A couple of months ago, my 74 450SLC apparently broke a chain rail and damaged the left cam gear pretty badly. I limped in to a local mechanic who replaced both cam gears, chain, tensioner and the broken rail. Amazingly, the car seems to run decently after this disaster. I assume this means I lucked out and avoided bent valves on that head (chain jumped only one tooth?).

I have since learned from this forum how common an occurance this is with these engines around 100K miles, and the usual preventive measure of replacing all the rails, tensioner, and checking chain stretch. This is what worries me -apparently only the one broken rail was replaced! Presumably this was the left inner (straight) rail- is this always the one that fails catastrphically? Or am I driving around with a time-bomb with another rail about to fail? Thinking about it, this might be the only rail positioned so that a broken piece would get between the chain and gear, or is it?

Hopefully somebody out there has seen enough of these to enlighten me, thanks!

Try this website for the article from ImportCar magazine. Looks just like my car except for the one broken valve and holed piston.
http://www.import-car.com/ic/ic40024.htm

__________________
Gregga
'89 420 SEL "Moby Dick"
'30 Ford Coupe
'31 Ford Slant Windshield Town Sedan
'33 Ford Pickup
'34 Ford Pickup
'01 Honda Passport

Yup, I saw this article too. Notice that it is the left inner guide that is being cited as the culprit, the one that I think also went out in mine.

I'm hoping somebody has seen enough of these to know if it is always that particular rail that fails, as this is the only one that was replaced by the mechanic.

If there was just slight piston/valve contact, would the car still run? Or does this failure tend to have a binary outcome (entirely catastrophic, or entirely okay?) Perhaps a related question -what is happening when incorrect valve clearances lead to rough idle?

thanks gregga!

You might try a compression test on the left bank. That'd give you an indication of bent valves. As to mis-adjusted valves, I just replaced 11 broken ball studs (the head rebuilder over-torqued them and the aluminum expansion popped the heads) and am waiting for a back-ordered adjustment tool. The car shakes at idle but smooths out at 300 rpm over. It has a vibration at highway speed when I have to cross town on the I-state. Hopefully, the tool comes this week and I can cure it.

__________________
Gregga
'89 420 SEL "Moby Dick"
'30 Ford Coupe
'31 Ford Slant Windshield Town Sedan
'33 Ford Pickup
'34 Ford Pickup
'01 Honda Passport

Good luck fixing that idle, gregga. I'm resigned to living with a slightly lumpy idle on mine for now.

Good point, compression still is okay on that side. At least until one of those unreplaced rails breaks and the chain jumps again! (hopefully the new chain and tensioner will keep chain slap to a minimum...)