Deck Height 1mm - base gasket thickness

[P911S]

What are the advantages/disadvantages of using a 0.5mm vs 0.25mm cylinder base gasket in terms of deck height? I know stock is 0.25mm but many builders like 0.5mm.

Anything else I should know regarding thickness on the base gaskets?

[Flat6pac]

Thicker equals lower compression..
Bruce

[PeteKz]

Thicker equals worse combustion chamber squish and less efficient burn.

That said, the important number is the clearance between the piston and the head. Use base shims to get that in the right range. I'm a minority, but I want as tight a clearance as I can get without contact. Generally around .030"/.75mm. It takes a lot of test assembling and measuring to get that accurately, so engine builders will not do that unless you specify it.

[69 911s]

Thicker may be necessary if your machinist took .010" off of your cylinder spigots when he leveled them or if the case halves themselves were machined after align boring. There you may need thicker base gaskets just to have the correct piston to head clearance.

[Jeff Alton]

Your post is a touch confusing.

You have measured 1mm deck height? or you are trying to achieve it?

Cheers

[stownsen914]

I've not heard of one gasket being better than the other. You pick the thickness that meets your need, to achieve the piston to head clearance you're aiming for. Every engine is a little different, especially if the heads have been cut, case decked, etc. First decide what piston to head clearance you are aiming for, e.g. .040", .050", etc., and then a mockup assembly tells you which thickness base gasket you will need to achieve that.

[Tippy]

Originally Posted by PeteKz

Thicker equals worse combustion chamber squish and less efficient burn.

That said, the important number is the clearance between the piston and the head. Use base shims to get that in the right range. I'm a minority, but I want as tight a clearance as I can get without contact. Generally around .030"/.75mm. It takes a lot of test assembling and measuring to get that accurately, so engine builders will not do that unless you specify it.

I had my heads cut and ran the original thickness and it appeared the pistons may have been touching the heads.

During refresh, I ran thicker and reduced squish. I swear squish made better performance everywhere.

[PeteKz]

Originally Posted by Tippy

I had my heads cut and ran the original thickness and it appeared the pistons may have been touching the heads.

During refresh, I ran thicker and reduced squish. I swear squish made better performance everywhere.

It probably did. I can't demonstrate that with dyno charts for Porsche air-cooled engines, but on American V8 engines I've built it makes a significant difference. The Porsche heads are open "hemi" chambers, which really suck for chamber efficiency. The pistons have big domes on them to poke up into the head to increase CR and to push the charge over into a smaller space near the spark plug. Henry at SuperTec and several other engine builders have come up with better shaped pistons and heads to make the chamber more compact and efficient. Those are custom jobs though.

If that were my engine, I would have left the shims the same, and simply clearanced a few thou off the piston where I thought I saw contact with a hand file. Then I would get the just necessary clearance and better squish.

For data collection purposes: How much was cut off your heads?

[Tippy]

I wished I could remember, but “cgarr” did them over a decade ago. Sorry.

[PeteKz]

When heads are cut, the machine shop will often stamp something like ".25mm" or ".010" on the heads.