Well, for those that have seen all of these timing chain failure threads lately, you will find myself posting in a lot of them trying to determine the cause of failure as I had always thought there was improper valvetrain geometry or something of the sort that caused the failure.

Well, yesterday at the dragstrip my car had timing chain failure on the return road after making a pass. I put the car in second gear and pegged the throttle for a bit(this is a very long return road with nothing on either side but fields) and at ~3500rpm the timing chain broke.

Details on my setup. Vengeance Racing cam installed by them - 231/243 .617/.623 115 LSA, Comp 921 valvespring kit, hardened pushrods, ASP Underdrive Pulley, 160 t-stat, etc. Car made 483/435 to the wheels and my rev limiter is set to 6800rpm. My tune was dead on via my hardmounted wideband 02 sensor and my HP-Tuners logging software.

This setup has had exactly 8k miles on it with mostly highway miles and the car has never been road raced. The rockers and valvesprings are in perfect order and show no failure. I will be having the valvesprings tested for pressure to determine if they were the culprit. I had plans to replace them once my combination hit 10k miles. My intake valves are not broken at the head, but merely bent.....which was surprising considering the nature of their design(2 piece welded hollow stems).

There was no indication prior to the break that anything was wrong. There was no sound preceding the break that indicated the valves made any contact at all with the pistons(I had the windows down). I think this is purely a timing chain failure and that being the case, I have absolutely 0 confidence in the car as of right now. How could I ever want to rebuild the car stronger or better when the only thing I can't make stronger is the damn timing chain. The Katech chain isn't stronger, but pre-stretched and heat treated I believe? Thats not enough to give me the confidence I am after.

Updates will follow as more details become available.

Update # 1 - Pictures of Passenger side head and pistons on Page 6

 

Maybe these chains simply can't take the added spring pressure. I'm not touching my cam....that's for sure.

 

If you never touched the heads and swapped to a cam with a smaller base circle, did you use longer 7.425" pushrods to restore the prelaod on the lifters? A smaller base circle of .060" would need a .030" inclrease in pushrod length. 7.425" would be close.

 

Great post with great information.

My Katech is holding up with 761/670 so far. Its almost like a crap shoot with these cars. Between the rears and the timing chains its like a disaster waiting to happen. I just thank the good lord I can fix my own junk. Even so, a chain failure is something I can't afford right now.

Good luck and don't jump ship on us!

I'd love to hear more and see where the issue is.

Additionally, I measured as close as I could what pushrod I needed with my combo. I didn't do any math, I physically set the motor up and used a pushrod checker. So far, so good with pushrods I found at various manufacturers that met my needs.

 

Vengeance did the install and I can only assume they knew to use longer pushrods for proper preload on the lifters. FWIW, the car ran dead quite with regard to valvetrain noise. It never made any more noise than it did stock. Improper preload should have made things very noisy.

 

Does anyone have any C6 specific directions for a head/cam swap. I am going to be pulling the heads and doing the rest of the tear down myself. Thanks,

 

I did my own swap with the help of another forum guy. It was pretty straight forward. the only thing is if your doing the cam you are going to have to unbolt the front steering rack and move it to the side. The heads are a piece of cake just make sure you use the proper loosening and tighening sequence. When you remove the heads loosen them up about 6-7 turns then lift them off the block so the water doesnt pour into the bolt holes. You can also use a shop vac at the front of the heads to suck the extra water out. Make sure you clean the surfaces for the gaskets to make good contact. You may also have to take the heads and have them milled to ensure they make a proper fit after being removed.

 

I'm sad to hear that another instance of this has occurred so soon after mine. No warning either. I've just finished tearing mine down and found nicked pistons and bent valves.

 

Everything you said about your failure was identical to mine when my timing chain went this past summer. No valvetrain noise, it broke under slight load in 2nd gear around 3,500 rpms and when the heads were removed it looked identical in damage to yours with bent valves stems and cylinder marking due to impact.

I had 12k on my cam and valvetrain, it was discovered the previous shop didn't have the right seats in there causing dual springs to bind. I don't know if the shop had replaced my original timing chain either. I don't know if you read my thread where Chuck Cow went into detail and posted some nice pics of the breakage and rebuild. Anyway, my current build is almost complete, short block was completely redone at a race machine shop, torque plate honed, and bunch of other goodies. At any rate I went with all new valve setup and dual timing chain from cloyes. Good luck with the new build, it does get frustrating seeing these types of breakages for such a high calibre sports car.

 

I just pulled a pushrod and it had 7.400" labeled on the side. The car ran exceptionally quiet, IMO, for a cammed vehicle so I always figured preload was right on. I thought I recalled the stock lifters had a pretty broad range for preload. Would having a 7.400" length pushrod throw the valvetrain geometry off significantly?

 

I hate hearing stories like this.

So do we have a definitive answer as to exactly why this happens? Can it be narrowed down to certain parts, or install methods? This makes me scared to drive my car.

 

Is there any clear way to test the springs to see if they are still good or were the likely culprit?

 

I think I missed something... Was it a stock chain? If so, was it your original and reused with the new cam?

 

What about this? The cam sprocket bolts may have started backing out on the drag pass. Then, when decelerating, they loosened up more. And the full throttle blip on the return road caused it to fail completely. I'm curious to see.

 

The stock chain was reused but the car only had 7k miles when the cam was put in. The stock chain shouldn't have had any problems after that few of miles.

I have yet to get behind the front cover on the motor. All I have done so far is pull the IM and the Valley cover to confirm that the timing chain broke. I am sure your average machine shop has a machine or press for testing valve spring pressures. If not, I will find someone to send them too for testing. Upon inspection, I find it very unlikely that they were the culprit, but they will be tested none the less. Comp 921's have a great track record and are largely regarded as one of the best duals available.

 

Although I am by no means an expert, but its my guess the cause of the chain failures is due to a change in the valve train harmonics that is causing the timing chains to whip violently at a certain RPM until finally a link break. It would be almost impossible to detect it during a tear down.

Could an under-drive pulley be the key contributing factor? Would an aftermarket different harmonic balancer prevent it? This is a tough one.

Seems like this problem should have surfaced at Katech or one of the other racing shops.

 

I'm wondering if all the above chain failures had underdrive pulleys. If so, is it possible that the smaller front dampener is affecting the torsional vibration of the crank?

 

Sorry to hear about your misfortune, I meant to tell you that in my first post.

That's what I thought. When we did my H/C my motor only had 10K on it, not hard miles either. No tracking, racing, etc, just some spirited driving now and then. What we found was there was significant slack in the stock chain compared to the new IWIS chain. I actually still have it, and if anyone has a new one, Id be happy to post a picture to compare the two. It would tell you alot...I'm sorry I didn't have the camera ready when we did the swap. I thought I was being overly **** in replacing anything removed with something new...it cost some more but I think it was worth it in the end.

Cameron's chain was also re-used and suffered the same fate under similar circumstances. From what I've seen, and for the relatively reasonable cost, I would never reuse a chain, or let anyone I know reuse one either, even if I had to pay for it for them.

 

This scares the sh*t out of me now. I have H/C + UD pulley on the stock chain.
Do we know exactly why this happens?

 

Can't you get a double roller? I know there were double rollers for ls1's I don't think the design has changed that much from ls1 to ls3 in that area.. ???