Identify 915 Gearset; count teeth?

[wprater]

I'm installing my WEVO gate shift and will be identifying the quality of the gearbox while I'm in there. I was going to rebuild it, but wanted to test it on the street first. It's in an unknown state.

I've been reading through Zimmerman's 915 tutorial which is full of information, but I've got zero experience with gear set quality and wear.

Everything appears to be in exceptional shape.

How can I determine my gear ratios and final drive, since there is no identifying number on the gearbox. I've got it opened up now!

[stownsen914]

If you are taking the gearsets off the shafts, it's easy - the gearset names are stamped on the gearsets. If you aren't disassembling all the way, just count the teeth on the gears. If you search on the forum or google 915 Porsche gears, you'll find a few different 915 gear charts showing the gearset options. Good luck!

Scott

[wprater]

Yeah, I found all those charts. I won't be dismantling it, unless they are in bad shape. I'm having a lot of trouble determining that.

Right now my goal is to get the gate shift installed.

I will count the teeth, but is there a diagram showing which gears are which? My Bentley manual does not show this information.

[Trackrash]

It is tough to know all possible problems without driving the trans. Here is my old Fifth gear that was very noisy. It looked OK when I first looked at it. I am thinking an experience pro would have caught it.

[wprater]

Looks like I have a stock 80 SC gearbox (BTW, I was able to see the numbers on the main shaft on gearbox)

AZ, HW, NT, QQ, SN

How does one determine the 8:31 rack and pinion ?

Thanks!

[boyt911sc]

Wprater,

I've done a couple 915 rebuilds but have not seen or done a Wevo gate shift. Could you document your Wevo installation for us? Thanks.

Tony

[stownsen914]

Originally Posted by wprater

How does one determine the 8:31 rack and pinion ?

An SC will have the 8:31 ring & pinion. If you want to satisfy yourself that's the case, you can just pull the differential side cover off. It's pretty easy, and no other disassembly is required.

[wprater]

Originally Posted by boyt911sc

Wprater,

I've done a couple 915 rebuilds but have not seen or done a Wevo gate shift. Could you document your Wevo installation for us? Thanks.

Tony

Already being done.

[KTL]

Not trying to be a Debbie Downer here, but it's easy to think that things look OK inside the trans. Takes a reasonably trained eye to know what you should be looking for

Some things to look more closely at:

1. Your picture doesn't show 5th gear & reverse since you obviously already took the tail case off + 5th & reverse, to allow you to get the center case section off. 5th & the reverse idler (two headed gear) is known have some problems since it's at the end of the gear stack and the splash oiling isn't as effective. This is usually due to under-filled gearbox. Take a good look at the teeth on 5th and also the shaft & needle bearings that the reverse idler rides on.

2. Look closely at the synchro bands inside the gears. They should have a decent amount of friction material on them. I can see from your picture that 3rd gear shows some wear on the synchro band.

3. Look closely at the toothed ring that is attached to each gear. These teeth should have a rather pointed, but not so pointed that it would be described as "sharp," tip on them. Teeth that are blunted or rounded off are showing some good wear.

4. Check the numerous M8 nuts that secure those two gear stacks to the diff housing. These M8 nuts are attached to studs screwed into the diff housing. Sometimes you'll find these studs are stripped out. Just put a wrench on them and check for tightness. Don't lean on them really hard. Just check for any that are noticeably not all that tight.

[wprater]

Thanks for the notes, Kevin. Here are some more shots. The pinion head gear looks a bit suspect.

My goal is to get this running with my new build and then research and acquire a new shorter gear set for a rebuild. I'm hoping these gears are fine to run for at least 5k miles. I didn't take them all off, so I don't know condition a brake bands or synchro rings.

Here are some close ups of dog teeth and more gears. They all appear to be in a similar condition. Wish I had a clue if this thing had been rebuilt, with bearings replaced etc.

[Matt Monson]

Based on synchros and dogteeth, I suspect someone has rebuilt that. It mostly looks pretty nice.

The one that concerns me is your SN (not SW) 5th gearset in the first picture. The picture Trackrash posted is what is typical of a 5th gear that sees a lot of track time and oil starvation. The more common cause of failure on most of these gears is wearing through the heat treat at the root of the tooth where the opposing tip contacts it. That's what your 5th gear idler shows.

See the shadow 2-3mm wide near the base of the gear teeth? It's a line that runs across it. That's the initial signs of it going. With more mileage it starts to pit and get pock marked. Eventually the tree gets chopped down and the teeth fall over like a row of dominos. Will it got 5000 more miles? Pretty likely if you don't find pitting on closer inspection. But you don't want to run that gear all that long if there if you car sees any significant number of highway miles.

Remember, a gear mostly only wears under load. If you drive the car 2-3-4 on back country roads you might not ever need to replace it. If you commute at 75mph 40 mi each way and use it as a summer driver your days may be numbered on 5th.

Regards,

Matt

[Trackrash]

You should also make sure the outer bearing races are tight in the trans case.

[KTL]

Well your reverse teeth on the 5th-reverse selector ring look like they may be in pretty good condition? Flip that selector ring over and look at the side of the straight teeth.

Some may laugh at me for saying that. But occasionally you'll find those straight teeth are boogered up because someone keeps putting the trans in reverse so quickly. Reverse is not synchronized and therefore you have to wait a little bit before choosing reverse after you push in the clutch pedal. If you put the shifter in reverse too fast, it'll crunch the gear. Or, if your shifter bushings are sloppy? It's possible to bypass the reverse lockout teardrop in the shifter and you can downshift from 5th to reverse as you're cruising along. Now THAT makes a really big crunching/grinding noise!

1st and 2nd synchro teeth and synchro band friction surfaces look pretty good. 3rd & 4th teeth and friction surfaces look a little worn.

As long as you've got it apart this far, I would suggest taking off the M8 nuts at the bearing retainers to slide out the main shaft and pinion shaft stacks. Doing so will allow you to see how tight the shaft bearing races are in the diff housing bores. You hopefully don't find a pinion bearing race like this one

https://www.youtube.com/watch?v=xmNmxm_AT0c

Full disclosure, the above video is a shameless theft of a line from a Seinfeld episode https://www.youtube.com/watch?v=y2RKmVqnNdw

[Trackrash]

It looks to me like that trans has been gone through before. Either that or the factory left rounded nuts on the bearing retainers. That would explain the generally good condition of the synchros and dog teeth. First and second synchros look fresh to me.

[Jon B]

Originally Posted by wprater

Wish I had a clue if this thing had been rebuilt, with bearings replaced etc...

It's obvious that the transmission has been opened for previous work, since the factory did not use blue RTV silicone on gaskets.
It appears as if the synchro parts have been replaced in 1st & 2nd gears, as Matt and others have said. They do not look 35+ years old.
The only possible downside to previous work is if the shift forks were loosened to remove the gear shafts.

As others have also said, you should check the roller bearing bores in the maincase. Wiggle the gear shafts to see if the bores or bearing clamps are worn, and allowing excess movement.

[wprater]

Originally Posted by KTL

Not trying to be a Debbie Downer here, but it's easy to think that things look OK inside the trans. Takes a reasonably trained eye to know what you should be looking for

Some things to look more closely at:

1. Your picture doesn't show 5th gear & reverse since you obviously already took the tail case off + 5th & reverse, to allow you to get the center case section off. 5th & the reverse idler (two headed gear) is known have some problems since it's at the end of the gear stack and the splash oiling isn't as effective. This is usually due to under-filled gearbox. Take a good look at the teeth on 5th and also the shaft & needle bearings that the reverse idler rides on.

2. Look closely at the synchro bands inside the gears. They should have a decent amount of friction material on them. I can see from your picture that 3rd gear shows some wear on the synchro band.

3. Look closely at the toothed ring that is attached to each gear. These teeth should have a rather pointed, but not so pointed that it would be described as "sharp," tip on them. Teeth that are blunted or rounded off are showing some good wear.

4. Check the numerous M8 nuts that secure those two gear stacks to the diff housing. These M8 nuts are attached to studs screwed into the diff housing. Sometimes you'll find these studs are stripped out. Just put a wrench on them and check for tightness. Don't lean on them really hard. Just check for any that are noticeably not all that tight.

Great info, thank you!

The bearings in #1 look shiny and nice. As do the idlers gears.

I do see some wear in the 5th gear tho, now that I'm looking more closely.

The the studs were nice and solid, there was once that I could tighten a bit.

[wprater]

Originally Posted by Trackrash

It looks to me like that trans has been gone through before. Either that or the factory left rounded nuts on the bearing retainers. That would explain the generally good condition of the synchros and dog teeth. First and second synchros look fresh to me.

The slim black lock washers were present on the main case plate. But as noted, that blue rtv was present, so it's been gone through. I was told this was used in a DE car but was "good to go".

[wprater]

Thanks for all this info. I was able to see what you're speaking about WRT the line in the base of the 5th gear. I did not see any visible sings of pitting yet, however.

I was told this was used in a DE event car, so what you're all saying is starting to make sense.

Just finished my motor build, so it's starting to sound good that I can run this gearbox with it, while I find how/if I will alter my gearing.

It'll be a mounting carver and street hot rod, not many road trips for this one, so 5th will see little use until I can get a new one with other the other gears I'm looking for.

Originally Posted by Matt Monson

Based on synchros and dogteeth, I suspect someone has rebuilt that. It mostly looks pretty nice.

The one that concerns me is your SN (not SW) 5th gearset in the first picture. The picture Trackrash posted is what is typical of a 5th gear that sees a lot of track time and oil starvation. The more common cause of failure on most of these gears is wearing through the heat treat at the root of the tooth where the opposing tip contacts it. That's what your 5th gear idler shows.

See the shadow 2-3mm wide near the base of the gear teeth? It's a line that runs across it. That's the initial signs of it going. With more mileage it starts to pit and get pock marked. Eventually the tree gets chopped down and the teeth fall over like a row of dominos. Will it got 5000 more miles? Pretty likely if you don't find pitting on closer inspection. But you don't want to run that gear all that long if there if you car sees any significant number of highway miles.

Remember, a gear mostly only wears under load. If you drive the car 2-3-4 on back country roads you might not ever need to replace it. If you commute at 75mph 40 mi each way and use it as a summer driver your days may be numbered on 5th.

Regards,

Matt

[wprater]

As some of you suggested that I check out the bearing races in the main case. I've checked them both now in the differential cover near the shims plates and the top cover.

The lower races on top of the differential housing are rock solid.. so business will be good there, Kevin

The upper races in the top cover had about a 0.3mm play in them. Is this something to be concerned about?

Here is a shot of the bearings and lower pinion gear.

[wprater]

Regarding the shift forks:

1/2 and 3/4 shift forks seemed to line up per factory spec and have 2.6mm of room when they're resting parallel.

I had to adjust the 5th/reverse fork as the gap was a bit too large. I adjusted it to be at 1mm between the top of the idler gear and top 5th gear.

Picture is before the adjustment