Tranny Questions - Here!

[red-beard]

I figured with that other thread I'd start a Q&A on the trannies. To answer a couple of questions:

Flipping the ring gear: Yes, it is easily done. I've even done it by accident! What you are doing is removing the differential and turning it around so that the ring is on the "outside" instead of the "inside". The pinion shaft needs to be pulled back to do this, so it means pulling the gear stack out of the transmission.

No. I did _NOT_ install the "super reverse" tranny into a car.

Replacing a "Bad" Synchro. Unless it's just 1st gear, you will need to pull the gear stack and dissasemble the stack. If you're going to do this, you will need a couple of special tools and a couple of unusual tools.

And speaking of Bad synchros ($40 each!). You need to look at the sliders ($180-400 each) and dogteeth ($110 each) as well. The 3 parts work together. To completely rebuild one of our transmissions with all new replacement parts costs about $2000 in parts alone. Even a reputable tranny rebuilder is going to charge you parts and labor and it will cost lostsa money.

Now. With that said. I've been contacted by several people who are having an engine rebuilt and want a tranny rebuilt. They are spending $5000+ to have a monster engine built, but want a cheap tranny to go with it. If you are going to spend $5000 to have an engine rebuilt to better than new, etc, spend the money and have a really good new tranny built for you! No, this is NOT an advertisement.....

Bring on the questions....

James

[JWest]

Flipping is easy, true (I did it and drove the car around the block with 5 reverse gears!), but one of the most difficult procedures in transaxle work is setting up the ring and pinion mesh.

If you flip the R&P then this setting in gone and you have to start all over. The pinion depth must be set and the ring gear lateral position must be set. This is a difficult and time consuming procedure that is beyond the capabilities of the majority of people.

[red-beard]

Did you "hear" any changes in the tranny with the reversed pinion? Clash?

James

[JWest]

Yes, it sounded like a freight train was running me over. While the pinion depth should be OK, for the ring gear to be in the right place would be very coincidental.

P.S. I'm not arguing with you, I was trying to add more info to your statement. I won't say anymore on this topic.

[red-beard]

Sorry - I wasn't either - I was just wondering what you heard. Maybe came across too strong...

James

[JWest]

OK

I didn't want to get into an argument.

Yeah, it really was bad sounding - like the whole thing was crunching into bits. Flipped it back and drove it (silently) for thousands of miles.

[Dave at Pelican Parts]

I hear that changing the gearsets also changes the ring and pinion engagement. Do you know what other pieces also have an effect? In other words, can you come up with a list of things that will require you to set the ring and pinion up if you change them?

--DD

[sammyg2]

I've set the backlash and wear pattern on many gearboxes over the years, the way we did it was just to first set the thrust, then measure the wear pattern with prussian blue, then measure the backlash. Set it and repeat. is this consistant with the 901 box? if so, anyone know what the correct backlash should be and where the measurement is taken for reference (diameter makes a huge difference)?.

Another question: the ring and pinion is set with gaskets (shims) on the front side (when installed in the car) of the intermediate plate. the 1st/reverse endplay is set with the shims on the other side of the intermediate plate. Does anyone know what the endplay should be set to for 1st gear?

BTW, for those who are economically challenged (way too cheep) the syncros for most if not all the gears are interchangeable and the 4th and 5th gear syncros wear much slower than the 1st and 2nd do because they don't get used as often. you can swap the syncros from an old tranny and put the 5th gear where 1st is on the good tranny and save some money, assuming it isn't worn out. The dog teeth and slider can quickly rule out and savngs as red Beard mentioned. Moral of the story, don't throw out those old trannys, especially the tail shifters. The internals are identical except for some of the shift linkage.

[red-beard]

Each gear, the physical gear, is truely unique. It will be slightly different than the other gears even for the same ratio. There is a "tolerance" in the build. It's the "Stack up" that causes a problem. Add up all of those tolerences and you'll find that your +/- now has 5 or 6 components. So anything that is "fixed" onto the shaft, any of the non-floating parts, will change the stack up.

Synch bands, gear teeth and sliders do not affect the stack dimension.

The actual gears, the spacers, the triangle thing that the slider moves on. All of these will affect the pinion depth.

Sammy, the prussian blue check is a better way to check the engagement than the proceedures in the Porsche manuals. The manuals have you check the depth with a dial indicator and measure pinion itself. I'll grab the tranny book and check and post the specifications.

James

[JWest]

The process is outlined in the factory manual, however special tools are used. The is no data given on the measuring diameter (you could guess based on the picture probably) to get the desired backlash of 0.005 to 0.007in.

The manual shows the dial indicator touching the boss for the clutch tube on a 911 trans - the 914 side cover does not have this boss!

If you know what you are trying to achieve, you could make it happen, but the info is not there for someone who is inexperienced to do this without the special tools in hand.

Here's an idea: Check the contact pattern and backlash (pre-flip) with prussian blue. Flip and try to achieve the same results.

I like the 4th and 5th to first and second dog teeth swap. Also 4-5 to 2-3 slider. The one thing that is a killer is the 1st gear slider - can't swap it for another gear and they are often chewed up.

James - have you been having good success with the "repointed" sliders? Been a while since that was discussed, do you have some miles on one you did that to?

[sammyg2]

Thanks, the .005" to .007" backlash is probably meant to be taken at the centerline of the ring gear, that's the accepted practice in the US but we all know them Germans can be "unique".

BTW, prussian blue works good on gears and bearings, but it's original intention was for use in practical jokes

(I bet some of you know what I mean).

[JWest]

FWIW here is a pic of the setup - looks like center of the ring gear to me:

[red-beard]

Repointing the sliders:

I've been running a repointed slider in the 914/6 since June of last year. Works much better than it did before.

Here is what the slider/dog teeth interface is supposed to look like:



Notice the teeth are pointed. When you shift, if the 2 parts are not lined up, the points direct themselves into the slots. Also, I'm just showing a side view of the pointed parts. The slider area between triangles is actually a "slot" and is pointed at each end. Just note that once the slider wears down to the level of the bottom of the triangle, it cannot be repointed.

When things start to get a little worn, and this happens even with good synchro's, notice that you don't have as good direction when things aren't lined up and that you're starting to get some "mushrooming" which makes engaging the parts harder. This is also the way things would appear if you just replace dog teeth and not replace the slider. This can be cleaned up by removing the mushrooming and then flattening the triangle. It won't be as good a right triangle, but it will work better than the rounded point.



Look at this drawing where both sides are worn and it's getting harder and harder to get the slider and dog teeth to engage.



Finally, look at a non fixable one. It is ground down so far, that you can't repoint it. It's flat and flush. Also note that the mushrooming has reached the point where you may not get meshing at all.

[red-beard]

Uhh - sorry guys - my slider drawings are almost porn-o-graphic.....

James

[scg]

...but they sure do tell us why there's grinding upon "engagement..."

[scg]

Oh, and how would you rate rebuilding a tranny on the 1-10 pelican scale... and how much time would you estimate?

[red-beard]

I expect it is easier than rebuilding a type 4. I can pull one, pull it apart, clean, inspect, replace the parts that need replacing and put it back into the car in 8 hours or less. If I just pull the gear stack out, less. Much less.

James

[elwood-914]

James,
Thanks for the info, even though I'm not in need of a rebuild yet I find it very interesting to follow. "I'm soaking it up like a sponge" Thanks

[Ron Meier]

James,

when they refer to a "short gear" 901, which gears do they change? Does 5th become 1:1?

[red-beard]

Depends - could mean anything really. It could be 3rd 4th and 5th shortend to be closer to 2nd and used for Autocrossing.

James