Hey guys, thanks for taking the time to read this. I need some help building a olds 425. The pistons are broke, the crank is bad, and parts are exspinsive. I have a local guy who will sell me the crank, rods and .30 over pistons for a 455 motor for $425. Will this be a direct swap in a 425 block, or will there be clearence issues? Also would i just have to buy a 455 rebuild kit or will i have to mix and match bearings and stuff?

Not sure what year the 455 parts are from but the 425 block and "A" heads are 1965. With the .842" lifters, and 10.25/1 compression. What kind of compression will i be looking at with the 455 parts installed?

 

The 455 parts should fit into the 425 block, and you would need a 455 rebuild kit.

Since you have a '65 block it is likely a 45º cam bank angle (CBA) block. This means you would need a cam for the 45º engine vs. one for a typical 39º 455 block. Any of the cam companies should be able to grind one with whatever profile you want, you just have to let them know it's a 45º block.

I'm not sure what you are referring to about the compression - that will be dictated by the size of the dish in the pistons. This could be anywhere from huge soup bowls to flat tops. You gotta know what pistons they are so you know what the compression ratio will be.

 

Thanks for replying. I just got the parts this evening. the pistons are L2323 with 18cc dishes. im worried with these 455 pistons having a different compression distance, and using felpro headgasket instead of the stock headgaskets, I may have lower compression than I would like. also whats a good size cam to go with? I was thinking of something with around 280 duration and 500-520 lift. any suggestions?

 

Well, the 455 pistons have a different compression height than the 425 pistons, but the 455 rods are also a different length to match the pistons.

So what CR would you like? You can get different thickness head gaskets and / or mill the heads to alter the CR. It all depends upon what you are trying to accomplish.

 

Ahh, i didnt think about the rods making up for the compression distance. I was hoping for around 10.25:1 like the original 425. Im just not wanting to spend all this time and money rebuilding this Frankenstein BBO and then have 9:1 compression. Lol!
Any idea where to get the thinner head gaskets? I believe they were .017 of something.

 

Unfortunately you've probably picked the worst piston you could use in which to achieve your goals. That piston is shorter than most of the other aftermarket ones, and has a larger dish than most as well. That, coupled with the excessive deck you're already going to have, and you really need that thinner shim head gasket.
And as far as camshaft goes, you'll need to know what Springs you're going to use, if the guides have been cut, and so on in order to be able to use the lift that you mentioned. Other things need to be considered.

 

You can use the 7" 425 rods on the 455 crank if you buy 455 Pontiac pistons. It probably requires decking but its close. The longer rod moves peak power up a few hundred rpm and reduces cylinder side loading. You would need dished 455 Pontiac pistons as the 80cc Olds combustion chamber with a regular Pontiac flat top piston will give a very high CR.