Busted dog teeth

Jerome74911S · 08:45 AM

I have had my 911 since last September, and although I topped-up the trans oil earlier, I just now got around to changing it. The magnetic drain plug had this nice surprise stuck to it; broken dog teeth. No wonder shifting into second is inconsistent - I take it very slow and mostly have no trouble.

The old oil was jet black and there was a lot of glop that came out with it. My guess is this: Since five of the teeth show very little wear (except for being broken off), but the sixth shows a great deal of wear in addition to being broken, could it be that the unworn teeth were broken a long time ago (the oil maybe never was changed) and the worn one was broken recently? All the teeth are magnetized, by the way.

How serious is this? I can't shift quickly now, but I also need to change the shift coupler bushings and the engine/transmission mounts. What do I face in the short term?

yelcab1 · 06-25-2010, 04:57 PM

In the short term you should stop driving pending a full tear down. Or, in the medium term you risk doing much more damage to the transmisison. These things are now getting very expensive to rebuild so keep the damage to the minimum by not driving it.

sc_rufctr · 06-25-2010, 08:30 PM

You need to remove the shift fork plate at the bottom of the trans.

With a good torch you should be able to check the condition on the dog teeth for 1/2 but you need to be able to rotate the pinion shaft to get a look all the way around.
There may be more broken bits on the shift fork plate and surrounding area.
It's possible to see 3/4 but difficult.

There is no good news. The bad news is If you keep driving it as it is it will only get worse.

What do you know about the car? Did you abuse it?

Jerome74911S · 06-25-2010, 09:08 PM

I haven't abused the car, but I don't know it's history. A local "Porsche mechanic" who had my car for some work evidently let his son drive it (!) and the son reported that it was hard to shift. God knows what that story turned out to be. This is not good news in any case. I really don't like the idea of stopping driving - that's what I have the car for and summer here is short. Time to do some pondering.

Oh Haha · 06-26-2010, 02:40 AM

Stop driving the car until you inspect the transmission.

You will only cause more damage if those get into the other moving parts.

Do a search using "broken" and "dogteeth" to find lots of information. (Not being a smartazz, links I have posted recently don't work.)

You should be having a talk with "Mr. I let my son drive a customer's car and it is now broken".

yelcab1 · 06-26-2010, 05:25 AM

In my experience, if you find 6 broken teeth, then there are 12 more in the transmision that have not worked their way to the magnetic drain plug and are resting someplace else. Your broken synchro hub has allowed the synchro ring to expand bigger than it is supposed to be and making shifting more difficult.

If you stop driving now and fix it, you may get away with $500 (very rare for a transmission). If you drive it more, you may end up with $1500 partial refresh.

The other alternative (I don't know what your finances are like), you could buy one of the three 915 boxes for sale right now on the For Sales column. One was $1500 ready to go, and swap it in then continue driving it.

In the end, it is money and you are the one to decide how much you want to spend to drive your car. Good luck.

Matt Monson · 06-26-2010, 05:31 AM

Quote:

Originally Posted by Jerome74911S

My guess is this: Since five of the teeth show very little wear (except for being broken off), but the sixth shows a great deal of wear in addition to being broken, could it be that the unworn teeth were broken a long time ago (the oil maybe never was changed) and the worn one was broken recently?

Quote:

Originally Posted by Jerome74911S

I haven't abused the car, but I don't know it's history. A local "Porsche mechanic" who had my car for some work evidently let his son drive it (!) and the son reported that it was hard to shift. God knows what that story turned out to be.

Make up your mind. Has the problem existed for a long time or are you going to blame the mechanic and his helper son and kill the messenger?

Sorry. I just get tired of the tendency people have to need to blame someone for every little thing that breaks on their 30 year old vintage sports car. The mechanic is trying to help you and look after your car and this is the thanks he gets. If you didn't feel any substantial difference in how the car drove and shifted between the time you left it with the mechanic and the time you drove it home, I'm going to go with your initial instinct that this damage was already done to the gearbox when you bought the car and you are just discovering it now. You have said nothing above that points to an abusive test drive while your car was in their care.

I would consider yourself lucky. With the chunks I am seeing there, it's a good thing that your gearbox isn't locked up and siezed. The right thing to do is stop driving the car and tear down the gearbox at your earliest convenience. Finding this out now before the next chunk breaks off and runs through there could be saving you thousands of dollars in repairs. That gearbox is a timebomb and it's not a question of if it's going to explode, but when. It's a sure thing. You now have the opportunity to put the fuse out before it blows.

LeakProof · 06-26-2010, 06:29 AM

I had basically the same thing happen to me.
Took the trans out of the car, brought it to the Porsche shop for repair, two weeks later went to pick up my transmission and the owner, who did the work handed me a bag of old parts, and said I had $2200.00 in new parts plus labor and tax.

Put transmission and engine back in car, I don't have a fifth gear!
Played with the shift coupler for hours and days. No fifth.

Lessons learned the hard way.
#1, If something happens to the trans, stop the car immediately, just by trying to drive in to garage, I further damaged the transmission beyond belief.
#2, Don't ignore, a transmission thats hard to get in to gear, it's a definite sign.
#3, Get your transmission repaired first, before tearing apart your whole engine. If your on a set budget.
#4, Now the Porsche mechanic has to remove everything, (free of charge) and tear apart the transmission to rectify the problem. He is also very busy now, because it is race season at Road America.

Good luck,

Leakproof.

red-beard · red-beard's Garage

Quote:

Originally Posted by Jerome74911S

I have had my 911 since last September, and although I topped-up the trans oil earlier, I just now got around to changing it. The magnetic drain plug had this nice surprise stuck to it; broken dog teeth. No wonder shifting into second is inconsistent - I take it very slow and mostly have no trouble.

The old oil was jet black and there was a lot of glop that came out with it. My guess is this: Since five of the teeth show very little wear (except for being broken off), but the sixth shows a great deal of wear in addition to being broken, could it be that the unworn teeth were broken a long time ago (the oil maybe never was changed) and the worn one was broken recently? All the teeth are magnetized, by the way.

How serious is this? I can't shift quickly now, but I also need to change the shift coupler bushings and the engine/transmission mounts. What do I face in the short term?

Those don't look like dogteeth.

Here are some dogteeth, bashed and bad, but still "whole".

Can you put a dime in the picture to help with scale?

No matter what they are, you've had a catastrophic event and you should pull the transmission and have it torn down.

Jerome74911S · 06-26-2010, 07:15 AM

For scale, and to show the one tooth that shows any wear, here it is in my fingers.

A couple other remarks: I'm not blaming my mechanic - or his son - but I didn't tell that whole story either. A number of times that mechanic and his employees mashed the transmission mercilessly, making me cringe. Still, he maybe didn't "do it". I'm simply bringing up a possibility. I have never mashed this trans; I know how to drive "crash box" transmissions with no synchromesh in any gear.

Therefore, I believe this problem is longstanding and precedes my nine month ownership. I can make it shift smoothly, but it is slow doing this and not much fun.

Seeing the size, are these dog teeth?

red-beard · red-beard's Garage

It is possible, but in the several dozen transmissions I've rebuilt, I've never seen them broken off the hub. Beaten flat, yes, but removed, no. John Walker has rebuilt hundreds and probably has seen worse than I've seen.

If those are dogteeth, the top (pointy section) is torn off, then afterwards the lower portion was corn-cobbed. To do the top portion would require extremely large mismatched speed, a bad synch band and shoving the shifter into gear very hard and fast. To corn-cob the lower portion would require extreme overtravel. They would have to be 2 separate events.

I have pictures of broken ball bearings, for the brass Intermediate plate bearing. I've fixed a couple with the bad steel bearing, but I don't have any pictures to compare of the broken steel cages.

You know, the bit in your fingers looks like part of something else, since it has a number etched into it.

Matt Monson · 06-26-2010, 08:19 AM

Jerome,
Sorry if I was a bit hard on you. If you've witnessed the mechanic and his helpers not show compassion for your gearbox you do have grounds to be a bit miffed, though I still don't think they did this. They would have to be really shady people to do something like that (which would be impossible not to hear and feel) and not come to you and tell you that an incident occurred on a test drive. But that's not really the issue, so we should probably just lay it to rest.

I too noticed what Redbeard said about those not being dogteeth but didn't want to say anything until I could figure out what they are. I've been out in my garage looking through my pile of parts attempting to figure out what part has that sort of striping on it. I am at a loss. The first five are clearly different than #6.

And number six is even more confusing. On a porsche gearbox there are very few parts that would be hand etched with a number like that. Basically it's the gears themselves and the ring and pinion. That's because they match them all as pairs, plus the R/P has set up numbers and measurements written on it. But sliders or dogteeth or bearings or any other manner of parts only have stamped numbers on them. Never something hand written like that.

I am a bit at a loss here, but still think this box should come apart immediately. There's something very wrong inside of there that has just not let itself be known yet.

Matt Monson · 06-26-2010, 08:21 AM

I was just about to hop in the shower and it came to me. Is this an early 915 gearbox? I think it might be the spline end where 2nd gear slips onto the output shaft. That end gets hammered on on those early shafts and the stops break. I think those are the stops.

Jerome74911S · 06-26-2010, 08:35 AM

Thanks for these detailed replies, I really do appreciate them.

The "number" on the worn piece turned out to be a bit of lint. Sorry. No number there, but there does seem to be an "H" on one of the other pieces.

As for those other 5 pieces, here is a front and back photo of one. All these pieces appear to have a triangular "hole" on the bottom side where they broke off.

I haven't checked, but as far as I know at the moment this is a '74 915.

red-beard · red-beard's Garage

I just pulled a good set of dogteeth off a random gear...

That chunk of metal is WAY to big to be dogteeth, unless you have a baby holding it. It is the wrong shape to be a gear. It is magnetic, so it isn't care or intermediate plate.

I'm leaning toward part of the shaft or maybe one of the triangle parts the slider slides on.

dentist90 · dentist90's Garage

I'm with redbeard on this too. The shape of those just doesn't look right for dogteeth. There is absolutely no indication they were pointed at one time. I'm just not sure what else they would be. My busted 2nd synchro teeth looked like this:

And the hub looked like this:

Broken dogteeth occurs when one continues to drive after it becomes noticable that the synchro ring is failing: ie, crunching shifts. It can also happen if one tries to 'speed shift' or slam into gear. Regardless, you will eventually have to do something. It might as well be now. BTW, it can be a do-it-yourself job. I know this first hand

red-beard · red-beard's Garage

No, seriously, the dogteeth shouldn't break like that. When the sync band is worn, the slider starts to contact the teeth, with some rotational speed. That is the "ZZZZZzzzzziiippp" sound you hear, the slider and the dogteeth.

Something stange had to happen to have only a few of the teeth engage and for them to be ripped off.

Jerome74911S · 06-26-2010, 12:01 PM

Quote:

Originally Posted by red-beard

I just pulled a good set of dogteeth off a random gear...

That chunk of metal is WAY to big to be dogteeth, unless you have a baby holding it. It is the wrong shape to be a gear. It is magnetic, so it isn't care or intermediate plate.

I'm leaning toward part of the shaft or maybe one of the triangle parts the slider slides on.

The piece shown is 5mm, or about 3/16" wide.

red-beard · red-beard's Garage

Yep, the dogteeth are about 1/16th, whole.

Geary · 06-26-2010, 04:26 PM

In older 915 transmissions, the weak synchro band won't straighten a slightly ID-enlarged shift sleeve as it slides toward engagement. This slight cocking (and the fact that two or three teeth are first to see partial engagement) is why you'll usually see adjacent teeth missing, rather than random teeth.

However, Jerome's pieces are part of the stop for 2nd fixed gear on the early style 915 mainshaft. The factory modified the design after a few years (I don't remember which year, but the PET will tell you).

Quote:

Originally Posted by Matt Monson

Is this an early 915 gearbox? I think it might be the spline end where 2nd gear slips onto the mainshaft. That end gets hammered on those early shafts and the stops break.