I was given a 1966 425 and when I tore into it I was hoping it was a steel crank 425 high compression. From what I have found someone put a 455 crank and rods, pistons. I know this is mostly the same block as the 455, it is a D block. My question, and I searched first is I have heard of the need to clearance the block for the longer stroke but I can't find any info on where this needs to be done? I have the small diameter lifters and it appears to be a 45 degree block. I noticed it looks like a recent rebuild, no cylinder ridge and the lifters came out easy, no sludge. It broke a piston and I am wondering if it may be related to the clearance not having been done? The bearings all look good to me. I will be checking all the clearances before proceeding. Oh, I also have the big valve "B" heads. I will try to attach some engine pics and a pic of what the engine is going in. I am putting it in place of a running '74 Olds 350 with a Turbo 400.

 

I also was wondering if the big valve heads sacrifice low end torque and throttle respone. My experience with other engines tell me that depends mostly on the cam and can't see these valves causing the loss of power in any way. Am I wrong about them causing a loss of low end torque and throttle response?

 

The ring land took a dump and that would have nothing to do with clearance issues.
As far as the valves go, these things make soooo much torque that it most likely wouldn't be noticed.

 

That is exactly what I thought. The big valves should make more power overall. I am mainly building a street motor and will probably upgrade to better (higher compression) forged pistons now. Before I took it apart everything looked like it had plenty of room to me. Any idea what caused the piston failure? Probably just weak cast pistons, right? I did notice that piston's pin seemed tight compared to the other 7. I don't know why but the bore that piston came out of is not scored or scratched, I guess I just got lucky for once.

 

Hmm, reminds me of my 425 piston...

 

That is nasty looking I can't wait to save up and rebuild it for my '42 Chevy. I think it will make it alot quicker.

 

Here's an old thread that may cover the clearance issue you asked about.

https://classicoldsmobile.com/forums...425-block.html

I would stay with the 455 internals that you have. But if you decide you'd rather have a 425 I've got a couple 1966 Toro 425 engines available. John

 

Thanks for the link, I probably will run it with the 455 but if not I will keep that in mind.

 

I will say that the piston failed because of detonation. High compression and low grade gas.
If you do go with the forged pistons for the 455, be sure to use .005 piston the cyl wall clearance.

Gene

 

Thanks for the info. I got the engine from a young guy who doesn't know much about cars. I bet he ran bad/ low octane gas in it. The premium should be Ok if timed right, correct?

 

425 has a cold light for the ultra high compresion. It has to unlight before driveing hard. retard the timeing a little if useing low octain gas makes it burn hotter. Gas needs lead in it with old engines. premium leaded is best. Got three 425's nice power plant!

 

Good to know. Now I have to save up to go through it right, I want at least 400 horse and that sounds pretty easy to do.

 

I hated that 'cold' light. It was blinding at night. I have gauges now, no more idiot lights.

 

I hate all idiot lights, give me a real reading of what is going on.

 

I have a 425 with tripower in a '66 442. It is a 389244D block with B heads. I have not had it apart - it was in my car when I bought it. I understand it has a W30 cam in it. Should I be using a lead additive in the fuel?

Pat Mills

 

I use it in all my old cars pre-1974 I think. Better safe than sorry

 

hey pat, your car sounds sick with that tripower and 425, mine should be fun with a 425 4 speed, and yes you should have lead unless you have hardened seats, or you could eventualy burn a valve, if you dont know what seats you have then run the lead just in case.