Skimming heads for compression

[someguy360]

Hi All,

I'm doing a basic refresh of my spare EFI 304 in the new year ready to put in storage as my backup/hot swap engine. Before anyone asks this is a street engine that won't see much past 3500-4000RRM.


It's a reasonably low km engine (sub 100,000km, no ridges, bores look good etc). I'm going to replace the bottom end bearings, cam bearings, dingle ball hone, new cam, new lifters, rings, new valve springs and new gaskets and thats about it. While I'm getting the block dipped and cleaned at the machine shop I've been recommended to get the heads skimmed .030" to bump up the compression to suit the cam I'll be using (Crane 276).

Can anyone tell me what compression a .030 skim is likely to give me on it's own and also whether I'll need to get the intake manifold machined at the same time or whether I'll be safe at that amount?

Thanks in advance!
Merry Christmas

[COM081]

Think on standard bore and stroke .030' off the heads will raise comp by 0.7 or thereabouts.

As for manifold, my guess is you'll be OK FWIW but more learned folks may have a firmer view on that.

[Horaldic]

Provided the block and heads are original and untouched from factory spec then my estimate would be 0.5 to 0.8 raise in cr.

Stock they were 8.4:1 so provided the engine is in good nick it would make it around 9.

At stock compression they are great candidates for supercharging, of course you would want to either be brave or have inspected your bottom end beforehand.

Lots of these have lost some vim over time so, realistically you’re probably going to have lost a bit and will not get the full benefit of your skim.

With your intake you are really going to need to measure it up once assembled. I’ve had experiences where I’ve skimmed and needed to machine the intake and also the opposite.

I use engineers blue to ensure that the surfaces are going mate uniformly. Critical thing is to make sure it sits flush on both sides and is not cocked up on one or the other otherwise the gasket might initially give you a seal but then, down the road, you end up with dramas.There’s tolerances in all these parts and sometimes they work in your favour and other times they don’t. Like as not it will be fine but you do need to check.

[guy incognito]

If the piston is 8 miles down the bore you may as well use the thinnest head gasket you can find as well.

[EH179]

Taking .030" off the heads, will lower the chamber size from 64cc down to 59cc. Will you need to machine manifold? Most likely, but trial fit first.

You will still need to know PTD height, to accurately measure compression.

[immortality]

Anyone know what the dish volume on standard 304 pistons?

[Horaldic]

Without having recently measured one my memory is that the dish volume is 12ml.

I should add to this discussion and say that, given a choice, I wouldn’t use head skimming as a method to raise compression. Sure, if you need to skim a head for flatness then that will raise the compression and it is a sometimes welcome side effect. However, the changes in dimensions and lack of subsequent “meat” for doing work mean I keep skims to the minimum that produce flatness.

I’d prefer to steer people toward piston and rod changes changes when bumping compression; much nicer from an engineering standpoint.