Mystery of the timing chain failure...

[bdw18_123]

Me and Dynomite have been exchanging PM's discussing the possible reasons for the timing chain breaking in my '90 ZR-1 (as well as things regarding reinstalling his dashboard).

I'm pasting his reply here in a new thread, since having further discussion in my first restoration thread would just be too messy. Anyway here's what he posted at the end of that thread:

Originally Posted by Dynomite

Just have to ask

It appears that only 2 or 3 rollers are badly worn on each side of the break. All other rollers shown in that broken chain and in the unbroken chain show no significant wear. What would cause that

You would think that there would be equal wear on all rollers?

It also appears that one link pin had sheared off on left side (fourth roller to the left of the broken link). Could that have happened first causing the rollers to separate a bit more in that area resulting in excessive roller wear due to change in chain pitch between adjacent rollers? Then resulting in excessive link stress and a fatigue break in the link because of change in chain pitch in that localized area?

I was thinking at first the break was catastrophic meaning it happend all of a sudden with an otherwise uniformly worn chain. The chain breaking because of a chain guide bolt dropping between the chain drive sprocket and chain (the guide bolt that I dropped when I removed my heads ).

It appears in this case however, that several rollers are worn in the immediate area adjacent to the link break. I assume the chain drive sprocket does not show any particular catastrophic damage on any individual tooth indicating an object got caught between the tooth and chain. A few worn localized roller(s) like shown would hit a different set of teeth (two or three teeth) each rotation on the chain drive sprocket (the drive sprocket would show equalized wear then among all its teeth).

So....what to look for if you have the front cover off when checking timing chains?
1. Check primary chain guide wear and primary chain tension (finger tight against chain).
2. Check primary chain guide bolts holding primary chain guide in place.
3. Check secondary chain guides wear (or broken pieces).
4. Check secondary chain guide bolts that hold bushings in place.
5. Check secondary chain guide bushings for wear.
6. Check primary sprocket and secondary sprockets for wear or tooth damage.
7. Check secondary sprocket (idler sprocket) bolts for tightness (three).
8. Check secondary sprocket (idler sprocket) shaft bearing for excessive play.
9. And...check what on the secondary chains
.....a. damage/wear to any rollers.
.....b. All pins in good condition.
.....c. Check for wear on any linkages or pins.