Is it safe to bore a 250 out .60 over?

[Anonymous]

cliffords sells .60 over pistons for the 250. Is this safe? has any body done this? I do not think the log head will flow enough for this big engine. How much do the 2v heads cost? Are the combustion chambers big enough for a 9.5:1 comp on this 338 cid?

do you think a holly truck avenger is worth it on a large displacement small six?


thanks...

 

[Anonymous]

I got it wrong, it is .060 adn .080 over, is this dangerous?

 

[Anonymous]

060 over is about as far as i'd go--080 is for selling a wore out engine. The only real advantage to boring is good fitting pistons. there is very little actual cubic in gain. Can't remembe the actual inchs per .010 of bore but it ain't much. On a 2 cylinder a .125 overbore results in about 7cubic in. Thus it seems a 040 overbore on a 6cylinder ought to be about 6&3/4 inchs. Harley twin cam engines have really thick jugs.

 

[Anonymous]

as i recall it works out to a 205" engine @.060

frankie

 

[CoupeBoy]

stude are you sure about that number, I used stock 250 bore/stroke (3.68/3.91) and then added .06 to the stroke bringing it up to (3.74/3.91) using the calculator at
http://falconperformance.sundog.net/compcalculator.asp
and I got 258 cu/in

And boring it out .08 was (3.76/3.91) and only increased size to 260" which isnt a whole lot when you figure the block is done at that bore..

-ron

 

[MustangSix]

My machinist and I were bench racing and figured once that we might be able to completely bore out the cylinders and install wet liners, increasing the bore to almost 4" on a 250. Then by offset grinding the crank to a 4" stroke, you end up with a 300 ci engine.

Only problem is, there is not a cylinder head that will support that kind of displacement. You would be many, many dollars ahead to just start with a 300.

On a 200, the same technique yields about 236 inches, but again, why not just start with a 240?

 

[CoupeBoy]

Only problem is, there is not a cylinder head that will support that kind of displacement.

Even if a person were to prep the block for a crossflow head it would still be insufficient?

-ron

 

[MustangSix]

Probably not. The crossflow has relatively small, high velocity ports for high efficiency. They would be overworked on a 280 inch engine. Again, by the time you got done with the high cost machine work and the high cost head, you probably could have built a couple of hipo 300's.

 

[mikeyo]

Hi all,
Again my 2 pennies.
To find the displacement change for an overbore.
The formula for CID is Bsq.XsXpi/4X#cyl.
Example: (forgive the chevy blashphemy, its off the top of my head)
4.0"Bore squared =16
multipied by 3.48 stroke=55.68
multiplied by 3.1416 (pi)=174.92
divided by 4 (constant)=43.73
multiplied by 8(#of cyl)=349.85 I.E...a chevy 350 small block.
Not real helpful but fun to show off with
MikeyO

 

[Anonymous]

The original issue here -i think- was will a stock head support a >.060 over 250. sure no problem- the misconception was that a .060 overbore was gonna yeild a big engine which it doesn't. ( maybe .6 over was figured) If you start out with 400 cu in and bore .060 over you actually gain some in's but on a small engine the gain decreases proportionally. I bored my Harley 1/8" or .125 and went from 88" to 95". This is starting with a 4" stroke and 3.75 bore. So it seems if these dimensions were on a 6cyl. you would start with 264" and with a .125 bore end up with285". Do that much bore on an I-6 and you'll be pumping water with the starter motor (well maybe not the first day).