Pistons and cylinder bores

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eschroeder1

eschroeder1

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This question is for all those engine techies out there. Here's the situation. I bought a balanced rotating assembly for a 340 that has KB243+30 pistons. I'm looking at purchasing a block that has been bored +30 over and the guy I'm looking at buying it from thinks that the block was bored for forged pistons. Knowing that hypereutectic pistons have a lower thermal expansion rate than forged, will these pistons work in this block?

Thanks FABO Team.

Eric
 
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Normally the machinist will finish each cylinder to match each piston?? Without the pistons (having them in advance) you can't properly bore and finish the cylinders.
 
eschroeder1 said:
Guy says that bores mic out at 4.070 inches and the KB243 +30 pistons also mic out at 4.070 inches.

Link to pistons. Keith Black/KB Pistons KB243-030 Keith Black KB Performance Pistons | Summit Racing

Eric
Click to expand...
Don't take anyone's word on anything- mic the bores yourself before exchanging money.
If the bores and the pistons both measure out at 4.070, you're bordering on an interference fit- zero clearance. No room for expansion of any kind.
Sounds like the block was rough bored/honed and still needs to be finished to size. Take the block and pistons to your machinist and have the bores finished to KB's specs for your pistons.
 
Any good machinist will request the pistons to fit them to the bore. This is typically done by cylinder honing. The roughness of the finished hone will be determined by the piston ring material. 65'
 
If the bores haven't been finish honed you might get the pistons to work but I wouldn't buy the block until you get a "good" machinist to mic your pistons and mic the bores on the block. If the bores are just a .001" over the recommended bore size it won't work.
 
I’d wager the pistons are not bringing measured right. Most pistons are designed for the overbore they are sized for. But some pistons need more wall clearance. KB hypers and some forged want them tight. Like easily within a percentage of error. the solution is buy the pistons, and measure each bore and piston properly. If they need to go bigger have it jones to the right piston to wall clearance and go.

Edit: if you have them a little loose it’s not going to hurt anything. Too tight will.
 
I have used them on several engines with standard machining only thing to be careful of they come out of the bore .018" as i used closed chamber e/q heads i had to machine 20 thou off the top of the piston
 
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