Essex Mike

Essex Mike

Member
Hello all,

By any chance does someone on here know the amount the standard SD1 3.5 heads have got to be skimmed to take composite head gaskets to retain the original compression ratio...?

Ive read many different posts from good ole Google, and from what I read 30-40 thou should be skimmed, that seems to be a lot.

20 thou is 0.508mm
30 thou is 0.762mm
40 thou is 1.016mm

Tin gaskets compressed thickness 0.45mm /18 thou + Composite compressed thickness 1.0mm / 40 thou.

So am I right in assuming only 20 thou need to be skimmed...?
 
Essex Mike said:
Hello all,

By any chance does someone on here know the amount the standard SD1 3.5 heads have got to be skimmed to take composite head gaskets to retain the original compression ratio...?

Ive read many different posts from good ole Google, and from what I read 30-40 thou should be skimmed, that seems to be a lot.

20 thou is 0.508mm
30 thou is 0.762mm
40 thou is 1.016mm

Tin gaskets compressed thickness 0.45mm /18 thou + Composite compressed thickness 1.0mm / 40 thou.

So am I right in assuming only 20 thou need to be skimmed...?
Click to expand...
yes its just the difference in gasket thickness
 
Replaced the tin on mine - RRC 3.5EFi - last year with Elrings. The heads had around 8 thou taken off to true.
As far as performance is concerned I've noticed no difference whatsoever. Which on a vehicle with the aerodynamics of a garden shed isn't really a surprise.
 
8 thou, that sounds like just a normal refacing skim.

I think I will Have 20 thou 0.508mm taken off my heads as the composite gadget is 1/2mm thicker than the standard tin gasket, that should ensure my inlet manifold will still fit, snd my compression ratio will stay slmost yhe same.
 
Another question,

How can I tell if my heads have been skimmed previously by someone else, and I take 20 thou off when someoneelse has tsken 8 thou off, so unknowingly ive enddd up with a 28 thou skim...?
 
upload_2018-8-3_8-34-16.jpeg

the shiny dot in the chambers is a raised spot used to measure head face depth,theres a nominal size in the manual that tells you how much can or has been skimmed off
 
Ratae said:
Replaced the tin on mine - RRC 3.5EFi - last year with Elrings. The heads
As far as performance is concerned I've noticed no difference whatsoever. Which on a vehicle with the aerodynamics of a garden shed isn't really a surprise.
Click to expand...

I found the same after changing to composites, without any skimming as that was not needed. The slight drop in CR is no bad thing as 9.35:1 is probably a bit high for 95 unleaded anyway.
 
norseman said:
I found the same after changing to composites, without any skimming as that was not needed. The slight drop in CR is no bad thing as 9.35:1 is probably a bit high for 95 unleaded anyway.
Click to expand...
My engine is in a Land Rover, and I think my compression ratio is quite low anyway something like 8.13.1, I don't think I can afford to lose anymore...
 
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Essex Mike said:
My engine is in a Land Rover, and I think my compression ratio is quite low anyway something like 8.13.1, I don't think I can afford to lose anymore...
Click to expand...

'Oh I see, in that case you've got plenty of wiggle room when it comes to octane ratings ;)
 
Dunno if this is going to help anyone, but I discovered this whilst searching.

The composite type heads can be checked by taking these measurements:
Cyl-head-dims.png


I measured my tin gasket heads with a vernier and my heads were 1mm thicker.

Ive just bought some HRC2210 heads, which supposedly they are already factory skimmed to take composite head gaskets.
 
Some information taken for "How to power tune Rover V8 engines, states 40 thou is the difference between tin and composite heads...

So it seems I will be skimming 40 thou off them...
 

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Bitten the bullet, heads are in the machine shop having 40 tho (1mm) skimmed off them...

I will report back once I have them fitted...
 
I know it's been a few years since my last post, but here us an update...

DONT DO IT...!!!!

Skimming 20 thou off the head, and using a composite head gasket altered the compression ratio, and made the engine run very juicy, it was almost impossible to tune, I dread to think how it also altered the valve train.

So the long of the short, 20 thou is far to much to skim off, I now have a pair of useless scrap heads...

I've since gone in the total opposite direction, and fitted standard heads with a composite head gasket, and it runs sweet as a nut.
 
Essex Mike said:
I know it's been a few years since my last post, but here us an update...

DONT DO IT...!!!!

Skimming 20 thou off the head, and using a composite head gasket altered the compression ratio, and made the engine run very juicy, it was almost impossible to tune, I dread to think how it also altered the valve train.

So the long of the short, 20 thou is far to much to skim off, I now have a pair of useless scrap heads...

I've since gone in the total opposite direction, and fitted standard heads with a composite head gasket, and it runs sweet as a nut.
Click to expand...
It was a successful experiment, though. Glad you got it sorted.
 
Shop took 12 thou (0.3mm) off Otto's heads when we did the gaskets. Everything went back together fine, new tappets at bottom limit, old in the other side at top limit (had to do half set as there had obviously been some starvation or something and the left bank ones were all worn / dished / damaged).
Runs lovely, now we have also sorted the MAF sensor crud, IACV clag and duff plug leads. Transformed him, where before he was running as a 6 (blown between 2 and 4) and we didn't know til the coolant started to pressurize.
 
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