Shafted twice on transmissions. Should I stay or should I go?

[Matt Monson]

Originally Posted by Jeffreypang911

Thats the plan!

Also check out these two forks. One is like off centered. not sure if this would actually affect anything though. Anyone seen something like this before? Probably swap over to the straighter one. But I don't think thats the cause of my problems.

Thats what I've been saying! Search here. There are many threads about fixing it. That may be the only thing wrong with gearbox number one. You may be able to bolt on the plate from gearbox two and be fixed.

[yelcab1]

In my experience, a bad transmission is relatively easy for someone with your level of wrenching ability. good luck. It's fixable.

[Bill Douglas]

The two gearbox thing (one working one, one reference one) is working out already. You can be our new gearbox expert

Good luck with the plate swap.

[Trackrash]

Originally Posted by Jeffreypang911

Thats the plan!

Also check out these two forks. One is like off centered. not sure if this would actually affect anything though. Anyone seen something like this before? Probably swap over to the straighter one. But I don't think thats the cause of my problems.

That is what Matt was trying to tell you about.

You have a great opportunity here to end up with basically a new transmission.

BUT, TAKE YOUR TIME AND SPEND SOME TIME READING AND LEARNING. These transmissions have some unique features and take some special tools. If done right you will have a bullet proof, great shifting, long lasting transmission. Make a mistake and you are right back at square one.

Here is your homework. How-To: Porsche 915 Transmission Repair Tutorial Part 1 - Porsche Wiki

and 915 Transmission Rebuild

[Jeffreypang911]

Thanks Trackrash. I actually read that over for the heck of it a few weeks back. Good refresher for sure.

So heres the progress for the day. Started taking apart my original known blown trans.


So check this out. This is from my "new" trans. Midcase is aluminum on this one. found some ribs broken. So something must have happened before my ownership because I never dropped it or anything. Maybe this impact caused the entire case to warp? Which is why my shift rod is binding. Just an idea...



Its also a bit of an eye sore. So I planned to use the Magnesium midcase from my original trans.


...But one of the bearings is spun. No play, Just turns. So I could loctite 660 it back in place. Not sure at this point.

So I haven't taken apart my newer trans yet. But here are some pictures through the shift fork hole.

2nd Gear:


3rd/4th gear:


5th gear:


And thats all i got. Cant see 1st without taking off the midcase.

So I still don't know why my case is binding during shifts. Thats the big mystery. My plan was just to swap midcases and the selector shaft. But now the case has a spun bearing it makes my decision harder. Maybe once I take apart the other trans later on I will have a better idea...

Let me know your thoughts.

[Matt Monson]

Did the rod give you any trouble when removing the tailcone on box #2? A common rookie error is to drop the engine/gearbox assembly with the rod still attached to the coupler. This bends the rod and makes it hang and bind on shifting like you describe.

[Tippy]

Didn't you say you heard a loud bang? Maybe you hit something while driving?

[clarkd]

General advice from someone who has only opened up one 915 box (1973 mag case).

You can leave the nut on the pinion shaft and parts wash it as a unit. Parts wash all pieces thoroughly.

Mounting the housing to an engine stand will make assembly/test shifting much easier than having your 915 on the floor or a bench. I did this alot before sealing mine back up.

Replace all seals and gaskets.Use Curil T on the gaskets.

The races in my case would turn and had no slop. I used loctite 640 sleeve retainer. It is a green liquid.

[Jeffreypang911]

Originally Posted by Matt Monson

Did the rod give you any trouble when removing the tailcone on box #2? A common rookie error is to drop the engine/gearbox assembly with the rod still attached to the coupler. This bends the rod and makes it hang and bind on shifting like you describe.

I removed the coupler. But I have suspicion that the rod is bent. It's the only real explanation. The rod didn't give me any trouble taking it off though just a bit stiff. Once I got the tailcone loose it slid off. But there's much more friction sliding it on and off compared to my tailcone from 72 since there's no bearing in the later one.

So perhaps that bearing can mask a bent shaft much easier which is why when I try that one, it seems to be much smoother.

I just don't understand why porsche would remove the bearings in the later models.

[Matt Monson]

I wasn't suggesting you bent it. I'm suggesting that like many "ran when parked" gearboxes whomever pulled that box did it.

With the tsilcone off and both inspection plates removed you should be able to pull those rods. One the bench even a slight bend will be obvious.

Not sure why the bearing change. You will likely want to use the 74 one with mechanical Speedo. And front housing with 7:31 if its in Good shape. Unless your car is an electronic Speedo conversion. Not clear to me how you fitted the later gearbox?

[Alex599]

I'm realy happy to see where this thread is going. Don't kid yourself Jeffrey, you are an airhead! I'm cheering for you.

Alex

[Jeffreypang911]

Originally Posted by Matt Monson

I wasn't suggesting you bent it. I'm suggesting that like many "ran when parked" gearboxes whomever pulled that box did it.

With the tsilcone off and both inspection plates removed you should be able to pull those rods. One the bench even a slight bend will be obvious.

Not sure why the bearing change. You will likely want to use the 74 one with mechanical Speedo. And front housing with 7:31 if its in Good shape. Unless your car is an electronic Speedo conversion. Not clear to me how you fitted the later gearbox?

Yeah its a bit confusing. The car is originally a mechanical speedo car. The new box is electric speedo, so I planned to just run the wires and convert it. I still have the mechanical speedo and cable right now though.

I can see that the shaft is slightly slightly bent at the end. Seems like it would be much more finicky if there is a slight bend and its being held in place through tightly fitting holes with near no-wiggle room or bearings.

Here is box #2 that is having the binding issues. Again the other side has no bearing. Just rides right on the magnesium case.


A closer look:


Here is box #1.




If I put the shaft in, and cock it a bit then try and slide it, the one with the bearing will still slide with ease. but the one without will bind up and feel "sticky". Almost like it is galling.

Why would Porsche change to an inferior design? Really makes you wonder.

Also the midcase on Box 2 also has a spun bearing. Spun bearings for everyone!

Perhaps I could use the entire casing (bell housing, midcase and nosecone) from Box 1 and transfer the 8.31 gears over from Box 2. But then I would have to reshim the diff/pinion and thats really pushing the limits of how comfortable I am with this stuff.

Easy way out is to just swap shift rods and button it back up - hope for the best.

Always appreciate your ideas.

[Mark Salvetti]

Just an idea here, but if you use the mid-case and diff case from Box 1, you can do some math and figure out how many shims to use with the Box 2 R&P.

Measure the total shim thickness of Box 1, and note the N-number etched on that pinion shaft. When you move the ring and pinion over from Box 2, compare that N-number to that from Box 1, and use the difference between N-numbers to adjust the total thickness of Box 1 shims.

You'll need to check how the gears mesh with gear marking compound after setting the pre-load and backlash of the ring gear. Which could require you playing around with more shims for the differential. Even if you move the diff bearings and shims from Box 1 to Box 2, you may find you'll still have to make adjustments.

Mark

[Jeffreypang911]

Wow. No need to swap anything... found the problem.

Box 1


Box 2


Didn't notice it until I brought it to a glass surface. Severely bent upwards.

Thanks everyone for the support. Enthusiasm back to high!

[Matt Monson]

Well done. Neither sounded catastrophic to me. Running it dry seems scary but based on the pictures there's only slight galling on the gears in box 1. Sounds like you are just going to move the rod over? The center cases can be repaired if you aren't looking to finish this today.

[72911]

Very good trouble shooting. I just went through a similar issue. Had a 915/40 shipped to me and the shift rod got bent during transit. Got another shift rod and a tail housing (shift rod bearing surface was galled). Assembled and have had a happy couple of hundred miles since.

[JossP80]

Well done, Jeffrey! Now bask in the feeling of accomplishment.

[Jeffreypang911]

Originally Posted by Matt Monson

The center cases can be repaired if you aren't looking to finish this today.

Thinking of getting it re-welded. Sadly I can't weld aluminum myself but I think its more cosmetic.

So heres part 2... this one might be a bit more difficult, and hopefully someone can chime in. When I had this trans in the car, it also made a slight noise like something was riding on the gear. I figured it was the shaft but now I am not so sure. take a look at the synchro ring hitting the three collar slide guides, sounded exactly like this when I was in the car too. I took a video:

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

So maybe my trans is really fubard

Doesn't seem normal. Also it seems like leaving marks on the rings. Hmm..

Heres a photo of where it is hitting.



Just when I thought I was in the clear. What could even cause the guides to hit? Or maybe it's normal and I'm going crazy? Heh.

[Matt Monson]

No, those marks are normal. That's from the slider slipping on the synchro.

I'm not talking about the fins. That's cosmetic. I mean the spinning bearing races. I don't subscribe to the loctite solution. They need to be overbored and sleeved imo.

[patkeefe]

Just had my (4) bearing races bored and sleeved by CMS. You don't want to know how much.