Rebuilding a 425

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.

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.

 

I’m not sure of the purpose for using the 76cc heads. If it is for compression there are pistons that do it better and since valve size is restricted by bore size we don’t gain much there. If I had any intention of trying to make the 425 into a high performance motor I certainly would need more than boredom for incentive. Greg Surfas

 

Greg, the shop is a great place for "social distancing". The thread is a coil read. Thanks for sharing.
PJ

 

The purpose of using the 76cc head is higher compression and much better breathing.

 

Just for conversation's sake regarding the 425. The intake and head ports are the same size, and the 425 is a single plane intake. The extra tortuous path was designed in to speed up the intake charge velocity to reduce fuel puddling and since the head's intake ports and valves are sufficient for 500 inches that would make them roughly 16% "oversized" for this motor. Just like the big brother this is going to be an essentially stock motor with a (valve train) rev limit of about 4500 RPM.
As far as compression ratio, with the flat top pistons, zero deck and just enough off the heads to keep the chambers at 96cc, static compression ends up just about 9.47:1.
Greg Surfas

 

Did you cc the heads? Are they really 96 cc? I think Ted cc'd his at 108 cc. I'd cc mine if I had a graduated cylinder. Zero deck is the way to go and shaved heads to get the CR up.

My 425 engine was a runner I got for $100 and I am not going to rebuild it. I also bought a rebuilt 507 for a $100 from the same guy with 4 bent pushrods and 4 valves rusted in the guides. The cylinder walls and piston tops were perfect though. It has late model pistons but with 76 cc heads . Whoever rebuilt it didn't know what they were doing. I am going to use the 425 heads and give them a mild port and put them on 507 so I can at least make the 507 a driver. That leaves me with a well used 425 and a set of 76 cc heads to play with........... I will slap them together and see if they will work. The 76 cc heads will get a port job and un-shrouded to the 4.08 bore just for fun.

 

Crank back from the machine shop. They found it to be correct (10/20 undersized). This time I perhaps did a better job of putting it together. All mains torqued, thrust bearing checked (0.003 clearance) now waiting for the rings.