TIMING CHAIN ISSUES

[Black and Blue]

I drag race a 4.2 engine with a 4-speed in a car. When I get to the finish line I let off. Simple right? As soon as I let off the cam gear shears off the cam. The bolt was still tight, torqued and lock tited in place. I don't let the car run out to the end of the track against the engine, I clutch it as soon as possible. Anyway, after replacing all the valves, I replaced the dowel pin with a new one from drill stock and added 3 grade 8 bolts around the center one making 4 bolts. Problem solved right. On the first run after this I let off and it broke the chain and sheared all the teeth off the crank gear.
I'm running Cloyes billet gears, 352# spring pressure at max lift, .140" piston to valve clearance. It's done it on two different cams (COMP and Bullet), two different timing chain kits and two different cranks. Crosses the finish line at 6200 rpm, 1/8 mile.
With just the lifters installed, I can turn the cam by the gear with one finger. The cam bearings are still original to the block.
Any ideas, this is getting expensive!

[Randy Bobandi]

As soon as I let off the cam gear shears off the cam. The bolt was still tight, torqued and lock tited in place.

Has to be cam walk. What are you using to control camshaft thrust/endplay?

[Black and Blue]

I use a nylon thrust plug through the timing cover. It had .006" play. Just didn't trust the factory spring/plunger setup.

[dwg86]

I know this is an old thread. From what I understand and for whatever reason the 4.0 will break timing chains right at the 6000 rpm range. I've read you have to go through that 6000 rpm range and can't stay there or run a gear drive setup instead of a timing chain. Do some internet research and see what you can find on the subject.

[Cheromaniac]

I know this is an old thread. From what I understand and for whatever reason the 4.0 will break timing chains right at the 6000 rpm range. I've read you have to go through that 6000 rpm range and can't stay there or run a gear drive setup instead of a timing chain. Do some internet research and see what you can find on the subject.

Yeah that's basically correct. Since the camshaft is only supported by four bearings, it's prone to a harmonic at ~5700rpm that can break timing chains. Racers did solve the problem with a custom gear drive and just put up with the extra noise from it. Others merely accelerated through that harmonic.
Newcomer Racing have been building race 4.0 and stroker I6s for years and haven't had this problem, so it would be interesting to find out how they solved it.