14reamdalton

Hi All,
I had originally posted about building my 425 into a stroker back in 2020, but the older I get, the more I realize I just want a good durable engine that will perform the way I need it to. I still plan to put this engine into my '80 3/4 ton chevy, paired with an NV4500 5-speed and looking for around 400HP for towing and being able to pull trailers semi-comfortably in overdrive. I had the engine blasted, tumbled, new frost plugs, new cam bearings, new mains and rod bearings. The 5-6 rod journal was ground due to a spun bearing that marred up the crank, and that journal will be running oversized bearings. I managed to find some stock bore 10.25:1 pistons that don't cost an arm and a leg, I think the originals were the 9.0:1. Would the higher CR have any effect on the stock heads? I wouldn't assume so because they were built with all 3 CRs and I can't believe the heads were changed for each style except maybe porting for the Toronados? Also, has anyone ever married up a manual trans to these motors to know what pilot bearing/bushing works best and how deep the crank needs drilled?

P.S. this engine was originally a red 2bbl motor with a "Super Rocket" air cleaner, if that helps distinguish the year and type. I will get the casting number and the Head castings this weekend when I dig the motor back out.

Thanks,
Dalton

Oldsmaniac

A red air cleaner if original would be on a HC engine. Pistons will have a slighter dish than the 9.0cr engine. CR on the Olds is determined by the pistons. Stock stick cranks used a pilot bearing, specs can be found on this site thru a search.

matt69olds

I have a 81 GMC 1/2 ton with a basically stock 71 455. I’m still using the factory TH350C trans and 3.08 gears. It pulls a car trailer just fine. With the overdrive manual trans, put some 3.73 or 4.10 gears in it and it will be even better.

Now that this truck is mine, there will be some changes. I built. 4L60 trans for this years ago, just never got around to putting it in. As soon as I find some 3.73 gears I plan to swap it in. Then some headers to replace the factory manifolds, replace the dash pad, and…….


Good thing my wife doesn’t read my stuff here!!

android 211

I don't think it would be wise to build a 10.25:1 iron head motor with a stock or mild cam and use it for towing. I think it would ping like crazy. The poster with the 71 455 has the right idea. That engine has low compression and the extra cubic inches increase the torque. Sell that forged steel 425 crank and get a 455 crank and stock rods and build a low compression 455. You can put the 455 crank in the 425 block. Finally you have to figure out if you have the 45 degree or 39 degree cam lifter angle blcok and if its a .842 lifter or .921 lifter.

NTXOlds

It sounds like you have a lot of time and money invested in the 425 already. If you don't want to spend a lot more, you could have someone add volume to the heads by removing some material and polishing the bowls. Also keep in mind that new head gaskets are thicker than the gaskets that came on a 10.25:1 big block. You will drop some compression there, too.

I am sure that someone like @cutlassefi can give you an even better answer.

14reamdalton

I found some info over the weekend, I have a "D" code block and "B" code heads. The block has .842 lifter bores and as far as I can see, the angle is 45°. (Also there is no drill spot on the front of the block). With the depth of the dishes on the original pistons, I can only assume they are 9.0:1, and I figured with modern head gaskets being huge, even the 10.25:1 pistons would drop to 10 or less by the time it's all said and done. I only want to go with these pistons to boost the power a little, instead of winding up with 300hp with an 8.5:1 CR, I might as well go for 9.5-10:1 since I'm buying the pistons anyway. I would venture to bet the iron heads could handle anything up to the 10.5 high compressions, right?

I want to refrain from changing everything over to 455 as much as possible, partly because of foolish pride and partly because of price and I already had the work done to this crank.

Keep in mind this won't be a daily driver, its a sunny day truck that I run occasionally when the roads are dry and the salt is long gone. I only want the capacity to trailer with it because it's a 3/4 ton with 4.10 gears and it has the load range to do so. I haul some with it now, but I am also running a1980s Smog Pump 350, so it has a quick 0-60 time of "maybe next week"!

v8al

See this post: 67 olds 330 cam for how to check your cam bank angle.

Oldsmaniac

A B headed 1966 425 is always a 45 degree block (non large lifter). Doesnt hurt o check