Head Thickness

[William2]

If a head has a depth overall of 4.609" does that mean it is still useable and could it take another skimming if required?

[seniorchristo]

Factory specs for head thickness is as follows:

Small valve - 4.638 / 4.643
Big valve - 4.598 / 4.603

Max. permissible metal removal:

Small valve - .045
Big valve - .010

It appears you are within limits with either head.

[512BB]

QUOTE:Max. permissible metal removal:

Small valve - .045
Big valve - .010

It appears you are within limits with either head.

But in reality, both types of head can have a lot more removed than quoted by Lotus, with no ill effects. They just wanted to sell a new head.

Leslie

[rgh0]

QUOTE:Max. permissible metal removal:

Small valve - .045
Big valve - .010

It appears you are within limits with either head.

But in reality, both types of head can have a lot more removed than quoted by Lotus, with no ill effects. They just wanted to sell a new head.

Leslie

Yes I agree you can remove a lot more than the factory specifications.

cheers
Rohan

[seniorchristo]

As a general rule, how much is compression ratio changed for a given amount of material taken off the head? Is the higher compression ratio of the Sprint achieved just by milling the head?

[wbmoore]

What is the minimum thickness (non-factory spec.) after skimming for a completion head ?
And at the risk of changing topics... but still head related. How and where do you check the head for hardness ?

[rgh0]

What is the minimum thickness (non-factory spec.) after skimming for a completion head ?
And at the risk of changing topics... but still head related. How and where do you check the head for hardness ?

There is no real absolute limit but things just get harder and harder to accommodate the thinner it gets. Down to about 4.52 / 115mm inches, which is about where all the cast lettering on the underside disappears I have seen done.

As you approach this thickness you will have issues with the edge of the inlet valve seat coming close to the milled surface of the head face especially if you have larger 1.62 inlets in a competition head. The inlet valves edge will also be protruding above the head face but this is no issue in itself as long as you have adequate clearance pockets in the pistons. The head chamber may need increasing in size and any piston intruder reduced in height, though in a competition engine you will already be playing with these to get the compression ratio you wanted.

Around this thickness you will also come close to breaking through into the water jackets that sit between the exhaust port and the face of the head. Depending on the exact core locations you may need to do some repair welding in that area. At this point it is probably uneconomic versus sourcing a replacement and you risk softening the head if you have to do a lot of welding anyway!

Typically you want to measure hardness at half a dozen points on the top and bottom faces of the head. The rebound testers that most good machine shops have, need a smooth flat machined surface to work on. Measure along each side of the bottom face outside the fire ring location at a few locations and along the top face near the head bolts. Your looking for over 50 Brinell minimum, ideally up around 70. At 40 Brinell the head is definitely to soft.

cheers
Rohan

[rgh0]

As a general rule, how much is compression ratio changed for a given amount of material taken off the head? Is the higher compression ratio of the Sprint achieved just by milling the head?

40 thou / 1 mm milling on the head will increase compression ratio by a bit less than 1 unit. The increase from 9.5 to 10.3 compression ratio in the Sprint was achieved by just milling the head thinner by 1 mm / 40 from 4.64 inches to 4.60 inches as far as I am aware.

cheers
Rohan