9 replies to this topic

[adam25]

How much can be skimmed off of a 998 head safely without any valve/piston contact?

[Bungle]

has it been skimmed before and what compression ratio are you after ?

[Ethel]

The problem would be making the head face too thin to hold a gasket. By the time the valves reach full lift the piston will be well out of the way - assuming the timing chain never snaps. Cylinder heads are sand cast so no 2 will ever be the same to give a definitive answer.

[adam25]

The problem would be making the head face too thin to hold a gasket. By the time the valves reach full lift the piston will be well out of the way - assuming the timing chain never snaps. Cylinder heads are sand cast so no 2 will ever be the same to give a definitive answer.

Do you mean there won't be enough threads on the head bolts to compress the gasket?

Edited by adam25, 02 January 2008 - 07:29 PM.

[Ethel]

Nope - those threads are in the block in any case...

A cylinder head is largely hollow due to the cooling water passages. Make the bottom face too thin and it can flex: Imagine bolting a strip of sheet metal on to a flat surface by each end compared to using a chunky bit of bar.

[adam25]

So what can i skim off my head if its standard roughly, i want the compression as high as poss, its for a race engine.

[Ethel]

If you're building a race engine buying the Vizard book would get you the best power increase for your money. Just checked my copy: 3/16ths would cause problems but there should be enough metal to meet the needs of a 998 running 12:1 CR which could well be too high for crappy unleaded anyway.

[adam25]

Does anyone know the measurement for a standard mini head from the headgasket face to the rocker cover face so i know whether the ones i have have been skimmed or not?

[Big_Adam]

Right,

Measure down the oil hole, then mesure the thickness of the head.

Take the head thickness and take away oil hole depth.

THAT figure is the amount left on the head, if you remove this amount you WILL bust into the oil way on the bottom of the head.

So leave like 1-2mm and you're fine.

Besides, you need to work on ratio, not just lumping off amounts. Too high and the car will run funny.

[minimender]

Does anyone know the measurement for a standard mini head from the headgasket face to the rocker cover face so i know whether the ones i have have been skimmed or not?

Depth of a standard cylinder head is 69.9mm approx