8 brow pistons in rebuilt 400
 

I found a rebuilt 400 engine that may not have even had break-in oil changed yet. The price seems fair considering cost of machine work nowadays. But it has the 8 valve relief jobber cast 411np pistons. Would it be worth buying and changing out the pistons to at least l2262f?

It depends on what compression your looking to achieve and if you want a more efficient motor.

The last time I saw these pistons it stuck in my mind how many sharp edges they have which could make for a far greater chance of having pre-ignition and knock taking place for any given compression ratio.

It depends on what you consider a fair price as you didn’t say the actual price and what year block. When I see those 8 valve relief pistons I start to think it’s a cheap build and who knows the quality of the machine work. Does it come with a machine shop invoice indicating what was done and parts replaced? Or is it a dingleberry hone rebuild?

Desired compression ratio
 

I just read the 411np are minus 14 cc pistons. I also now see they are 1.7 CP so if the deck was zero there is not much chance of any other piston being used. Though if someone uses those pistons I doubt the deck would have been shaved or zeroed out. Ideally I would like to get or build a 400 at least 8.5 to 8.8 compression ratio.

I would satisfy myself first on quality of parts, machining and assembly than worrying about the pistons. Those pistons aren't "bad." It is the details inside the engine you can't see that I would wonder about. If you need a driver and all is good run it, and don't change the pistons. I rebuilt a 400 over 20 years ago with them and 1969 46 small valve heads. Its good with 87 octane in WA and 85.5 in MT. I have cast rods too gasp. : )

Is the engine able to be run? If so can you do a cranking compression test on it? Do you know what cam is in this engine?

Stan

Not to much information available. It is at a shop having been removed for a LS swap. 100 percent complete carb, alternator,ps pump, starter, etc minus exhaust manifolds. it looks like it was just rebuilt. But the story told by owner to the shop is it was rebuilt in 2014 by Pontiac driven 20 miles then parked. He said it has a Competition Cam but not which one. And of course the 411np pistons. My calculations is it was a 7.1 to 7.4 compression ratio. Probably a real dog who knows, but bad enough the owner did not want to drive his 1975 Firebird after rebuilding the engine.

I'd set the value as if it were a core block and some parts.
 

Presume that you will have to replace everything as in a normal rebuild.

And if the block is a 500557 casting, that value would drop to close to zero.

No it is an early 75 block 988

If you're going to tear into it, you're going to
build it. First there's the pistons. But are you
really going to hang them on the cast rods?
Now you have new rods/pistons. Are they the
same weight as the old stuff? Might as well
balance it. And have the crank checked. Was
the break-in enough to assure trouble free
miles with the used cam? Is this particular
grind even the one you want?

You're either rolling the dice and buying a
runner, or you're buying a core. Bird in the
hand, or two in the bush. Eight relief pistons
aren't the end of the world, you just don't
want to settle for them when you have far
better choices. But that bridge has already
been crossed.

Depends on what you can buy it for. I wouldn't pay over 1K, since I'd gut it like a fish and start over. If you're willing to run those cast pistons and slap a set of high compression heads, maybe 1500-2K if the carb, sheetmetal and accys are all there and desirable pieces.

I'd use a set of cheap forged rods and autotec pistons. Speedmaster or ported iron heads and a HFT and let it eat.

Unless it was dirt cheap there is pretty much zero chance I would buy a "rebuilt" engine with the intention of then changing the pistons. If you are going to do that, just buy a core and have it built.

Now on the other hand if you are just going to run it as is, I would do that, again if the price was reasonable. Unless you are buying it with a ton of documentation, I view pretty much every facebook marketplace/craigslist engine purchase as a gamble. Sometimes Im willing to gamble, sometimes Im not. Depends on what I out of it and how much money Im out if I lose the roll of the dice.

You are WAY more generous than I would be. I'd cut those prices in half.

Remember that this engine was so disappointing that the previous owner cut his losses after twenty miles.

I did that. (cuz i broke one of the 8 vr from detonation) The l2262's were just under $400 usd. I added ARP bolts to the rods. The machine shop was another $350 CDN to swap the pistons, install the bolts and resize the big ends. Then another few bucks for gaskets and new rod bearings. You might get a way with reuseing the rings but inadvisable. Then you're close to about $1k if you do the disassembly, ball honing, cleaning, assembly yourself. Older engine that I had. It is worth it for me.