I have a SBC 400 bored +0.020. I bought new SpeedPro coated skirt pistons, supposedly +0.020. I placed one in the bore, WITH NO RINGS, and it was about as snug a fit as you can get and still slide in. I don't know a great deal about bored motors and pistons, but I have a little common sense, and feel that the piston should have a little more clearance in the bore. I'm hoping that I was just sent the wrong pistons...maybe +0.030? What are your thoughts? Thanks...

 

The coated pistons are supposed to be a VERY tight fit. If you can barely get them in, they're probably just about right. Be careful not to damage the coating by rocking them from side to side or catching them on the top of the bore.

 

If you had +0.030" pistons, they wouldn't be in the bore at all. The coating is about a thousandth thick, or just slightly less, and givent the nominal skirt clearances for cast pistons of 0.002-0.003", that extra 0.001 or slightly more is going to make then feel very tight. You could check the skirt clearance right under the oil ring groove at the top or bottom (not pin sides) of the piston, but would need to be very cautious to avoid scratching the coating. That would be bad.

 

You should have the pistons and bores measured by someone with the proper tools (micrometers and a bore gage) and confirm the proper fit, it's best to buy the pistons first then have the block bored to fit them.