Kevin in Atlanta

This what remains of the fifth gear on a 100k 90 GT transmission that spent some of it's life in a twin screw supercharged.



This is what it should look like:


Can this be repaired?

If not, are other 5 speed transmission final drive ( hope I have that right) compatible and if yes, what is the downside? 2.74 to 2.20 for example.

TIA

dr bob

Somebody turn on the Grail Beacon. Seems like a lot of the late failures, especially in high output applications, include that fractured fifth gear and shaft.

Daniel5691

andy-gts

calling greg brown.......if anyone can custom make something he can !!!!!

Kevin in Atlanta

Well, the Grail Beacon worked! A late model fifth gear has been located and should be on its way East shortly.

Kevin

Rob Edwards

That's impressive, the only direct replacement for a GT 5th gear is a G28.12 unit from '89 in the US or 87-88 ROW. All hail the Grail Beacon!

Kevin in Atlanta

Well that is interesting. 1990 GT has a 11:29 ratio and the 87 USA/Canada is a 21:33 from what I've been told.

This is the latter. So, not the holy grail after all. I guess that changes things.

Kevin

Rob Edwards

You're confusing the ring and pinion gear ratio with the 5th gearset ratio. 90GT has a 11:30 ring and pinion gear ratio, or 2.73:1, G28.12 has an 11:29 R&P. Not what you need.

You need a 22:32 5th gear set, part number 928 302 905 32, which is used both in the 90 GT's G28.55 and the G28.12.

Kevin in Atlanta

Rob, thanks for the clarification. I was parroting what i was told.

Next question: Can I weld a good 5th gear set 'tip' be welded onto the otherwise good 5th gear set?

DR

Drive line Repair Specialists would be my first thought. There is a big one in Atlanta, I think.

I hope you can fix it!

Cheers,

Dave

dr bob

That one came from Dr. Rob.

Rob Edwards

Yes, I am certainly no Dr. Bob.

DR

I just thought it was my meds and let it go...but it did give me pause. :-)

Dave

GregBBRD

That's a fresh break. There is no sign of a stress riser or pre-existing crack, in your picture.

You can thank the guy that removed this transmission for this "service".

Any side movement of the transmission or the torque tube will allow those "leaves' to flex and break. Pretty common 928 "rookie" mistake....although I do go by and remind my guys to pull the transmission straight off of the torque tube, whenever we pull one. We also turn the "clamp" 180 degrees and leave the clamp on 5th gear once the bolt is removed, in order to offer some additional support.

Kevin in Atlanta

As eluded to above this was the result of the owner that used the car in question as a test bed for his twin screw supercharger in concert with loose clamp. That missing piece was long gone before the transmission was dropped to be replaced with another GT transmission I provided the PO.