Lotus Elan

Valve clearances and timing chain tension

PostPost by: mbell » Fri May 17, 2024 12:22 pm

I've been doing the headgasket on my engine and so adjusting valve clearance on re-fitting. I am familiar with the clearances changing when the head is fitted and torqued down. So I waited until the head was on and torqued down to adjust the clearances. I did this one cam at a time with pistons set mid stroke. Found that both number 1 cylinder where just slightly overspec, so changed shims.Couple of checks to confirm clearances where in spec.

I then fitted, timed the engine and tensioned the timing chain. I then also checked the clearances again and found that both number 1 cylinder had closed up around 2 thou.

Which thinking about it isn't too surprising and I am not concerned about it but don't remember reports of this in past and didn't find anything in a search. So interested if other people have experienced this?
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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PostPost by: promotor » Fri May 17, 2024 1:11 pm

mbell wrote:I've been doing the headgasket on my engine and so adjusting valve clearance on re-fitting. I am familiar with the clearances changing when the head is fitted and torqued down. So I waited until the head was on and torqued down to adjust the clearances. I did this one cam at a time with pistons set mid stroke. Found that both number 1 cylinder where just slightly overspec, so changed shims.Couple of checks to confirm clearances where in spec.

I then fitted, timed the engine and tensioned the timing chain. I then also checked the clearances again and found that both number 1 cylinder had closed up around 2 thou.

Which thinking about it isn't too surprising and I am not concerned about it but don't remember reports of this in past and didn't find anything in a search. So interested if other people have experienced this?


Yes, it's regular: the tension on the cam chain affects the cam to a degree particularly on no.1 cylinder and to a lesser degree on no.2 (the others are usually negligible) - if you loosen the tensioner off you'll probably find the setting goes back to roughly what it was. The cam is likely flexing slightly too.

Slight play in the bearings due to wear will influence this more - if you're a little worried by it, a check on the cam journal sizes and condition of the cam bearings may show issue. However, there are some problems that cannot be solved. Your tolerances may be spot on and you can't do anything about it - just set the clearances with the load on the cam chain.
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PostPost by: mbell » Fri May 17, 2024 3:52 pm

promotor wrote:Yes, it's regular:


Thanks, I thought it probably would be but don't remember reading it and couldn't find anything.

I figure with chain off there is only upward loading on the cam from valve springs. But adding the chain will add some down tension which will close the gap if there is one. Especially on cylinder 1 as you say.

promotor wrote:condition of the cam bearings may show issue


Cam bearings are new, so not concerned there.

promotor wrote: just set the clearances with the load on the cam chain.


It moved them from upper end to lower end of the specs. So plan is to leave alone and come back once its run a bit see where they are then maybe adjust them.
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PostPost by: Davidb » Fri May 17, 2024 5:11 pm

I have found that the valve clearances change with the torque of the cam bearing cap nuts.
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PostPost by: mbell » Fri May 17, 2024 6:28 pm

Agree. I was very careful to torque them all to the correct 9lbs/ft to do the measurement. One time I missed torquing one down and caused increase in clearance. Torqued it down correct and clearance was back to expected value.
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