When it comes to head gaskets, the most often talked about features are material and overall compressed thickness. Both are critical specifications in the overall engine-building equation, but they are not the only specs you need to concern yourself with when it comes to head gaskets. There’s also the matter of gasket bore size vs. engine bore size. We know enough to know that we don’t know enough on the subject, so we enlisted the help of our friends over at Cometic Gasket.
The basic geometry of the cylinder head, head gasket, and the engine block is a sandwich of two components with a gasket as the filling. We all know that as you change the thickness of the gasket, you change the volume of the quench area, which in turn alters the static compression ratio. But, there is another variable that can alter the volume of the quench area – the diameter of the head gasket’s bore size.
“In a lot of cases, the minimum gasket bore size is dictated by the combustion chamber configuration, as opposed to the cylinder bore size,” — Jeff Evers, Cometic Gaskets
For that reason, the sizing of the gasket’s bore diameter needs to be tailored to the engine’s configuration as tightly as gasket options will allow. “In a lot of cases, the minimum gasket bore size is dictated by the combustion chamber configuration, as opposed to the cylinder bore size,” says Jeff Evers, Automotive Product Line Manager at Cometic Gasket. “Generally, we recommend .030 to .060 inch over the cylinder bore size. The bore embossment will need to be outside of the combustion chamber.”
Evers expands on the acceptable range, saying that the exact fit of the gasket bore size can really vary with the engine’s application. “For an NHRA Super Stocker, it could be as tight as the actual cylinder bore size, and for a performance street rebuild it will probably be around .040 inch.”
Too Much In Either Direction Is A Bad Thing
Much like a lot of things in an engine, too tight is more dangerous than too loose. If you arbitrarily size the gasket bore too large, then you are giving up compression, which isn’t immediately dangerous to your build. Go too tight, and you could run into problems. “If the piston sits out of the hole, then certainly mechanical interference would be a factor,” says Evers. “Even if the piston isn’t out of the hole, you can have hot spots in the case of an MLS gasket. Neither option is acceptable, obviously.”
Evers also points out that the type of gasket might dictate what’s an acceptable oversize. “With a composite gasket with a fire ring, there are fewer bore size options in comparison to our MLS gasket.” So, if you had a 4.030 bore, but the closest oversize was 4.100 bore, then .070 inch of clearance is what you’re going to have.
Doing The Math
So, while Cometic recommends shooting for a range of .030 to .060 inches, going excessively large really only hurts your compression ratio. How much? Well, let’s look at the math. We know the volume of a cylinder is its area ( π·radius2) times its height. And if you look closely, a head gasket’s bore is just a very, very oversquare cylinder. So let’s compare two hypothetical LS gaskets that are both.051-inch thick. Let’s assume we’re running an LS1 bore of 3.898, so we’d opt for Cometic’s 3.930 bore size. That offers .032 inch of gasket bore clearance. Perfect, right? (It should be, as that’s exactly what the 3.930 gasket was designed for.)
But what if the 3.930 gaskets are out of stock and you’re at a race, trying to get the engine back together for eliminations, and your buddy happens to have a set of .051 gaskets that have a 4.155 bore? Well, you know that it’s not going to be unsafe, but what’s it going to do to your compression ratio. By applying πr2·h, we can see.
3.930-bore, .051 compressed thickness gasket
(3.14159) (1.9652) (.051) = 0.619 cubic inches (or about 10.14cc)
4.155-bore, .051 compressed thickness gasket
(3.14159) (2.07752) (.051) = 0.692 cubic inches (or about 11.34cc)
If we plug that additional volume into a static compression ratio calculator, we see that by swapping to the larger bore gaskets on our hypothetical LS engine, it’s worth almost 0.2 points of compression, going from 11.28 to 11.11.
The next thing to consider, is that while the general trend holds true regardless of overall gasket thickness, a thicker gasket will see a bigger difference with bore change than a thinner one, as the overall numbers are larger, and farther apart. Similarly, a .030 change on a 3.700-inch bore will be less dramatic than a .030 change on a 4.250-inch bore.
Now, whether or not that change in compression is a big deal is totally up to you. In our hypothetical thrashing-to-make-next-round scenario, it’s probably not a big deal. If you’re just being impatient and trying to complete your build, you probably don’t want to leave that extra compression on the table.
At the end of the day, the decision always falls on you (and your engine builder) as to what you’re comfortable running. But, by being better informed, you can make better decisions. And really, that’s what we’re all about.