Care and Feeding of a 915 Transmission

[djmcmath]

While I'm stuck here at work, waiting to go home to finish rebuilding my transmission, I thought I'd start a thread on the proper care and feeding of the 915. Had I recognized the warning signs of my tranny failing, I probably wouldn't have been quite so surprised when it completely died on me. I'm sure there are things that I didn't know that would have prolonged the life of my 1st gear synchro, as well.

So here's the thread -- post your "shift like this to help your tranny," or "don't ever do XYZ, or you'll suffer the following consequences" or "If your trans is doing PBC, then you're close to a failure" stuff here, please. Then the poor blokes who don't know the warning signs and symptoms (that's me) can have a single consolidated thread.

I'll start:
My warning signs:
1 - Shifting into first only works when moving very slowly. Not when stopped, not when moving any faster than "barely rolling". This indicates that the synchro band for first gear is no longer working -- when I pulled it, in fact, the friction material was all gone. That means that the tires and the input shaft have to actually be travelling the same speed, or you won't get the teeth to engage.
2 - Funny material in the tranny oil when draining. Filter it every single time, look for bits of stuff on the drain plug. I had dog teeth on my plug -- dead giveaway.
3 - 1st gear felt a little uneven, through the shift linkage. I'm betting that was caused by gaps between dog teeth on the synchro ring catching and releasing the operating sleeve. Had I realized that, I would have known that the first gear synchro was missing some teeth.
4 - Second gear wouldn't shift when stopped, either. Same problem as #1.

Some tips I've figured out, mostly from reading other people's posts:
1 - Never ever speed shift. Don't force it in, just gently pull. If you hear something that sounds like a grinding noise, you pulled too hard.
2 - Only shift into first when stopped. If you can't, see warning sign number 1. The 1st gear synchro is specially designed around only being shifted into when stopped.
3 - Swepco? Debatable, as no back-to-backs have been done, but most of us agree that it's a cheap price to pay for peace of mind.
4 - Bushings etc. seem to help with some of the precision, which reduces the guessing of gears. There is some discussion about how less play in the shift linkage can actually cause marginally accelerated wear in the trans if an out-of-alignment condition exists. The consensus seems to be that the precision shifting allowed by better bushings reduces the damage more than that which may be caused by the out-of-alignment condition. If the experts would like to discuss this one for the enlightenment of neophytes like myself, I'd be thrilled.
5 - Double-clutching? I know it's supposed to help, but I'm not sure why, or quite how. Again, experienced thoughts?


That's about all I can think of. Thanks in advance for those who'll post their practical wisdom to add to my humble list.

Dan

[anthony]

I'll add to that proper clutch cable adjustment. I just had a PPI done and that was one thing that came up. The mechanic said a properly adjusted cable will help the synchros last longer.

[MAS]

Hey, I've never been able to always shift into 1st when stopped... even after I had the tranny rebuilt. I've had several older cars that have been similar. However, I can shift into 1st easily at rolling and more importantly at higher speeds... not that I ever make a habit out of it (for obvious reasons). So I'm not sure what synchros would have to do with the problem... especially when you are stopped.

-MAS

[djmcmath]

MAS, I have a theory -- if your clutch isn't quite fully disengaging (improperly adjusted), then the engine will always be spinning the input shaft, to some extent. That would make the 1st gear synchro pretty ineffectual, as it's trying to perform more friction work on it's 500 square millimeter surface area than the clutch is, with significantly more surface area. But that's just theoretical for me, as I've never done the experiment. Anyone?

Dan

[SpeedracerIndy]

Perhaps a discussion for another thread, but I am starting to wonder if the use of the factory short shift (FSS) kit is hard on the syncros. I was always a believer in the FSS kit and struck down all who said it is bad, until yesterday. My gear shift lever that goes with the FSS kit started spinning in its housing making it very hard to shift, especially into the gated gears. The other day I swapped back in the original shifter while I get my FSS shift lever welded back together. WOW, what a difference. While I HATE the mile-long throws, the effort required is much easier, and much smoother. I also noticed that it takes quite a bit longer for the gear shifter to fully engague the gear, forcing you to shift a little slower. I am still going to get my FSS lever repaired, but I am still up in the air about whether it is hard on the syncros or not.

[djmcmath]

Monday morning bump -- this thread seems like it has great potential for archival value; I'd hate to let it go without one more shot. Thanks, all.

[Grady Clay]

Good posts all.

I will add timing technique to the list of “How to preserve your transmission.” The timing of shifts (up and down) is critically important. Too fast (speed shifts) makes the syncros work too hard in one direction, too slow makes the syncros work too hard in the other direction. When timed correctly, it takes only three fingers on the shifter.

Most of the life of a transmission is determined by driver technique. If you have a 911 with a PO, you are not in control of any former abuse. If you rebuild your transmission, you get a “fresh start” – sorta.

Proper double clutch down shifting greatly extends the syncro life. I double clutch downshift every time I go down a gear. I “match revs” whenever I go down more than one gear. I heal-and-toe constantly.
Is all that absolutely necessary? No. It is good consistent training.

The first gear design is asymmetrically predisposed to downshifting. I downshift into 1st just before I come to a stop. If you sit in neutral, with the clutch out (engaged), at a stop light and then put in 1st – that is an up-shift into first. The syncro is not as powerful in this direction.

Every time a gear “crunches”, you further damage the gear dogs and sliding sleeve.

When you drain the gear lubricant, also pull the shift pivot plate. For the cost of one inexpensive gasket you get a better “window” into your transmission.

If you find inappropriate parts in the oil or the shifting changes while driving; STOP. Do not drive more. Pay for a flatbed tow home or to a shop. Diagnose the problem even if you have to rent an econobox for a week. If it turns out to be a simple problem – Great, money well spent to be safe.
Nothing can exacerbate the problem and the ultimate expense more than driving with parts floating around inside you transmission.

Best,
Grady

[bigrubberjeep]

Well I have the same issue (cant shift if rolling over 4MPH) even at a full stop I get a small grind. So I always make sure to shift to second before I go to first ( a trick I learned with my Jeep Wrangler) This give me a smoth transition and no grinding even at a full stop. My wrench says it's the syncros, but he also said it's a comon problem or "issue" depending on who you ask with the 915. He basicly said, take it in whenever I am fed up with it. He has seen 915 with this issue go for over 50K miles with no further issues. (as long as you baby it)

[Bill Verburg]

3 - Swepco? Debatable, as no back-to-backs have been done, but most of us agree that it's a cheap price to pay for peace of mind.

I disagree. Over the last 30 odd years that I have been fooling w/ 915s I have tried several different fluids. Back to back, some I have only kept for a few months before rejecting them(Mobil 1), others are as good. I no longer experiment, Swepco has proven to be at the top of the field

[RoninLB]

Originally posted by Grady Clay

I will add timing technique to the list of “How to preserve your transmission.” The timing of shifts (up and down) is critically important.

I've never been able to master that discipline. I think that seperates the guys who like the 915 and those that don't.

great post G

[fredmeister]

Double clutching is imperative to being smooth at speed on the track as well as easier on the engine and tranny. Matching revs is the key to allowing the differential in speeds of the gear shaft being shifted from to minimize the work the synchros must do in speeding up the gear being shifted into until the dog teeth align with each other and engage. Your clutch and differential and axels all will like it as well.
Driving the 911 fast is a challenge we all savor, the 915 box adds to that, if it was easy what fun would that be? Though Porsche even fessed up with the 950 box and used some other companies synchro design, admitting that Ferry's was not the best out there.