Everything I have ever read, and every number I have used for Cometics, is that their specified thickness IS the compressed thickness. The uncompressed thickness all over the map due the multi-layer make up of these gaskets, plus the embossed sealing ridges. One example of an actual measurement done in the field:Bob, is Cometic Gasket's part listings for the head gaskets the uncompressed height or compressed height?
We have now been able to get accurate measurements and volumes from this engine "as built", and here are the results, as summarized in the attached table (input and calculated results from the html calculation file provided by Port City Engines) and related photos:Definitely get all your hard numbers in place first.
-calculated swept volume (actual bore x stroke)
-combustion chamber volume (using the actual plugs run in the engine)
-deck height (positive or negative)
-top ring land volume (usually about 1 CC but easily calculated)
-compressed head gasket ID and thickness
This is really the only accurate way to get the true compression ratio.
Why (aside from using the later non-cork version)? We are milling off most of the piston domes off, which achieves the net combustion chamber volume (and SCR) desired.I think you might want to include a different gasket as part of your solution, unless you make the pistons flat tops.
Bob, on the topic of head gaskets, could you please provide advice as to what size of head gasket to purchase?A standard 1.9 gasket usually specs out to .039” new, and .031” compressed with a 94.5 mm ID.
You don’t need a 2.0 head gasket for a 94 mm bore.Bob, on the topic of head gaskets, could you please provide advice as to what size of head gasket to purchase?
The cylinder bores on the engine are 94 mm, so exactly in the middle of the 1.9 (93 mm) and 2.0/2.2/2.4 (95 mm) cylinder size. The head gasket that was in this engine was a Fel-Pro, with a measured ID of 95.1 mm. Visually, it appears that the sealing ring is JUST barely larger than the cylinders. It was the 10-bolt style with the cork spacer gasket at the front, which was incorrect, as this engine has a 12-bolt style chain case, negating the cork gasket. Clearly it sealed the cylinders, even at the elevated compression pressures this engine created. Perhaps not so much the high pressure oil passage, due to the cork gasket, as evidenced by the oil found in the coolant.
My inclination is to purchase a 2.0 (95 mm ID) head gasket, to ensure sufficient diameter for the 94 mm cylinders. Or is a 12-bolt (non-cork) 1.9 head gasket a better choice, as it provides less lost volume around the edges of the cylinder?
I will also ask Gil what head gasket he suggests, based on the actual ID of his head gaskets.