Here is my scenerio. Last winter I built a new NA 565. I targeted and achieved a static compression ratio of 10 to 1. I am using ,040" Cometic headgaskets, have 118 cc heads, +4 cc domes on the pistons. All worked out real well with the new power.
Now to the problem. I am stuck to only being able to fuel up at local fuel docks. Jugging, octane boost, etc is just not an option. I have to use whatever fuel is available on the water. In the 20 + years I have been boating here, 93 octane was always available. Unforunately, not anymore. At least not anywhere local or practical for me. I am stuck using 89 octane. I am concerned that running 10 to 1 on 89 octane will cause detonation, so for this season I am runing the engine with the timing set back a bit and also am running only 1/2 to 3/4 throttle. So far, frequent plug checks show that all is well, no detonation is evident. This winter I want to knock the compression down to 9 1/4. I have run 9 1/4 on 89 octane on a previous engine, successfully. After doing the math, I calculate I would need a .095 thick headgasket to achieve my goal of 9 1/4 compression. I have never built an engine with that thick of a headgasket. My concern is will the gasket hold or possibly blow out? Cometic says no problem, they have done it before. What do you guys think? Anyone ever run a gasket that thick before?

You are going to lose you quench which might be a bigger problem. If you had the heads decked and wanted to make up the difference I wouldn't see a problem cause you would be keeping the same quench. But you are trying to bandade a potential problem (and I understand why)
Aluminum heads or steel, might not be so concerned with AL and running it colder? Can you just pull the heads and have the chambers reconfigured?
This is one I want to hear the experts chime in on.

I would need a .095 thick headgasket to achieve my goal of 9 1/4 compression.
OUCH !!!!!!!!!!!! anything over 40 just dont like it , good luck

How far are the pistons in the hole? I try to keep total quench to .040-.045. I have run as much as .060 in a pinch. Any piston to deck clearance you have, combined with an .095 gasket, increases your total quench to a dangerous level. The engine is probably equally, if not more, likely to suffer from detonation with a lower static compression ratio and an .095 gasket than it is at 10:1 with an .040 gasket. I'd go a different route.

IMO, I would yank the engine and swap the pistons. Rememebr that Cometic is in business to sell gaskets. They will do anything you ask them to do.

A customer of mine ran 10:1 steel head engines for several years on 87 octane before selling the boat last year. I had the timing set back to 32. I feel like he was very lucky though, as I had told him that I thought it would be crazy to run anything but 93 in them.

I would set the engine up to run 87.

I was thinking that as well.

My pistons are .005 in the hole. I didnt even think about losing quench area. I was solely thinking about killing some compression. I should have thought of that. Thanks for getting my head out of my a$$. Looks like a psiton swap is my best answer.

From your numbers, I come up w 10.18CR. I used 118 for the head volume. Most 118's I cc'ed come in around 121 or so. Plugging that in comes to 9.95CR. Going to flat tops w -3 dish, same head gasket, comes in at 9.49 with 121 heads. Open them to 125 that is not that much, your at 9.25CR. Good Luck

Seems like swapping heads for alum. would be cheaper and faster than pulling engine and replacing the pistons. You could probably keep your comp ratio and performance.

Vinny, we play with head gasket thickness all the time in our drag race engines, at the compression we are talking here a thick head gasket will work fine....we've run .060+ @ 15-1 comp..no issues...so at that comp you will be fine....summit has a good seclection of diff thicknesses..good luck Rob