Lotus Elan

Front cover - timing chain interference.

PostPost by: elanner » Fri Nov 15, 2019 12:11 pm

Oh gee, many thanks to all for the replies. This forum once again proves its worth. I suppose that it's good (uh, bad) to know that back plate/chain interference is not particularly uncommon. Nevertheless it would be nice to fix it with this build.

There's clearly plenty to check. I'll report back on progress in due course....

Nick
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PostPost by: jaman » Sat Dec 28, 2019 5:04 am

One other possibility is that the chain was run "slack" ie not with proper tension adjustment. The previous owner was not ware that the noise was not usual. Common issue in Jamaica where I was in the 60's & 70's.
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PostPost by: elanner » Sun Dec 29, 2019 1:56 pm

Yes, you're right! I've been meaning to add a bit more to this thread now that the engine reassembly is almost complete.

Despite considerable over-thinking about the issue (see below) the only logical conclusion is that the wear is caused by a chain that is both slack and worn. As mentioned above, I guess that many Elans have suffered this at some point in the past half century, which is what makes the damage quite common.

Nick

----

With the help of forum member Avro I have had three backplates to examine. All of them with a small amount of chain related damage. One showed marks on the top flange, by the exhaust sprocket. Another showed marks mostly in the middle of the chain run, by the chain guide (vibration damper). The last showed marks on the bottom flange, by the crankshaft sprocket. So, weirdly, the damage is not consistent.

On doing a trial assembly of the back plate, head, chain and sprockets (not fitting the front cover) it's easy to see that there seems to be adequate clearance between the chain and backplate. For the chain to foul the backplate, especially by the crankshaft sprocket, a surprising amount of deflection is needed.

The damage requires the chain to deflect sideways, not on its natural bending axis. I think that for this to be possible the chain links would need to be worn (in addition to the chain being slack).

The run from the crankshaft sprocket up to the exhaust camshaft sprocket is the driving side of the chain run, not a section where some slack could be expected. Perhaps tension changes as the camshafts rotate can set up vibration harmonics that allow the chain to drift off a straight track?

Anyway, in the end we simply moved on and put the engine back together with a new chain. Clearances all looked fine.

Gary Anderson noticed rub marks on the inside of the cam cover between the camshaft sprockets, so I know that it has been slack at some time. I don't know what the clearance between the chain and the cam cover should be but I suspect that it's not much. Which would seem to be a good reason to not over-tighten the cam cover down onto its cork gasket. A small spray of engine primer paint on the inside of the cam cover where the chain has rubbed will act as a tell-tale for any future rubbing.
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PostPost by: StressCraxx » Sun Dec 29, 2019 5:09 pm

Elanner,

Glad to hear you have been able to resolve the issue. In my limited experience with power transmission chain, when the chain wears, it not only gains length due to the pins wearing and side plate holes elongating, the chain can now move more side to side when the tension is lost. At 5000 rpm (that translates to LOTS of meters per second! in chain speed) and above a loose chain is capable of all kinds of gymnastics, particularly when the engine slows quickly in a rapid downshift.

Looking forward to hearing when you have your car back on the road again.

Regards,
Dan
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