Home away from home
From Melbourne Australia (and likely under the car)
The valves will be slightly cloer to the block and pistons, but their height 'within' the head won't be affected by the surface grind of the head. You will need to re-set the tappet clearances since the head has 'effectively' moved closer to the block. It will affect rocker geometry to some small extent, but nothing to worry about unless you were taking off major amounts of material (which I gather i not the case).
If you were hypothetically running a really serious cam, then you might want to put clay on the piston crowns, and re-fit the head with a dummy head gasket, or washers the same thickness as the compressed head gasket, and turn the motor over (by hand) and see just how close it currently comes to piston to valve contact (and the 'risk' areas are usually around or between 10 degrees either side of top dead centre, as the exhaust is still closing as the piston heads for tdc, then the intake valve is opening as the piston still travels up but then opens more and more as the piston starts travelling downward. And so forth.
If it is currently _very_ close to making contact, you have to work out whether or not a surface skim might put you into a danger area. There's some debate as far as to how close is safe or not (remember you are checking at room temp) - but if there is something like 100 thou, you'll be nowhere near danger with a minimal head skim just to clean it up.
If it is getting too close for comfort, then you'd need to enlarge (or create) valve reliefs on the piston crown.
It's all a bit of an unknown without knowing how severe the cam timing/lift is, and the piston crown shapes etc.
I just re-read it and you mention fitting new valves. Sight unseen (by me at least) I'm assuming they might be slightly larger diameter than the previous ones? In which case you'll haqve to check clearances even if you didn't skim the head surface. It's also possible the new valves will sit 'lower' in the head, since they'll 'seat' at a different point than the previous smaller diameter ones will. If they sit lower in the head (when viewed as if the head was fitted, though most head porters talk about high and low with the head upside down as that's how they work on the seats, so to speak) then that would alter the valve spring height. It'd actually (if the collet grooves are in the same place on the old and new valves - and that isn't a certainty) increase valve spring pressure a little. If the valves sit higher in the head, it would reduce it.
So the short story of all that - if you are just skimming the head, no change to valve spring intalled height will occur, so no concerns. IF you are putting in new (esp slightly larger) valves, you are going to have to physically check for valve to piston clearance AND also valve spring installed height and shimming may or may not be required. No way anyone could answer that one with any certainty, we'd all have to measure it :)
Posted on: 2012/8/18 7:27