timing belt issue...

[Thighs]

so ive got a 1990 325i with an 87 i motor, and it wont stop snapping timing belts. it snapped one around 2 weeks ago, so i replaced the head with my old used 90 head, new t belt and tensioner, and it snapped again an hour later.

what could cause it to snap belts like this?

[yert315]

Just throwing this out there, but is there such thing as tensioning the belt too much? Just a thought.

If you're snapping belts, are you bending valves every time as well?

[Thighs]

im not tensioning it too much AFAIK, just getting it as tight as i can without straining much. IIRC the haynes manual says that the deflection should be between .25 and .5 inches, so thats where i got it.

yeah, im destroying heads. lol

[dashboardmonkey]

Snap a pic of your snapped timing belt, I would like to see this.

[Hallen]

Is the tensioner new? How about the roller? On my old belt, I noticed that it was partially running off of the rollers and the gear. It was scraping against the timing belt covers. I replaced the tensioner and roller and the new belt had no problems with this.

Possibly, your belts are riding wrong and getting pinched or chafed and that is causing them to break. That's my guess anyway.

[SpecM]

maybe a sharp burr on the cam or crank sprocket?

[Liam]

That is kind of odd. I have probably done something in the neighborhood of 1000 M20 timing belts over the past 20 odd years. I would take a very hard look at the sprockets and tensioner etc.

Also take care not to kink, twist or otherwise maul the belt prior to or during install.

It is possible that it was installed too tight, I give the tensioner a little nudge to take up the slack side and let the spring tension the belt.

[SpecM]

you're using a good quality belt, right

namely, Continental?

[dashboardmonkey]

you're using a good quality belt, right

namely, Continental?

[bmwlover0725]

I'd check the crank sprocket. also, don't worry about tightening the tensioner. Simply bolt it down while the new belt is being installed and to tension it, just loosen the bolt real quick and let it snap onto the belt. As long as it's loose, it will tension it the right way on it's own. Believe me, I've done three timing belt jobs in the past 2 months

[Devil Inc]

Continental belt + the right direction = solved problem!

[bimmerboy12]

it isnt possible to over tighten the belt if you are letting the spring do it's job. You should buy a new spring and install with the tensioner. Sounds like you are overtightening by manually tensioning per your comment about .25 inches of slack etc.
the overtightening bends the tensioner pulley and makes the timing belt rub/bind/break.

I speak from experience :)