Engine inspection result for rebuild

[Kim K Kaiser]

i got call about engine after review.

He told me i required 30 over pistons ring.

Standard main bearings. He only had to polish the crankshaft.

My question is if the hole is bigger now.  Woukd that affect compression.  He suggested just standard as he felt it would be minimal at best.  

Any thoughts on that.   

[William R Conte]

Standard bore x stroke for a 390 is 4 x 3.875.

Overbore of .030 gives you 4.030 x 3.875.

Radius of cyl is now 2.015. r^2 = 4.060225.

Pi = 3.14

3.14 x 4.060225 = 12.7491065  (Area of circle formed by the bore)

multiply that by the stroke  12.7491065 x 3.875 = 49.4027877 = cu in per cyl, x 8 = 395.222302

Your engine is now 395.222302 cubic inches, or rounded down to 395.

[Kim K Kaiser]

So no need to have any concerns regarding camshaft?  

[Jason Edge]

Cubic Inch Displacement will increase a bit (Bill Conte has done the math for you), and compression may slightly alter from the stock 10.5 to 1 compression  with the new standard pistons ... but marginally at best and absolutely nothing to worry about.  The aftermarket pistons are most likely designed to dial on to the stock compression ration given bore and stroke, unless you could even find low or high compression pistons. There were low compression options for these engines back in the day but have not looked to see if you could even order these if you wanted to.

[3a93lc0athkg1]

Marvelous arithmetic by Mr. Conte!  I'm glad someone remembers all that stuff they learned in school, I lost it all within a year or two.

This is one engine I wouldn't worry about boring larger holes in - the new block was rolled out in '63 with plenty of extra room for that; GM knew they were going to be boring it out further and further over the next 5-10 years to accommodate more accessories and performance demands.  This basic same block ultimately wound up at 500 cvi before the oil embargo prompted the government to force manufacturers to forget looks and power, in favor of economy.