[grparen]

I was driving my '63 Tempest with it's '65 421 engine slowly out of my neighborhood when I heard a bang and then a noise like a bicycle chain dragging. Looked under the hood and saw my harmonic balancer at an odd angle on the crank. I towed the car home and started digging into it. Found out that the balancer collar had broken and after taking the timing cover off, saw that the chain had snapped also.

Here is the question. These Pontiac engines are non-interference, correct? Which means that if the crank and cam are not turning at the same time that there is no interference with valves hitting pistons. I tried cranking the motor after the chain broke and the crank turned with no interference. Even though I have a high lift cam in the car. Which means that possibly there has not been any damage; bent valves, push rods, etc. Should I just replace the chain, put on a new harmonic balancer, set the timing, and see if the engine runs right? Or, do I need to pull the motor and open it up for inspection? I just had this motor rebuilt and I don't really want to pull it out of the car again.

How did this happen? I believe I am the cause because after the rebuild I put the balancer on tightly but did not torque it to spec. How stupid was that?

[tom s]

First of all not TQ to spec is more than likely the cause.The military way.never have time to do it right but always have time to do it over.I will be surprised if you dont have bent valves.You should not have to pull the engine unless it broke the crank snout.Start there.Tom

[gtoguys]

Many years ago I broke the balancer on my GTO 400. The chain snapped and the pistons hit the valves. I bent 6 valves and 8 pushrods. So for you I would take the valve covers off and look at the pushrods.

[hurryinhoosier62]

Originally Posted by tom s

First of all not TQ to spec is more than likely the cause.The military way.never have time to do it right but always have time to do it over.I will be surprised if you dont have bent valves.You should not have to pull the engine unless it broke the crank snout.Start there.Tom

X 2, Tom

[bankbook]

Originally Posted by tom s

First of all not TQ to spec is more than likely the cause.The military way.never have time to do it right but always have time to do it over.I will be surprised if you dont have bent valves.You should not have to pull the engine unless it broke the crank snout.Start there.Tom

X3
Most common cause. Don't ask me how I know.

[DOC]

Loosen all the rockers. The valve heights should all be the same. If not, You have bent valves... While they are loose run compressed air in each cylinder using a compression tester. you should not hear any massive air leaks. If you do, It's gotta come apart.

Good Luck.

[Goatracer1]

Years ago my balance broke at the key way and the piece caught in the chain and snapped it. Bent 1 valve, just 1 but still had to take it apart. Pain in the butt.

[grparen]

Crank snout and keyway look fine. I removed the valve covers and valve train looks undisturbed. I was able to twist 12 of the push rods where lifters are not loaded and none are jammed. I wouldn't be able to turn them if they were bent, right? Do you think I should still take apart valve train for inspection? Or just replace timing chain and balancer?

[tom s]

I would take all the push rods out a roll on a 100 % flat surface and be 100 % sure.Tom

[grparen]

I pulled all of the push rods out, rolled them on a flat surface and none of them are bent. I put them all back with the same lash as when I took them out. I have a question. I was able to turn the camshaft with a ratchet about 3/4 of the way around and then hit a stopping point or some very strong resistance. I was afraid to push it to see if it would continue to turn. Is that a problem? Everything else seems to be fine except the broken balancer and timing chain.

[DOC]

If the timing chain is off and no pushrods were in the motor that is not a good sign. The only thing else that should be touching it is the distributor on the other end. Recommend to pull the intake and valley pan and take a look at the cam and see what is going on and what is holding it tight from turning easily. That may be the cause of the timing chain breaking...

[grparen]

I turned the camshaft both clockwise and counter-clockwise and it stops close to the same place. I wonder if it is broken!

[DOC]

Thats what I am thinking.... Pulling the intake and valley pan will tell you right away.

[grparen]

Well, I pulled the intake manifold and valley pan and all looks fine. No broken camshaft. I put a ratchet on the cam bolt and turned it past the sticking point and I it turns fine. I guess I was a little apprehensive turning it before. All looks right to me. Seems like not properly torquing down the balancer and it backing off caused it to break and also break the chain. Not sure if I should just replace the chain and balancer and put this thing back together and fire it up and see what happens. What do you guys think?

[GruntedCatalina]

Sorry to hear about what happened man! that sucks big time. I am personally for the Gung ho method of grip it and rip and see what happens but, I think taking the heads off, or going the compressor method would be best. you may be lucky but, you might not be lucky a second time either.

[Heybuck]

I'd look at it this way......

If I was out on the road, a long way from home and I had little to no alternatives, I'd put it together and hope like heck that it will run and keep running.

But if it was in my garage, now that engine would not fire another shot till I KNEW it was all in perfect shape. It's your choice and risk.....

Ian

[tom s]

if 100% sure there is no bent pushrods I would put it together and fire it up.Tom

[DOC]

now is the best time to at least check the intake valves with some air into the cylinders. you will hear real easy if an intake valve is stuck open. Its the biggest valve to get hit.

[hobbygto65]

Had the same thing happen to at about 4000 rpm. Replaced the balancer and chain and its been running fine ever since.