Re: the consequences of running a head gasket that doesn't precisely match the bore/combustion chamber.

Head gaskets are usually listed with their compressed thickness in decimal inches and volume in cubic centimetres (cc's).  I say "compressed" because once the head is torqued down, the engine heat cycled, and the cylinder head fasteners retorqued, the head gaskets will usually compress and thin out compared to the out-of-the-box condition.  Some gasket (materials) are designed to compress more than others.

The volume is calculated as the area of the combustion chamber opening times the compressed thickness.  This would be straight forward if the opening were round.  Fortunately the manufacturer provides this information for us.

As you have eluded to in your question, you are aware that head gasket volume does indeed play a role in compression ratio.  Static Compression Ratio is the measured volume of the combustion chamber, cylinder, et al when the piston is at Bottom Dead Center (BDC) divided by the volume while at Top Dead Center (TDC).

Compression Ratio is important to performance because the combusting air/fuel mixture will expand around 3-5x the initial compressed volume.  What that means is that, with more compression, more force will be exerted on the pistons, more output (HP) and greater efficiency result.

Of course with more compression comes the possibility of exceeding the detonation limits of the fuel.

What can be done to help our engines push the limits of detonation resistance?  Specifically pertaining to head gaskets, along with the entire combination of parts, is the quench distance.  This is the distance between the top of the piston and the bottom (flat) surface of the combustion chamber, usually the head gasket surface of the cylinder head. Every effort should be made, in a naturally aspirated (NA) engine, to get this dimension below .055 inches.  Typically this is accomplished by decking the block and using the head gaskets to establish final clearance.  For example, zero-decking the the block and using a .046" thick gasket to arrive at a .046 quench distance.

I have seen and felt, first hand, the positive (and negative) affect of tight quench distance.  It is entirely possible to actually increase compression and, at the same time, increase resistance to detonation.  A tight quench distance (<.055") will push the air/fuel mixture into the volume of the combustion chamber.  Mixture motion is enhanced, facilitating a faster more complete burn.  The narrow cross-section of a tight quench distance doesn't promote preignition but allows a controlled burn originating from the combustion chamber.

The next issue is crevice volume.  To be honest, I hadn't thought much about this, beyond contact area of the sealing surfaces of the gaskets and reduced compression numbers, until I attended an engine building seminar.  One of the topics was crevice volume.

The issue is that liquid fuel is forced into the crevice volume.  This effectively leans the Air Fuel Ratio (AFR).  Then this liquid fuel burns at a less opportune time, possibly detonating.

Crevice volume is created above the top ring towards the combustion chamber between the top ring land and the cylinder wall, in sharp corners of deep valve notches, and in between the cylinder head and block deck.

The problem specifically related to head gaskets is that there are so many bore possibilities for each engine family, the gasket manufacturers would have to produce an infinite number of bore sizes.  Not economically viable.

Ideally you want a perfectly cylindrical shaped cylinder all the way up to the combustion chamber to reduce crevice volume.  This is not probable because of combustion chamber shape relative to bore size.  The chamber will typically overhang the bore diameter below the valves (this is part of why larger bore blocks allow any given head to flow better).  Unless you are running custom gaskets, like Cometic, you simply don't have the options.

A quick search on Summit Racing lists the above Fel Pro 17068 as having a 4.500" bore, .039 compressed thickness, producing a 10.300cc volume.  Not much selection (except for Cometic MLS or SCE copper) in thickness and bore sizes.

Hope this answers some of your questions.  If I knew more about your build, application, and at what stage of completion you are at, maybe I could help further.

Cheers,

Joe

The parts you have are capable of supporting 500 hp.  Not my first choice in rods and pistons but up to the job nonetheless. Incidentally, the valve pockets in your pistons might not accommodate the big intake valves in yor head, so be sure to check piston-to-valve clearance.  (Actual clearance will depend on deck height, cam, cam timing, etc.)  Lastly, you can't have a 1.9" exhaust valve in your heads with those intake valves; 172" is more like it.