390 Cylinder Bore and Piston Diameter Issue

[davbell22602]

I was going over my 390 block today. So pulled everything down to the bare block. I went to measure the inside diameter of the cylinders with a telescopic gauge then measured it with a outside micrometer and I got 4.077 on all 8. I went to to inspect the pistons and found that they have "30" stamped on them meaning 30 over. So measured the diameter of the piston with 4-5 inch micrometer and got 4.077 on all 8 pistons. I have 4.050 for stock cylinder bore. Does that sound right?

There are some surface scratches on the pistons on the side where the wrist pin doesnt go through at.

Here's a pic of pistons

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[foamypirate]

I think you might want to double check your measurements with the mic. That's a HALF INCH larger than they should be if your measurements are accurate.

 

[89Suburban]

Dude, that tray is neat that holds the parts.

 

[davbell22602]

I think you might want to double check your measurements with the mic. That's a HALF INCH larger than they should be if your measurements are accurate.

I was thinking the same thing. I will recheck tomorrow on the piston diameter.

Dude, that tray is neat that holds the parts.

You get them here

www.goodson.com

 

[davbell22602]

Fixed in post #1 on the piston diameter.

 

[crazy4offroad]

Yes that sounds right... 0.003 wear on the pistons if they were 4.080" (minus 4.050 = 0.030" overbore). I would snap gauge 3 depths of each cylinder parallel and perpendicular to the crank and check for out-of-round and out-of-taper while you're at it.

 

[davbell22602]

Yes that sounds right... 0.003 wear on the pistons if they were 4.080" (minus 4.050 = 0.030" overbore). I would snap gauge 3 depths of each cylinder parallel and perpendicular to the crank and check for out-of-round and out-of-taper while you're at it.

OK, A .0015 piston feeler fits between the piston and cylinder wall but its tight fit.