Alexandre: Grant is right about the 54. The 54 and 55 1st 261's are the best of the 261 blocks as far as casting quality, (nickel/tin alloy) and thickness in wall and main web areas. But, there is a trade off, the later 58 and up blocks have the improved oil system, and better cooling. Also the later crank and rods are superior forgings to the earlier 54/55 1st. Life is full of trade-offs.
I also agree with Tony, in that I think for street use, stay with .080 over max. I think thats the biggest oversize 261 piston you can get anyway. And it's a cast piston at that. I would try to stay as close to stock bore as possible for wall thickness/integrity.
You run into all sorts of problems with a large overbore. And as mentioned, the core shift issue is ever present. I have bored several 261's out to .020/.040 and .060 oversize. I have not taken one to .080. I would not even try to take one to .125 over, at least with conventional boring bar set-up. Your out there in no-man's land.
The only way to safely bore to that size, would be to sonic test each bore at 12,3,6,9 o'clock positions and at about 3 inch interval levels throughout bore height. Then you could establish exact core shift, and wall thickness position on all bores. The bore centerlines would almost certainly be off center to crank axis. Would have to bore block on a horizontal boring mill. Then you could reposition for each cylinder offset. All the rods/pistons would have slight angularity to crankshaft centerline.And by the time you get to this point you will have marginal wall thickness and less than desirable cylinder wall integrity. As you can tell it would not take that much to make this set up fail, and after all that expensive machining. You could have casting cracks, or early heat failure from such a thin wall, once you got up to operating temps, or high compression and under heavy load.
After all that, your only out to what a 270 GMC is. Which is the piston that you would have to use. Then you have the problem of the pin size. As you can see the list of difficulties increases with bore size. You could just get a 302! It's a 4 by 4.
You can weld, and then offset grind the crank to adjust the stroke, but be prepared to spend lots of time and bucks there too. You have to weld, offset, rough grind, send crank out for heat treatment, undersize the mains, then finish grind the crankpins. Believe me, I have done this, and it's a big chore. All the welding just seems to pull the weights around and before you know it, the crank is all spazzed out!
I have safely taken .040 off the deck, with no troubles. Like I said, if you want power, keep that bore small and wall thick, and jack that compression up. Forged pistons, headers, porting and lots of head prep/flow and valve work are the secrets to more HP! Let me know if you find any forged pistons for a 261. Good Luck, and repost your ideas!
Think Chevy!
:rolleyes: :p
