Is it worth the effort

[steveww]

I have just checked the valve clearance of the twinc and No 3 is one thou tighter than spec. Should I re-shim it or just leave it

[garyeanderson]

Intake ore exhaust and what is the clearance measured?

Gary

[steveww]

No 3 exhaust valve is just one thou tighter than spec. Perhaps I am just being lazy here? May be I should pull the camshaft and reshim.

[mikealdren]

It's a while since my +2 was on the road but my memory is that they can move a bit anyway after you have set them and +/- a thou is ok. After all it was presumably running reasonably before you set it and I bet they weren't all exact then!

[poiuyt]

I had a tappet become chewed up by the cam when the gap went away. Also found all of the other intake valves tight.

I'd suggest keeping them at the middle or high end of the gap range because the gap will close as the valve pounds into the head.

Steve B.

[1964 S1]

Doesn't the low side of the cam prevent the valve from pounding into the head? On a pushrod engine, maybe that could happen?
I'd leave it alone and drive hard if runs good.

[rgh0]

Steve

1 thou tight is probably not an immediate problem but it needs watching and at 2 thou I would be wanting to change it. If I had the engine out I would restore all clearances within spec.

In general I run the exhaust from middle to top of range and inlets from middle to bottom of range. This is because the exhaust runs much hotter and tend to move faster in eroding the valve seat and valve and thus close up their gaps faster. Tighter clearances on the inlet also help breathing by maximising lift and duration of the inlet valve opening and this is not so critical on the exhaust for engine performance.

Rohan

[steveww]

If it is worh doing it is worth doing properly.

It does not take that long to reshim so I will pull the cam and do the job properly. Better to spend some time now than a lot of money later should the piston try to close the valve.

[denicholls2]

No, the low side of the cam does not hold the valve open. Unless, of course, the specified gap is not available to let it rest properly against the seat. When properly adjusted, the cam only touches the tappet within the "duration" period of the cam lobe.

No, a tight valve's issue is not hitting the piston. It is:

1. Without a proper gap, the valve risks not closing fully. Not only does this cause a loss of compression, but it prevents heat transfer from the valve to the head through the seat and therefore risks burning the valve.

2. Proper lubrication of the cam and tappet is more of an issue with tight valves. This may be of greater concern now that certain additives are no longer present in API oils.

[Dag-Henning]

No 3 exhaust valve is just one thou tighter than spec. Perhaps I am just being lazy here? May be I should pull the camshaft and reshim.

- if measured on cold engine, I would try again after warming up to working temp. You may find your one thou is gone..........Believe the clearence at work temp is the important one.

Dag

[rgh0]

Dag

Clearances as specified are measured cold. In a running twin cam the inlet valve is keep cool by the incoming fuel air charge and grows less than the warm alloy head so the cam moves away from the valve stem and the inlet clearance increases a little. The exhaust valve however runs much hotter than the inlet and it grows faster than the head so the valve moves closer to the cam and the clearance closes up a little. This is why the specified exhaust clearances and larger than the inlets as in operation the move towards each other to a similar operating clearance at around 8 thou.

Impossible to measure actual operating hot clearances as by the time you shut the engine down and remove the cam cover the heat soak from the head to the valves has equalised all the temperatures and you no longer have the running set up of a cold inlet valve a warm head and a hot exhaust valve.

regards
Rohan

[Dag-Henning]

- will not argue with that.....

Dag

[ggrenz]

I read that early Brit engines ( '67 & older?) have soft valve seats that unleaded gas tends to erode. My '67 S3 #! & 2 intakes are now (as of Mar09) .0025" or so, & have not checked clearances for maybe 15 yrs and 10k miles. I believe they were within spec then. In Tim Engle's 1/30/2006 post he suggests "Target the clearances to the top of the range... 0.012" for exhausts
(0.010 - 0.012") and 0.007" for intakes (0.005 - 0.007")" ........ My owners manual says .006-.007 for the exhaust. What should exhaust clearances be?
With the long storage/down time, (9 years) one problem I have is the engine spitting back through the intake every few minutes. Runs pretty strong otherwise. Any suggestions?
Also, my current valve shims look like small "hats." Should they? or should they look like flat washers without a hole, sounds strange to ask, but I used to work on my Alfas circa 1965-1970 and may have used/interchanged the wrong shims in the Elan.

[CBUEB1771]

Also, my current valve shims look like small "hats." Should they? or should they look like flat washers without a hole, sounds strange to ask, but I used to work on my Alfas circa 1965-1970 and may have used/interchanged the wrong shims in the Elan.

The "small hats" shims are probably fine. Standard Lotus shims are simple flat discs. Cosworth and some other suppliers produce the "hat" configuration. The Cosworth "hat" shims are available in a slightly wider ranges of thicknesses. I have a mix of both.

[AussieJohn]

Rohan, does the fact that the valve clearances change when running mean that if you try to set the engine up with offset dowels to the spec as per the manual then this will be wrong because the exhaust cam duration will be less than stated in the manual because of the larger clearance? I am now getting confused! cheers, John