Mercruiser HP425 Gen V Big Block pistons
 

I believe this is a standard 454 Gen V big block Chevy however the power comes from forged innards, crane cams. Bore is 4.25 ins, stroke 4 ins, rod length 6.135ins, head volume 118cc, std deck height 9.8 ins. Gaskets removed are Felpro standard Marine which I think are the 1071's 0.041 inch thick volume 10.6cc from their website.

Pistons removed are Speed pro 10215228 - High Performance Piston - This 454 standard, pressed pin, high performance piston has a 8.75:1 compression ratio. As I'm blowing the engine I dont want to goto higher compression.

I am rebuilding due to a seized piston and have little bore corrosion apart from an 8 thou pit running on the top ring at TDC which I think is too severe to leave. Ideally I would like to go plus 20 thou.

What piston should I go for?

I just put a set of flat top je's in a .060 over 454 that will have a. 6-71 on it. I can get the part number when I get back home tomorrow.

What sized blower are you using?

B&m 174

In that case i wouldn't lower the compression too much

It was great at 8.75:1. Idle little rough but very smooth at mid and above. Sounded good too , so would like something about the same.