VI Cutty

Block is a 1969 455, unknown mileage. Let's assume more than 70,000 miles and never opened up. I'll be aiming for up to 450 hp in the build.

- I've been told I should have it bored out even if a hone is all it needs, particularly given the intent for higher performance
- Does it make sense to bore 10 or 20 over, or just go straight to 30 over? (I'm leaning towards 30 over...I suspect same cost to go 30 as to go 10, and not much more than honing)
- Seems like most forged pistons I've found start at 30 over...is a standard bore piston used for 10 or 20?
- How much additional power does going 30 over generally contribute?
- What would the displacement become at 10/20/30 over?
- Apart from the pistons and CR calculations, does anything else need to be changed when boring 30 over?

Sugar Bear

Piston choice and available oversizes may be the determining factor. I wouldn't bore any more than needed as it reduces the number of times it can be bored and unnecessarily reduces cylinder wall thickness for very little gain.

Good luck!!!

matt69olds

The power increase would be negligible going from 455 to 461. The real improvement would be from better ring seal and increasing compression.

Im betting you could get aftermarket pistons is any overbore you desire. Some less common engines use oversized pistons from the same engine family but different displacement (I think a early 400 oversized piston is a very close to a 350 piston? I may be fuzzy on the details)

I bet the labor to machine the block would be basically the same regardless of final bore size. Most of the labor is in setting up the machine. However, the cost of a less common overbore might be more than any machine shop savings.

oddball

Hone-only would only be a problem if the bore, after honing, is too large for the pistons. Honing will remove material, though generally a quick freshen can take less than 1 thou. A bore that's larger than the piston is designed for will result in a noisy engine (piston slap) and possibly ring sealing issues. If the bore is straight, not damaged, and not worn too much, then just a hone is fine.
There's next-to-no performance change from a normal overbore. That only adds a few cubic inches to the total displacement.
Pistons generally need 2 to 5 thou clearance, depending on material and construction. So you need a size-specific piston for any overbore.
30 is the standard rebuilding oversize because that's quite likely to remove any common kind of damage on old engines. Newer engines you're seeing 10 and 20 over pistons readily available, but that's because they're newer engines. You can get a custom piston made to any size you want, but they cost more than standard part number pistons.

Gpc1

I agree with oddball, I’ve honed and reringed many engines with good results . When they bore an engine .030
Over Its more common it seems probably easier too because of the depth of cut being.015 verses .005 for a .010 overbore which doesn’t leave much to work with.

67OAI

Don't put the cart before the horse; in other words, have the bores evaluated before buying any replacement pistons! If the block truly has high miles, it most probably will require new pistons and a bore/hone to fit.

7314haywood

.030 over bore is very common shelf pistons. If you have the time and want to wait get .010 over if cylinders will clean up at. 010. It's just a guesstimate but every cubic each should be close to 1 HP gain. So .010 over is maybe 1HP gain. Ring seal, head, cam combo is where its at.

BangScreech4-4-2

With 70K, you're likely to find at least some kind of ridge. I would go with the minimum overbore for which you can readily find the kind of pistons you're going to want.

VI Cutty

Good info, thanks all. Trying to be well informed for talking with my machinist/builder once he's had a chance to assess the engine and all this is very helpful.