Pistons for 400ci 350hp 1970 GTO
 

Looking for some serious advice!!
I'm searching for pistons for my 400ci 350hp pontiac engine.
Took motor to a rebuild shop to get block cleaned and inspected and was told that I have standard pistons but need to bore larger and put in new pistons. Don't know where to get .20 over pistons.
Is there a mfg that makes .20 over pistons cause have no need in going .30 over just cause of pistons is there?
Wanted to know if anyone would give me some advice on this issue.
:confused:

Typically pistons are available in .030,.040 and .060 larger diameter than stock bore size. You may be able to find a .020" larger diameter pistons, but the challenge is finding a common ring pack to fit the pistons. the piston rings are also typically only in 30, 40 and 60 over sizes as well.

Also, which cylinder heads do you have and what kind of gas do you want to run? Typically we suggest using dished pistons to reduce the compression to 10:1 or lower to run on pump gas. This is often needed on the '72-earlier engines with high compression heads.

I'd also suggest using some decent forged rods, as they're cheap insurance against rod and rod bearing failure. The stock rods can be prone to failure when they're diassembled or run pretty hard in a hot combo.

Check with Ken at Pacific Performance, he is the resident Pontiac 350 expert.
http://www.pacificperformanceracing.com/

Northern Auto Parts offers Sealed Power pistons and rings in .020" over. Part #411NP20
Here's their website: http://www.northernautoparts.com

All the 70 350 hp 400 motors really check in at 9.5 compression at best, so I would not fret over having too high a compression ratio even if going to a .030" over bore size, and with a forged piston that draws heat away faster than a cast type there would be even less need to be concerned.
A step up to a one notch bigger cam would let you run 91 octane easy!

A set of lighter wrist pins stuffed into the pistons since you have to rebalance the motor anyway will make for less bottom end stress and a faster reving better performing car.

Those are the "jobber" cast 8 relief pieces of $#!t,I'd recommend staying far far away from those pistons. 2¢

They sit way down in the hole as SOP and the additional valve reliefs add a bunch of extra chamber cc's,so understand that those pistons offer absolultey nothing desireable.

There are tons of posts here about those POS cast 8 relief pistons,a search will bring plenty of that info up.

Just go .030" over,that .010" over bore difference is NBD @ all,and for sure use a forged piston,and yeah get a set of the "budget" aftermarket forged rods too.

You'll be glad you did.

I would probably recommend the KB/Icon #KB891 forged pistons myself.

I'd also highly recommend zero decking the combo with ^^^^ those pistons too.

Those #13 heads will likely end up around 76 to 78 cc or so w/o milling them.

But I'd CC those #13 heads just to be 100% sure where I stood in the CR department.

FWIW

Bret P.

Listen to Bret....he's absolutely right. Those 8 relief pcs. of junk are good for ashtrays and paperweights, and that's about all.

The only reason to go a minimum cleanup and odd 0.020 is if you are planning on custom pistons and can find a decent ring pack and may need to go bigger in the future on the block or the block is thin. A custom piston you can also order the compression height to possibly not have to deck it also-and they'll probably be lighter than any off the shelf ones.

With my 409, blocks are rare so we used a minimal cleanup boring and a Mopar sized rings pack from stock 4.3125 to 4.325.