Kevin Baker

I'm having a problem with the timing belt. When we changed it last week, we tensioned it to the proper new belt tolerance of 2.7 +/- .03 and the problem as I see it is that the belt is not stretching as the engine heats up. I belive this because the belt continues to "walk" to the end of the cam sprocket. If I understand this correctly, and I'm asking for thoughts, verification. As the engine heats and expands, the belt should "stretch" some thereby maintaining a correct radius, for lack of a better term. If the belt is NOT streching, then it would make sense that it would "walk" as the engine heats and expands, the belt, in a effort to maintain its "radius" would tend to move toward the front of the cam sprocket. Am I correct, or nuts?

83na944

Kevin,

Are you assuming the the camshaft sprocket or cranshaft gear is tapered so that the tight belt moves away from the engine? Interesting idea.

Is this causing a problem?

How did you figure this out? Did you go back in to check the tension after a week?

Kevin Baker

Have not checked tension, as I drive about 24 miles per day. I'm just a little bit compulsive about the timing belt. So I checked the inspection hole by the dist. and saw that the belt had moved.

goingboeing737

Kevin,

Is it still doing it, or is this from last week?

Also, I forgot to ask, how does the clutch "feel" since the bleeding?

Kevin Baker

Still doing it The clutch feels great, though I'm sure that the leaking heater control valve is shortening its life a bit, but I'll be fixiing that soon enough.

goingboeing737

I'm off Thursday 14th.

Kevin Baker

Me too! I'm thinking that I'll try to back off the tension a little, I mean LITTLE bit tomorrow.

PorscheDoc

A new belt should be tensioned to 4.0 +/- .3, it is only a used belt that is tensioned to 2.7 +/- .3 Check clarks-garage.

Kevin Baker

OK, yeah, that is the number. I was a bit tired when I posted the thread. Any thoughts on my "theory"?

PorscheDoc

Wierd, it seems like guys usually talk about the belt moving towards the rear of the motor and get close to the rear belt cover, not vise-versa. Is the triple square bolt on your cam housing tight so there is absolutely no wobble to the cam sprocket? Next question is, is the belt 100% rubber, or is there some sort of inner layer that prevents expansion? I have never cut through one to see.

Kevin Baker

Good questions, I'll have to check the cam sprocket.

944Ecology

The most likely cause of this walking is the incorrect installation of the sprockets. Check your work against a factory diagram to see if you haven't inadvertently swapped one of the sprockets that has a shoulder towards the front for one with the shoulder towards the rear.

gb

tod84944

George is right about this. One our 84 track car, after a belt job with new cam seals also, A seal was turned wrong. This caused the belt to "walk" foward. Take the sprocket back off and do like George said and check to make sure everything is put back together right. Get a good digram that shows how it goes back together.

Granite 944

As far as "walking" goes, yeah, it could be that you did install one of the shaft drive sprockets on backwards, the most likely one would be the cam drive sprocket on the crankshaft. It must be installed with the shoulder/flange forward, not against the belt cover. The cam tensioner will have shoulder towards the cover, pump shoulder of course is forward. When your engine heats up, this belt has no other option but to stretch tighter, and loosen back again, but it is kept in alignment by these shoulders (shouldn't be very much variance in belt position on drive gears, if at all). I've noticed my belts all seem to run even with the forward edge of the cam sprocket. I've also had a rebuilt 951 waterpump that had the older/smaller pulley installed on it, thus allowing the belt to run about an 1/8" off the backside of this pulley when pulley is installed even with forward end of pumpshaft. (it got replaced immediately). The pulleys for the older style pumps are smaller diameter, and shorter in length (IIRC). So make sure you visually inspect the alignment of all pulleys/shoulders, and have full belt face engagement on these pulleys.

Geo

Another possible issue here is the head work that was previously performed. It's possible that when the head was milled flat it may not have been perfectly square to the cam.

A little extra info for those playing at home...... Kevin's last belt did this as well.