I hope your all sitting down for this - but I've actually found some time to work on my car


I figured it was about time to finish building the engine and one of the first jobs on the list was to check the valve to piston clearance

So I finished off the head, set the valve clearances, stuck it on with some plasticine on the pistons (using a spare gasket and belt)
then timed the cam with my dial gauge (which was nearly 8° off using the nearest tooth on the belt!) and turned it over a few times by hand, stripped the head back off and measured the plasticine at a min of 2mm clear

its only at this point that I realised I don't know what the minimum acceptable clearance should be for these engines and despite searching for some time now I can't find any conclusive answers

the figures I've see range from just over 1mm (0.040" ) to more than 2.5mm - and some just say as much as possible.....


Any thoughts ?

quote:

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Originally posted by mcerd1
I hope your all sitting down for this - but I've actually found some time to work on my car


I figured it was about time to finish building the engine and one of the first jobs on the list was to check the valve to piston clearance

So I finished off the head, set the valve clearances, stuck it on with some plasticine on the pistons (using a spare gasket and belt)
then timed the cam with my dial gauge (which was nearly 8° off using the nearest tooth on the belt!) and turned it over a few times by hand, stripped the head back off and measured the plasticine at a min of 2mm clear

its only at this point that I realised I don't know what the minimum acceptable clearance should be for these engines and despite searching for some time now I can't find any conclusive answers

the figures I've see range from just over 1mm (0.040" ) to more than 2.5mm - and some just say as much as possible.....


Any thoughts ?

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I'm more interested in how this clearance can reduce while running?

Assuming the valve springs are strong enough, are we talking metal expansion or valve float reducing the clearance. I'd have thought the engine block would expand more than the piston or rod given it's greater length so maybe the clearance would actually increase?

[Edited on 15/7/16 by Mr Whippy]

All I can say is that my engine gets abused, its still good after 5 years. Maybe your over thinking the situation.

Piston to deck clearance is quoted as 0.9mm safe 0.75 race this is to account for rod stretch at high revs
Therefore 2mm is more than enough
If you change camshaft for 1 with more lift you should check again
Full lift is after TDC so chasing the piston down the bore and it won't catch it
Lift at TDC is the place you need to worry about and you can increase the gap by retarding the cam timing

I'd say 2mm will be fine too, presuming valve springs are up to the job.

quote:

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Originally posted by Mr Whippy
I'm more interested in how this clearance can reduce while running?

Assuming the valve springs are strong enough, are we talking metal expansion or valve float reducing the clearance. I'd have thought the engine block would expand more than the piston or rod given it's greater length so maybe the clearance would actually increase?

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quote:

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Originally posted by Nickp
I'd say 2mm will be fine too, presuming valve springs are up to the job.

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According to the guide Burtons give out they specify 1.5mm or 60 thou minimum clearance, so you're well within tolerance. I used a Piper 285 with a Victor Reinz 1.3mm gasket and 40 thou head skim and got 1.8mm clearance when I did the plasticine test. Hope this helps 👍

cheers guys, it gives me a bit more confidence


next up - I'd better check the actual combustion chamber volumes and spark plug tip clearance too.....