

[MikeD930]

.013" is a big difference! Have you measured the O.D. of the installed ring as shown below?




I see another bad design that may contribute to the problem is a lack of ring grooves within the I.D. of the ring. I suspect this cause the synchro ring to "grab" the anchor block which violently slam it into the hub before the synchro teeth has a chance to engage the slider.