Lotus Elan

Valve clearances

PostPost by: el torro » Fri Sep 21, 2018 10:37 am

The engine in my plus 2 is an earlier one 1968,it has been rebuilt by PO,modified for unleaded,workshop man gives spec for engines up to no -----, and after that I am confused what clearances to use
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PostPost by: alan.barker » Fri Sep 21, 2018 10:50 am

The later clearances and it was only the Exhaust that changed.
BUT if different Cams have been fitted follow the Camshaft Specialist e.g. Burton, Kent ,Piper etc :wink:
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PostPost by: el torro » Fri Sep 21, 2018 11:28 am

Ta,will have to find cam type fitted,should be stampeded on them ,got cam cover ready to come off ,ta
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PostPost by: nmauduit » Fri Sep 21, 2018 11:44 am

Miles writes in his book 6 thou +/-1 for inlet and 10 thou +/-1 for exhaust, unless you have a radical camshaft that would be a good starting point.

I should mention that for the last big valve head I upgraded with Q420 cams I opted to slightly larger inlet gaps, set at 8 (exhaust aiming at 10)
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PostPost by: rgh0 » Fri Sep 21, 2018 11:10 pm

Cams typically have a clearance take up ramp of 10 thou height. This ramp is a gradual slope that closes the clearance between the cam and the follower gently before the cam lift accelerates. If you have running clearances greater than the take up ramp you get the characteristic clicking as the contact between cam and follower occurs on the rapidly rising cam after the take up ramp.

The exhaust valve expand more than the head as it is heated by the hot exhaust gases so the clearance reduces in a running engine thus a cold setting of 10 thou gives a running clearance hot of around 5 to 7 thou. The inlet valve is cooled by the incoming fuel more and expands about the same amount as the head hence the 6 thou clearance cold and hot.

The different clearances Lotus specified in the early engines may have been due to different valve materials or may have been due to Lotus realizing they had them set to tight

cheers
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