nsyr

The other day as I was going back to work from lunch the teeth on the my timing belt stripped off right at the crank. When I initially tried starting the engine it turned over a little slow for a second then just spun freely as if the starter wasn't engaged to the ring gear. I am assuming that all the valves were closed when it broke since I never heard any tapping from the engine (i am hoping). However there seems to be no resistance for the engine as if there is no compression. It really spins free. So I am thinking that each cylinder has a partially open valve that is not causing interference or is this wishful thinking. I also need some input on what would cause this. The belt is a few years old but doesn't have too many miles on it. I will try to get some pics up from my phone.
btw it is an 86 stock 951.

KuHL 951

Sorry but if the engine turns over that freely, there's no compression, therefore bent valves. The valves are never all closed unless the head is off the car.

nsyr

This didn't happen when the engine was running, only when starting. Shouldn't I be able to hear the damage being done (tapping)? And if there is no compression each cylinder would need to have a valve open and ultimately bent. Or is there enough clearance to have a valve open a little with the 'piston travel'? Either way if I need to replace some valves so be it. I am just confused as to what would cause it to happen like this?

Cory9584

It couldnt hurt to try a new belt. As long as you didnt try cranking it for long i've seen people get lucky.

Chris White

The usual cause of belt failure is not the belt but the rollers / tensioners.
You have some bent valves now. Several. If you are lucky it could be just intake valves (cheaper).
Don’t underestimate the force the starter motor can create when it is geared down by the flywheel. It can bend connecting rods on a hydra locked engine – that’s a lot of force!
At cranking speed there is not much of any impact ‘speed’ so it doesn’t make much noise. The brief period of hard cranking was the valves being bent.

KuHL 951

It could be your belt skipped a few teeth first causing valve contact and that was enough to stop the cam long enough to take out teeth at the crank. How many times the engine turned over after it stripped will determine how many valves kissed. And yes the starter has plenty of torque to bend a valve or two...here's proof from a slipped belt at idle and every piston was the same. Maybe you got lucky, I doubt it though.

nsyr

Well I guess it is off with it's head then. I'll post what I find.

KuHL 951

You can rotate the cam a full 360 degrees or it just moves back and forth until it won't go any further? If the cam rotates completely you either sheared the shaft key at the cam gear or your cam snapped.

nsyr

It will only go back and forth about an 1/8th of a turn or so until valves start hitting pistons.

KuHL 951

That sounds more like it but you statement of "I can rotate the cam' threw me off. As Chris said hopefully you just bent intake valves. The sodium filled exhaust valves are very pricey.

nsyr

I was just looking at Lindsey Racing and they had the exhaust valves for the 951 at about $50 a piece. I thought they were more around $100.

MAGK944

Had the exact same thing happen to me last month, thought it would be ok, but two valves were bent.

My belt also stripped some teeth just from one area, I am convinced it was a bad belt as everything else was ok and the belt was about as old as yours with low mileage. From the date you changed your belt it seems like you might have gotten one of those bad batch of belts.

Here is the thread about my belt failure: https://rennlist.com/forums/showthre...ferrerid=54590

nsyr

Would a leakdown identify bent valves? It would be quick and easy.

KuHL 951

You can't do a leakdown if the cam won't turn to close each valve set at TDC for that cylinder. If you pull the camtower then all good valves are retracted if they aren't bent. Your head has to come off anyway because you have bent valves. I think the best way to confirm, if the damage isn't obvious, is laying the head CC up and pouring in alcohol or solvent. Any bent valve will leak and you can see it in the port. Usually a bent valve is easy to spot as they are offset and won't seat squarely. Sorry to say but you need head work.

nsyr

Yeah denial is a powerful thing. I will probably get the cam tower off tonight. Thanks for all the input.