- Sep 7, 2015
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I have looked closely into this head swap. I have a 500 and a 425 side by side with both heads off. It looks like it could work. I did a lot of research and could not find out why it wouldn't work.Aside from the cooling differences, which may or may not be major issue, just a rough estimate with a comp calc going from 96cc heads with 8.2:1 comp to 76cc would be more like a little under 10:1.... Very rough measurement.
IF it did work out with cooling ports and whatnot,
Assuming your 76cc heads are un-shrouded to the 4.08" bore and are now 80 cc, I estimate you would be around 10.25 to 1. this assumes the stock 425 heads are 108 cc, as found before on this forum and not 96 cc as listed. The 425 chamber is the same size as the 76 cc heads but there is a very slight core shift (up or down) between the two heads maybe 1/16" shift or so. Using the stock 500 head gasket (or 425 head gasket, can't remember which) will put the fire ring of the head gasket pretty close over the edge of the cylinder a little like a 1/16" so I was thinking the head gasket could be shifted a 1/32" to help center it in the chamber by elongating the head bolt holes on the gasket and spray tack the head gasket centered to the head chamber before assembly. The 425 ports are the same size as the 76cc heads, but the 425 heads have the bad sharp 90 degree bend on the intake right under the valve seat. The exhaust ports in the bowl look a little smaller on the 425, but I didn't measure them. The coolant passage alignment is really a non-issue. You can drill steam holes in the 76 cc head that line up with the 425 block by using the 425 gasket as a template over the 76 cc head. If you ever put the 76 cc heads back onto a 500, a new 500 head gasket would cover the drilled holes, so I drilled mine out. If you look at the 425 heads and block there are open water passages to connect all the cylinders together and the "steam holes" don't really look necessary since there is no trapped water like on a Chevy 400. I would say this swap would work if the fire ring of the head gasket doesn't end up partially exposed over the 425 cylinder wall causing a pre-ignition or a sealing issue. What you can't do is un-shroud a 76 cc head to the larger 4.30" cylinder of a 472 or 500 and then expect it to work on a 425 with the smaller 4.08" bore. The chamber takes it right up to the edge of the 4.08" bore.
I can take some pictures if you are interested.