915 rebuild: has my synchro been "flipped"?
 

Hi everyone,

I'm chasing the dreaded oil seal leak at the input shaft of my 1972 915 tranny and was wondering about the first gear synchro ring. The car shifts great in all gears, but sometimes (like 1 in 20) it will go clunk when selecting first from stop. It's not dependent on double clutching as I could tell and the car shudders a little.

Since I have the tranny appart, I'd like to take care of this as well :) No big issues that I can tell, dog teeth on 1st look pretty good:

http://forums.pelicanparts.com/uploa...1597701488.jpg

The synchro ring has a rough black coating that is worn asymmetrically on top:

http://forums.pelicanparts.com/uploa...1597701488.jpg

From what I read in Gordo's excellent post and Zimmerman's wiki, these are supposed to be square, or am I mistaken? Does this mean that a previous owner "flipped" the synchro and reinstalled it, or is this a normal wear pattern?

At least (I hope), I won't have transmission fluid all over the garage, the damm thing was a rolling exxon valdez oil slick...

Thanks for any feedback!

Your focus isn't great on the synchro pic, but it looks to me that all the wear is on the left side of the ring (from the center, high point left). My guess is this ring has not been flipped.

Put on 2 new syncros along with a new op sleeve and all will be good. Shiney areas are not good on syncros and old and new parts should not be mixed.

Thank you both for your quick replies and advice. John, I have read many of your posts and I really appreciate the time you spend sharing your knowledge. Solamar, I tried to focus on the edge of the ring to see the cross section. Here are a couple of better photos of the surface. There is indeed some sign of wear:

http://forums.pelicanparts.com/uploa...1597751651.jpg
http://forums.pelicanparts.com/uploa...1597751651.jpg
http://forums.pelicanparts.com/uploa...1597751651.jpg

The second gear synchro is very similar, maybe a bit less wear. Following John's advice, I took a better look at the slider between 1st and 2nd:

facing 1st:
http://forums.pelicanparts.com/uploa...1597751842.jpg

facing 2nd:
http://forums.pelicanparts.com/uploa...1597751842.jpg

I find it interesting that the 2nd gear teeth are a little more worn than 1st. I suppose that this is either due to too fast upshifting, or (like me on the first drives!) fishing for 4th when downshifting from 5th :rolleyes:

I will do exactly as suggested and replace the two synchro friction bands and the slider. It is not a cheap fix, but well worth it to do things right.

For reference, here is where I gleaned the knowledge (but not the skills) to take the 915 appart:

How-To: Porsche 915 Transmission Repair Tutorial Part 1 - Porsche Wiki
http://forums.pelicanparts.com/porsche-911-technical-forum/941069-gordos-915-transmission-rebuild.html
http://forums.pelicanparts.com/porsche-911-technical-forum/159059-cant-shift-into-first.html

Here is one that I liked because it has lots of tips and the fork linkages were not disassembled.
https://www.pelicanparts.com/MotorCity/marcesq3/trannyrebuild.htm

Thanks to all for the wisdom on this great forum!

Agree. You also have the option of updating first gear to the asymmetric dog tooth design as installed in later versions of the 915. Pelican lists the parts needed.

You might want to inspect the 3rd/4th syncro assembly too. The smaller shift sleeves get some serious wear on the area that rubs on the syncro ring.

I realise it's been a while, but the draft from an unclosed thread is getting chilly with the colder weather!

Final report: gearbox back together and working great! No more difficulties getting into first and all is well. I did not remove the forks and everything went back into the right position. Since all the bushings are new, shifting is good. I also changed the pilot bearing though it was new. It looked like it could have been binding some.

As others pointed out, it took some 50 Km before the new synchros bedded in and everything went really smooth.

Thank you all for your help and advice!

.
With an arrow, I've pointed out the small build-up of material that can be found on each tooth. This build-up causes increasingly balky shifts, as the gear's engagement tooth finds it increasingly difficult to pass through an ever-narrowing gap.
http://forums.pelicanparts.com/uploa...1605463000.jpg

It's been awhile, but I think you'll find on each synchro ring tiny etched lettering near the ring's gap. This lettering indicates which side of the synchro is to be installed upward (visible when first installed). So, if this lettering is downward, the synchro has been flipped, or it was installed upside down (which may or may not be a problem).
.

You sure about this? Got a photo? I've never heard this. Just checked what was is presumably a factory synchro in my shop, no lettering visible to me. Curious...

I'm quite far from any 915 parts, but the etching is quite faint, and NOT on all synchro bands. I'll take a photo in a couple weeks and post in this thread.

Thanks for pointing this out. I don't remember seeing anything on the synchro I took off even when examining it with a magnifying glass. I still have it (of course) so I will take a look next time I get the chance!

Paul, genuine GOETZE synchro rings have a small GOE mark or etching on one side.
The mark is merely a means of manufacturer identification. It's not meant to indicate the installed direction of the synchro ring.

http://forums.pelicanparts.com/uploa...1605586603.jpg

Thanks, Jon .. I'm not that experienced with the earlier parts, but for some reason (mistakenly) I had it in the back of my mind that factory put etched lettering upward.

Piston rings from GOETZE have similar markings, but are also marked "TOP" for the installation direction.
Synchro rings for later 356 models had a deep groove on one side to indicate the installation direction, and on these the GOE mark faced the gear.

Even with the early shift hub in 1st, I update with: new style 1st synchro, one piece brake band, updated thrust block (energizer) and anchor block for one piece brake band. It gets rid of all the 1st gear crunch. G

Thanks, Jon .. pretty much the reverse of what was stuck in my mind.