Fun with 915/02 - Another tranny thread...

[KevinG]

Yea, though I walk through the tranny of death, I shall fear no evil for other Pelicans have traveled this path before me (and left nice notes).

I have an early 915 trans that was leaking from both ends. The shift selector shaft seal is no biggie, but Porsche must have included a summer intern when designing the 915 cause you have to change this one from the inside. As I found out earlier from this bulletin board, you have to completely tear down the transmission to get there. So begins my fearful foray into the unknown.

Day one didn't turn out too bad, actually. I started about 2:00 and knocked off about 5:00, including taking 64 high-resolution color glossy photos, with the circles and the arrows and a note on the back of each one. I have included some resized ones below along with my first set of questions.

Question 1 - when I was removing the gear housing the shift selector rod came out with it, displacing the doohickey (technical term) in the picture below.

My question is, when I put the tranny back together, does it go here...

or here...


According to the diagrams of a 915, there should be a pin or something that activates the back-up lights when reverse is engaged. There was no such pin in my tranny, and Pelican lists them as NLA. Does anyone know of another source? You would have thought that I would have noticed that my backup lights were non-functional after 8 years, wouldn't you? Nope, not me.

Before I dropped the engine and tranny, I was having the somewhat normal notchy shifting between 1st and 2nd, with a fairly rare difficult shift into 2nd that popped back out. Am I safe in assuming that this is caused by worn synchro rings? Since I have the thing torn down to the shafts with gears attached, is it any big deal to tear these down and change out the rings?

I have included pictures of the dog teeth for 1st and 2nd (3 views) and 3rd & 4th (1 view). They look good to me, but what do I know. I would appreciate it is someone who knows what they are looking at could let me know if my fun is gonna get expensive. Here they are:

1st & 2nd view 1:

1st & 2nd view 2:

1st & 2nd view 3:

3rd & 4th:


Finally - the reason for doing all this to begin with. I assume I have to disassemble the differential housing as well to get this seal replaced? If anyone knows any tricks to avoid going in there I would appreciate hearing them.



Thanks for the help!

Kevin

[Eagledriver]

They don't look that bad to me but I've only rebuilt one transmission. My dog teeth looked about the same as yours and I put new syncro rings in on gears 2,3, and 4. Look at the mating teeth on the slider, they should not have rounded ends or chipped teeth. There should also be a "concave" area in the middle of the teeth. This area accepts the syncro ring and serves to lock the ring in place. If the teeth are worn so this concave area is compromised the thing will pop out of gear. I would replace at least the syncro rings and if in doubt on the others you should replace the dog teeth and slider for 2nd gear. The gears can be pressed off the shafts and you can send them away to put the new dog teeth on if you don't want to tackle that yourself.

-Andy

[KevinG]

Thanks Andy - I appreciate the reply.

Eagledriver, eh? Toyotas or fighters? Just curious.

I found the answer to my first question by further reading, but still would appreciate more feedback on the dog teeth. At this point I am planning on replacing synchros on 1&2. Is it really necessary to replace the shift sleeves / sliders as well? At $230 a pop it adds a lot to the cost. Just replacing the synchros and sliders for 1,2,3&4 adds to nearly $1,200 + tax & shipping.

Ouch!

[ChrisBennet]

Until the experts weigh in, here's my take:

One 1 & 2 the dog teeth look OK, but the synchros look worn (shiny) though that could be the light.

That said. ever since John Walker counseled us that those "good looking" 1st and 2nd dog teeth can be brittle at this age I've always replaced them. When they break off, the teeth can cause more problems if they get sucked into a gear.

Another thing, I notice that 1st gear dogs are the early type (symmetric /\). Later ones are asymmetric (like this |\). I don't know if the early kind are available any more. If you do replace the 1st gear dog teeth with the later style you will also need to replace the 1/2 slider, 1st gear energizer, stop and brake band.

Good luck finding a 1/2 slider. Last I knew, you couldn't get them. Maybe the situation has changed.

If money if tight and you are willing to go into the box at a later time now that you know how easy it is, you might consider just flipping the synchros so their unworn side faces the slider and stoning the burrs off the slider gears.

have fun,
-Chris

[Eagledriver]

Now that you know how easy it is to get the tranny apart, you can decide how thorough you want to be What I did was bought a second junker transmission and robbed the parts I wanted from it (I was switching to an 8:31 RP). I used the input shaft of the newer transmission and it had the asymetric dog teeth and a better slider. All I had to replace was the syncro's. The used transmission from an SC only cost 900 dollars.

I used to fly F-15's

-Andy

[KevinG]

Got the differential out and found my missing reverse switch activator pin. The sucker fell all the way through to the differential case. Now I can clean it up and replace the input shaft seal.

I also got the 1st and 2nd gear snychro assemblies apart. The synchros show a few spots where the finish is getting a bit worn (I went back to the 5th gear synchro to compare, but didn't see much difference), so I went and ordered a full set (1-5).

The teeth on the synchros and sliders just looked too good to replace. I really couldn't see any wear other than a few burrs.

Chris - Is there any benefit in switching to the assymetrical 1st gear? I imagine Porsche did it for a reason.

I am assuming that the 3/4 gear set is removed from the input shaft side - does anyone know how to remove the flange nut without an old clutch plate to use as a tool? My existing plate only has about 3k miles on it.

Thanks for the help!

Kevin

[KevinG]

Finally got snap ring pliers robust enough to remove the circlips on the synchro assemblies. Pics below are of the 1st gear assembly as seen when the clip has been removed. I don't know if the brake band assembly is complete. If you look in the pictures, there is no brake band on the right side of the assembly. Is this because of the early assymetrical 1st gear, or do I need to source a brake band for the right side? Dang - I had been hoping to get the tranny back together on Monday when the new synchros are supposed to arrive.

Thanks,

Kevin

[Grady Clay]

Kevin,

Yes, 1st gear syncro is asymmetrical. Porsche did this to get a little bit more syncro when downshifting into 1st at the expense of less syncro when upshifting into 1st. Say what … upshifting, that can’t be. Actually when you are stopped and the transmission is in neutral with the clutch ‘out’; when you suddenly depress the clutch and shift into 1st – that is the upshift. It will ’crunch’ almost every time because of the ineffectual survo (none) to the syncro.

I like to use the 2nd gear parts and have a symmetrical syncro.


On to the input shaft seal.

Do a Pelican Search on “dam seal”. This simple little ‘extra’ may prevent you from having to repeat the rebuild around the seal – and ruining a new clutch disc.

There is (or used to be) a company who would machine off the cast-in-place clutch release bearing guide tube and replace it with one for an externally replaceable seal.

The leak at the input shaft seal probably contaminated the clutch disc with transmission oil.

Carefully inspect the flat tension springs in the pressure plate. If there is any sign of ‘buckling’, replace the pressure plate.

If you have the flywheel off the crankshaft, remember to replace the flywheel bolts (they are ‘use once’).

There is some controversy about reversing the syncro rings to present a new wear surface. This works only when the sincro is very little worn. If done with a worn syncro, the installed diameter gets too large and shifting into or out of a gear gets difficult.

Your 1st and 2nd engagement dogs on the gears look just fine. I suspect the sliding sleeve dogs are the same. You probably should install new syncro rings. Inspect the other gears also.

Best,
Grady

[john walker's workshop]

901/914 transmissions used that single brake band in 1st also. if you changed it to two bands with the related anchor blocks, it did shift down better at a higher speed, but when at a stop in neutral and going into 1st, it was more difficult to get into gear. so that's the trade-off. it works fine with the single brake band. you just can't jamb it in gear quickly without grinding.

[KevinG]

Grady, John - Thanks for your valued input. It's nice to know, particularly since I can now proceed without sourcing another part!

I have ordered all 5 synchros, they should be here Monday. If I can manage to get the flange nut off of the input shaft so I can get to the gears there I will be happy indeed. It has so far defied all efforts I have directed at it. I didn't have a spare clutch plate to use as a tool, though.

I will have to inspect the one from the car though - it probably did get damaged by the leak. I remember cleaning a ring of semi-hardened oil residue from the inside of the trans housing that in hindsite was probably thrown there from the plates.

Thanks again -

Kevin