Stronger trans bits needed...

[cstreit]

Hey gang,

For the 3rd time in 3 years I completely grenaded my third-gear assembly in the racecar. I've had it with this problem and want to address it once and for all.





Basically this keeps happening... The 3rd gear dog-teeth shatter and/or the syncro assembly comes apart.

SO, I'm looking for solutions. I don't know if anyone offers stronger components for this area. The gears themselves are typically fine in this scenario, it's the rest of the components. Not my 3rd gear teeth are damaged, the shift fork is dinged up, and it even took out the guide-fork on the cover plate.

Any suggestions?

[mb911]

power hauss II should have the stronger parts you need

[Bill Verburg]

I'd call Hayden about one of these
XT915

Contact Windrush Evolutions Inc.
Tel: 650 595 5772
Fax: 650 595 5744

[cstreit]

Bill,

he's first on my list actually. Then Powerhaus.

unfortunately I've gotta be ready in 3 weeks.

[Grady Clay]

Chris,

I suspect there may be some defective parts out there. When the dog rings were originally manufactured, the teeth were machined all in one operation, not one tooth at a time. I think some aftermarket dog ring machining had poor indexing as the teeth were machined.

Take a new sliding sleeve and new dog ring and couple them together. With your thumbs and fingers get them centered and torqued in one direction. All of the dog teeth should contact the sleeve equally around the circle. If you find the contact/clearance varying around the circle, this is the problem.

What happens is the torque should be evenly distributed among all the dogs. If the parts are incorrectly machined the load is on only a few dogs and exceed the strength of the metal. This looks like the cause off your malfunctions.

Best,
Grady

[cstreit]

Grady,

That would certainly explain things were that the case. THis problem seems to start with the car popping out of gear, which in consisten contact surfaces would certainly do. Unfortunately it quickly turns into a completely shattered gear assembly resulting in an expensive rebuild, especially when it takes out 1 or 2 custom ratio gears.

I'm loathe to try this again as I will shortly have the worlds most expensive Porsche transmission in a few years. I know other people are pumping 300-325 HP through a 915 so I'm assuming it's either really bad luck, or something I'm doing. (or both)

[Grady Clay]

Chris,

Another issue that is contributing to this is the hub change
Porsche made in ’77. While the hub is stronger, the torque
is transmitted through three fewer dogs (green line).

Has anyone ever seen an early (’72-’76) 3-4 hub fail?



Looking at your image again, is the 3rd gear dog ring not
seated against the gear (red arrow)? If not, was it installed
this way or is this a result of the failure?






You said this was the third transmission failure. Do you have
images and descriptions of the other two? Can you trace
your parts source? Are there dogs and sliding sleeves in
other positions you installed at the same time? Do you move
dog rings from one gear to another? If so, how do you
remove them?


Do you have a thermostatically controlled pump, a cooler
and filter on the transmission?

I use three large Alnico U-magnets in the transmission
to prevent any damage from circulating broken stuff. I don’t
think they would have helped in this case.

Best,
Grady

[Geary]

I too am curious as to whether the synchro teeth backed off first (Grady's red arrow). The gear splines should be electrically peened whenever the synchro teeth are replaced to avoid this issue.

We have run into factory shift sleeves in which only a handful of teeth were transmitting the load, so it's not always the synchro teeth that can be irregular. You really need 30X magnification to properly inspect the wear pattern on the flanks on the shift sleeve teeth and synchro teeth.

[gestalt1]

i've driven dog-ring type racing transmissions and they are not that difficult to deal with on the track. you learn the feel (dogs aligning) of them quite well. i think converting a syncro type trans is not cheap but it is the ultimate solution. i'm sure you could call GT as well as Hayden about doing this.

[cstreit]

Grady,

They were aligned properly when installed with the standard press and heat/cold...

What you don't see is the spider/hub teeth (the triangular thing) has the 3 teeth that engage with the slider ring sheared off. I think this might have started the whole thing, I dunno.

[cstreit]

Talked to Paul at Guards transmission too. Getting schooled. SHould have done this last time!

[Grady Clay]

Chris,

With the hub spider broken there is another possibility. During
the ‘80s “bean counter” attack, many parts were manufactured
less expensively. A few months ago I was looking at the
remarkable difference in a shift linkage part that had broken.
There was no question that the later part hadn’t received the
radiusing of the former version. That was what lead to the
failure.

The original part is 915.303.115.01 in my ’73 paper parts manual
and PET 6 shows the current replacement as 915.303.115.09.
"

"
(C) Dr. Ing. h.c. F. Porsche A.G.


With all the carnage you have I hope you can spot the cause.
You should be able to find out if there are various “improvements”
in the 3-4 hub. That I don't know.
What year is the trans and various pieces?

Best,
Grady

[cstreit]

I bought this trans 3rd hand. It had been stock, was converted for a while to run transversely in a porsche powered Causey Special Can-Am, then I ended up with it. The case has the later style "Omega Spring" style and I made an adapter for the "long arm" style clutch arm.

The internals? WHo knows? I know it's a later main shaft with the collar instead of the "support feet" near 1st gear (cause those broke and killed version 1.0 of it).

Version 2.0 had a broken/loose snap ring and vomited guts into the trans. Bent the main shaft.

Version 3.0 (current) has a broken spider, bent guide fork, and sheared off dog-ring engagement teeth.

[cstreit]

[Grady Clay]

Chris,

I think this can actually become the most expensive 915 everif you continue. You now have the original cost, race gearing, other mods, maintenance rebuilds and four expensive malfunctions. From the history, every remaining part is suspect.

My sincere advice would be to start with a fresh, unmolested donor. With careful inspection you can transfer some parts. I encourage you to start with a case inspection, fresh bearings, check/set the R&P and add all the upgrades and back-dates available.

I know time and money are always tight for any racer. Starting fresh here makes technical and economic sense.

Best,
Grady

PS, Are you trying for the Aug 13 Gatway Natl in StL? Going to the Runnoffs?
G.

[cstreit]

Grady,

NASAProRacing German Touring Sedan Nationals at Mid-Ohio.

I've given this a lot of thought and will continue to mull it over for a few more days. I agree with you. Doing the same thing over and over again but expecting a different result is lunacy.

[Joe Bob]

When I grenaded my second gear a couple of months ago....my bearings in the intermediate plate were found to be a bit wobbly....everything else was fine as I was able to shut it down prior to flying parts interacting with others.....

Hayden at WEVO helped me out....so did Aase Bros.