Cylinder Base Gasket Omit

[justinp71]

Has anyone had experience with not running the cylinder base gasket with an aluminum case? Is it worth it?

Also has anyone used Curil T on the new Black Cylinder Base gaskets?

[Mo_Gearhead]

I'm pretty new here, just finished my first engine rebuild. Why would you intentionally omit those gaskets anyway? Trying to gain that thickness in deck height for a slight compression raise??...or what?

[justinp71]

You can raise compression from 9.3:1 to ~9.6:1. The Euro motors had 9.8:1 and they have 20 more hp.

I don't think you'll get 20hp but something and in wayne's book it says that you can leave it out if you have adequete clearance.

[ChrisBennet]

Before you consider it, do some a search on "deck height".
-Chris

[justinp71]

Agreed, but if it is not a wise idea to omit the gasket, then there is no reason to check the deck height.

That brings up another point... can you re-use the cylinder head to cylinder gaskets once you torq the heads down? Without running the motor of course, for example if you were checking your deck height.

[Jeff Alton]

Why would you not check the deck height? It is simple. I assume, from your aluminum case and head gaskets, that you have a 3.0 motor. If you have CIS pistons you can check it pretty accuratetly with a depth micrometer or even vernier caliper....

Cheers

[ChrisBennet]

Originally posted by justinp71
Agreed, but if it is not a wise idea to omit the gasket, then there is no reason to check the deck height.

That brings up another point... can you re-use the cylinder head to cylinder gaskets once you torq the heads down? Without running the motor of course, for example if you were checking your deck height.

I was hoping you'd do a search to save me some typing.

Ideally you want a deck height (the distance between the head to cylinder, measured at the very edge of the piston) of .035-.040" - a smaller distance than I've measured on factory motors.

If omitting the base gasket drops you below that figure, you risk piston to head contact. Remember, the pistons aren't always being pressed down, sometimes they are being thrown away from the crank.

964's and 993's don't have a base gasket. They have a sometimes ineffective O-ring instead. I use ThreeBond 1204 sealant to augment these O-ringed cylinders based on Henry's advice. If I was omitting the base gasket I would use 1204 instead of Curil-T. (Henry sells the hard to find ThreeBond 1204 if Pelican doesn't have it.)

The head gaskets don't need to be installed when doing a test fit. They do not change the dimensions of the motor. I wouldn't reuse them but that's just my gut feeling - I've never reused them so I don't know if they'd leak.

As a rule, I would never use deck height to change compression on a 911 motor. Set the deck height to where you want it and change the compression some other way. IMO, there is no "free" horsepower to be gained by raising the compression by shaving the heads or trimming cylinders, etc. It's a lot of work ('cause now you have to shave the chainboxes and do something with the extra chain slack) for little gain.
-Chris

[ChrisBennet]

Originally posted by justinp71
You can raise compression from 9.3:1 to ~9.6:1. The Euro motors had 9.8:1 and they have 20 more hp.

I don't think you'll get 20hp but something and in wayne's book it says that you can leave it out if you have adequete clearance.

Where do you get those compression increase figures?
Euro "9.8:1" motors probably have a true compression (as measured) closer to 9.5:1 I bet. The factory compression numbers are always larger than when I measure them. (It's not just me either.)

-Chris

[304065]

I don't know the dome height of a 9,3:1 SC, but here's the idea.

Eliminating the base gasket to raise compression is a bad idea. Certain professionals may be able to do so, but for the home rebuilder, I would advise against it.

[justinp71]

Thanks for the information guys.

I did a search on it and no one actually gave hard evidence to whether it is worth doing.