Intermittent 2nd gear problem

911Tfan · 06-21-2011, 06:46 AM

I have a 1970 911t with a 5 speed. Recently I had the gear oil changed to Kendall NS-MP hypoid and had a side cover replaced so the clutch cable could be restrained properly. the clutch disc was in good shape but the pivot bolt was working it's way out and kept putting the clutch out of adjustment. that is fixed now. A bunch of other new parts were unstalled like clutch cable end, fittings, etc... Recently I have been driving around in the country, mostly at slow speeds and after about 45 minutes I will try to downshift into 2nd and it grinds and won't go. this makes me think it has something to do with temperature. so I avoid using 2nd and when I do I use it extra gently and after a couple minutes it's back to normal. It seems that 2nd will grind at a much lower rpm than it ever did before. Sometimes when I downshift into 2nd at a stoplight for example and I am rolling toa stop, I can feel the gears turning unless I am going less than 5 mph or so. First gear has always been like this since I had the car. Any ideas ?

jstobo · 06-21-2011, 09:26 AM

Bad/worn synchronizers. Classic on early 901 transmissions. First gear usually goes then second gear next. Time for a rebuild if you can't live with it.

911Tfan · 06-23-2011, 08:27 AM

If the synchros are failing how come it doesn't act that way all the time?

Peter Zimmermann · 06-23-2011, 09:21 AM

Quote:

Originally Posted by 911Tfan

If the synchros are failing how come it doesn't act that way all the time?

A synchro ring is a round component that, when activated, is compressed by a round part called a slider, or shift sleeve. The shift sleeve has many teeth on its ID surface, and those teeth, during a shift, contact, then compress the synchro ring, which then brakes the gear to a stop so it can be engaged.

A synchro ring can't/won't wear out and remain a perfect circle, it usually wears a little here, and a little there, as do the teeth on the slider. Imagine looking at a clock. The ring can be within spec between 12:00 and 2:00, but worn beyond spec between 3:00 and 6:00. If worn teeth on the slider contact the extreme wear between 3:00 and 6:00, the slider will not be as efficient in stopping the gear, so it grinds.

If the worn teeth make contact between 12:00 and 2:00, for example, its possible that the slider will still work good enough to brake the gear to a stop; so, no grinding. Your synchros are at some point between functional and worn out, I would guess they are closer to the latter.

Hope this helps!

911Tfan · 06-26-2011, 02:43 PM

Wow thanks Pete ! That was a easy to understand explaination ! I took a ride with my wife todaynd did no downshift at all today,other than than at very slow speeds to be in the right gear when the light turns green. No grinding . I also a very careful to take my time shifting, ie 2 secon per upshift. I am not in a race anyway. Just antiquing. I would like to extend the weekend sightseeing rides a while until getting it rebuilt or maybe getting a replacement.