915 synchros

Alan L · Alan L's Garage

I have just pulled 1st and 2nd apart.
This box has had trouble changing down from 4th down, progressively worse to 1st, without crunching. You have to shift slower than normal.
For 1st the car has to be almost stopped.
The dogs look great.
Should I see any difference between worn and new synchro rings - they look pretty much the same to me.
Thanks
Alan

tab tanner · 10-13-2006, 06:20 PM

How is the pilot bearing (back of the crankshaft?How do the operating sleaves look?and the hubs?

Alan L · Alan L's Garage

I think what I was trying to say was it is difficult to change down without crunching. And it gets worse as you go down thru the gears. Have to change slower as you go lower.
The sleeves, hubs and gears , and dogs look great. The car actually only has 30,000 km on it. The gear box looks really tidy and you would be hard pressed, I think, to see a problem. The car has has a year on a track , at a guess, so the box has probably had some fast down shifts, by what may be an incompetent driver.
So, is the wear on the synchros normally visible? I was expecting some worn synchros. I was expecting to see some flattened/worn grooves.
Could the pilot bearing do this?
Regards
Alan

tab tanner · 10-14-2006, 03:52 AM

YES, If it is bad It will not allow the input shaft to freewheel when the clutch is ingauged. I had a 944 do this same thing as you discribe. I allways replace them when I have a clutch out. If you don't see any probles with the trans it's probably not the the cause of the shifting issue

MBruns · 10-14-2006, 04:32 AM

When they resist changing gears you need to replace the sliders (operating sleeves) the syncro,s most often the dog teeth as well, the problem with a visual inspection is most of the stuff LOOKS good but unless its been burned by an awfull overspeed downshift they look almost like new, I have rebuilt all of the porsche gear boxes for over 25 years and you will be doing it over if you dont change out the consumable parts, its as simple as that.

Its getting very expensive to do those boxes and when they need bearings as well it can get ugly. Be sure to inspect the bearing hold down plates for wear.

Have Fun, Mike Bruns JBRacing.com

tab tanner · 10-14-2006, 05:56 AM

Great advice Mike!!!!!!!!!

sww914 · 10-14-2006, 08:07 AM

As far as the appearance of the synchros, the wear area won't really look worn to the untrained eye, it will look smoother and even shiny compared to a new synchro. This smoothness allows it to slip.

greg bradbury · 10-15-2006, 12:18 AM

Alan,

A car with 30,000 k's on it souldn't be suffering gearbox problems, especially down shifts. Yes as the others have said worn synchro bands have a shiny appearance but it usually takes 100,000k's to start to be a problem. My gearbox with 130,000km had no down shift problems only upshift 1-2 and 2-3. You may have a clutch problem. Is the clutch disengaging properly between shifts? just a thought. You may have problems in this area instead.

MBruns · 10-15-2006, 05:43 AM

Driven like it was designed to be shifted I agree, but he stated that it spent a year as a track car. We have race/ track day customers that can go many seasons and have the usual wear and others that can not make 2 sessions with out killing the syncro's and sliders and dog teeth, if you treat the shifter like a wine glass stem they will be much happier, even in a track car there is no advantage to speed shifting with the style of syncro's that the 915/901 use. I just cost's alot of $

Also if it is/has been updated to the later style 1st gear assymetrical dogs/slider they DO NOT like track duty, they were designed to make it easy to down shift into 1st sitting still, when we prep for performance we back date the 1st gear to the symetrical earlier style (like the rest of the gears)

Mike Bruns JBRacing.com