How flat does the head need to be?

[Paul Coleman]

I've just had my series 1 907 cylinder head checked for flatness and I'm told it dips in the middle by 0.0015" but I can't seem to find any figures for what is acceptable. Can anybody please tell me if this is okay or do I need to get the head skimmed?

Thanks, Paul.

[lotus4s]

1.5 thousandths is not much at all and should be in spec.

If it's 15 thousandths that is probably too much but I don't know what the specs are for your 907. It also depends on whether the distortion is longitudinal or transverse.

[ralphw]

Paul

0.0015" is pretty good. Warning however, flatness in a unclamped head is not unusual. Most used aluminium heads bow when they have been removed from the block. I have seen a number of occasions now where people have had their head skimmed to get it flat and bowed the top face causing the cams to be tight or oil leaks to appear. My supplier does about 1500 heads a year for me and we only skim to clean the face up. We take a lot of effort in the fixtures we make to clamp the head so the head to bock mating face is flat before skimming as this is how it will end up once it has been reclamped to the block. Never had any problems and I have seen some heads with distortion much greater than 0.0015". We generally shim 0.008" (0.2mm).

Cheers

Ralph

[Vulcan Grey]

Here is what Lotus has for the 910 and 920 heads. I would use this as a guide.

[ralphw]

Well done Travis, was trying to remember where to look for this. 0.0015" is pretty damned good considering the age of the car. must have had an easy life which is always encouraging for the current owner.

Cheers

Ralph

[Paul Coleman]

Thanks all for the replies. The full story to this is that my original head had been skimmed so many times that the valve seats had actually been skimmed as well, so I took a chance and bought another head off eBay for £20 that just happened to be on there at the time and was local. It had sat in somebody's garage for about 20 years and by the looks of it had never even been on a car - so for once I may have had some luck?

Cheers, Paul.

[molemot]

Paul, you must be living right....jammy beggar!!

[Paul Coleman]

Yep, the sun always shines on the righteous

Paul,.