Lotus Elan

Cam adjustment clearance

PostPost by: holywood3645 » Tue Jan 24, 2023 6:04 am

Once cam shims clearance is set. Is the clearance gap likely to increase or shrink once engine is run in?

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PostPost by: rgh0 » Tue Jan 24, 2023 6:20 am

Hi James

The exhaust clearance will close up a little as the exhaust valve runs hotter than the alloy head by a signficant amount and expands more despite the lower expansion rate compared to the alloy head.

The Inlet valve clearance remains approximately the same as the inlet valve runs signficantly cooler than the exhaust due to cooling by the inlet charge.

It is hard to qantofy these numbers without measuring somehow the valve temperatures when running. However I believe the design intent is that both cams end up around 0.006 inch clearance when hot to ensure the follower contacts the cam during the initial take up ramp with goes to 0.010 inch high off the base circle before the rapid cam acceleration begins. It is when the follower contracts the cam during this rapid accelration phase that produces the cam on follower tapping noise when you have excessive clearance above around 0.010 inch

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PostPost by: andyelan » Tue Jan 24, 2023 8:21 pm

Hi James

I don't know if it's relevant but do be aware that the clearances will change significantly when the cylinder head is torqued down. So don't set the clearances with the head on the bench and then expect them to still be correct once the head is bolted to the block, they'll need resetting again once the engine is built.

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PostPost by: mbell » Tue Jan 24, 2023 8:52 pm

Hi James,

My expectation is that valve clearances would reduce overtime, as the valves and seats wear allowing the valve to sit slightly higher in the head and close the clearance between valve and cam.

I wouldn't expect any movement just from running in, but more just from wear and tear over a longer period. (I might be wrong on that thou.)

I agree with Andy that the clearance can be effected by torquing the head down, so best to set them once head is fitted. I found no1 exhaust to be out of spec (6 thou) on my engine and few other close to limits. The engine looked like it had a rebuild not many miles before I got it. My guess that it had valve clearances set before fitting the head.


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PostPost by: elanman999 » Wed Jan 25, 2023 9:17 am

Just my thoughts on this.
I have found that No 1 exhaust is effected by the tension of the timing chain, hence the difference between on and off the engine. You try leaning on the cam sprocket and you’ll notice the change.
On any fresh head I have found that the clearances of a few of the tappets will change somewhat, quite quickly, say in 150 to 200 miles. And so on a fresh head I always check the tappets at approx. 200 miles, then again at 500 miles but unless any have reduced to nearly zero I do not adjust. Then I check and adjust them at approx. 1K miles. I then check them every year (3 to 5K miles for me) but only adjust when required, which is normally about three to five years for me.
Checking the tappets is easy, it’s adjusting them that is a little harder to do.

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PostPost by: englishmaninwales » Thu Jan 26, 2023 6:35 pm

When I found the timing chain tension causing No1 exhaust clearance to change I’d assumed it was due to camshaft and/or bearing wear.

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