Cometic makes Custom Gaskets

[silas]

Just ordered a custom base gasket from Cometic. Quick and painless phone call. The technician that I spoke with was super helpful and knowledgeable.

Cometic Gasket, Inc.
8090 Auburn Road
Concord, OH  44077
(800) 752-9850   Call Toll Free
(440) 354-0777   Order / Tech Line
http://cometic.com

I got mine in copper at .027 thickness to make up for the .014 that I just had shaved off the top of the cylinder block. They offer spring steel and fiber as well, but they jump from .020 to .030. Only the copper was available in .027. Figured I'd be better off with .003 more clearance than .004 less.

They can cut to any overbore size as well as stock 1100 or 1200. There are .005, .010, and .020 stock gauges that you can order online, otherwise, just give them a call.

No minimum quantity too.

Turnaround time is 3-4 days and they ship via UPS. I'll update this when mine arrives.

[racerrad8]

Silas,

The copper base gasket is a great tool when building an engine for optimum performance where you need to adjust the deck height.

With that said, copper is a poor choice for the base gasket as they leak. I used to use them a lot in the engine work I did and the leakage issue was always the problem.

I tried many different sealers to try and help the sealing, but never found anything acceptable.

Then about 5 years ago Yamaha introduced their embossed coated base gasket in the .010" thickness and I have never looked back. I use these in conjunction with the copper gaskets when I need that extra .005". I can sandwich the .005 copper gasket between two of the coated gaskets and no leaks.

If I was you, I would install two of the .010" base gaskets and run it. You can safely remove .020" from a stock cylinder without having any problems. One of the most common upgrades many people do to their race engines is to change out the base gasket from .020 to .010 and that increases the compression ratio.

You can drop the cylinder onto the case without a base gasket and measure the piston height in the cylinder and that will determine the maximum amount of base gasket you can remove.

My goal is to not hi-jack the thread, but the copper base gasket for the FJ is a poor choice. I am a stock Cometic dealer, but the copper gaskets are not something I am willing to stock as they are just too problematic.

Randy - RPM

[WestOzXJR]

....You can drop the cylinder onto the case without a base gasket and measure the piston height in the cylinder and that will determine the maximum amount of base gasket you can remove....

A couple of questions about calculating the piston to head clearance, if anyone has the answers...

1) What thickness do you assume for a compressed Cometic head gasket? (I've not yet dismantled an FJ engine that utilizes these to be able to measure).

2) How close are people setting this piston to head clearance on their FJ engines?

[AustinFJ]

In the Ducati world, we call that "squish", and we set it at ~1mm.    We do this by using a crush-able material (certain plastic pieces or some solder) placed on the piston head, usually around the outer rim, torque down the head, attach cams/cam chain with tension,  and rotate through TDC once or twice.  Dis-assemble and measure the material.  With HC pistons (crowns to take up combustion space), we sometimes use (un-chewed) chewing gum and also check valve/head clearance at TDC. 
All I'm saying is for some engines, this setting can make the difference between a screamer , a dog, and a pile of junk.  Had some folks use 0.85mm and found that rod stretch and heat at high rpm was a bad combination.  It is AMAZING the damage that can happen when a piston just kisses the head my a hundredth of a millimeter...at speed...


Do these engines respond well to properly set 'squish'? 

[andyb]

Stock headgasket is 0.030".  Cometic comes in a bunch of thicknesses, depending on what you order.

Yes, setting the squish is appropriate anytime you're working on an engine.  Just be warned that you are also moving the piston/valve relationship as well.

[AustinFJ]

Understood on the valve timing issue when setting squish, though it is more a timing issue than a clearance issue, since it only effects timing by a few degrees at most. 
But the question still remains...

What is the optimal value to set squish to for these engines?

[silas]

More invaluable info. Thanks guys.

I *think* the tech at Cometic advised that the stock base is .010, in which case that's a FAIL on his part. He also said that .010 extra won't make a major difference in compression. Throwing that into question now too.

No major harm though if I've got the room to make a gasket sandwich following Randy's advice. I'll measure my actual height to be certain. Gonna have the head and block sandblasted and washed by a rebuild shop and get everything together next week.

[WestOzXJR]

The reason for adjusting squish is that to set the value tight without the piston hitting the head is to force or "squish" this dead band of air/fuel mixture towards the centre of the chamber where it can do some useful work, as this band around the outer edge is too far away from the initiation point of flame propagation to produce a useful "push" - by the time the flame "gets there" the piston has already begun downward travel.

Where this has been tested on the dyno on various engines it shows an improvement in power of about up to 5% as compared between an engine set at a nominal value and one where the dimension is optimized. It is "free" horsepower with no trade-off with the added advantage that less heat is transferred into the upper cylinder wall and that is why a good engine builder will pay particular attention to this dimension.