Inliners International Forums General Discussion Engines 327 pistons in +.125 chevy 250

I read an article in the inliners newsletter last night about a chevy 250 engine that's getting good gas milage performance. I didn't have time to read the story thoroughly but I did notice that the engine had been bored .125 over and was using 327 pistons. Can anyone provide additional info on this setup? I've heard about using 307 pistons but this is the first time I've heard of using 327 pistons. Does this require any other modificaions like different rods? What compression ratio would you get with this piston? Any additional info will be appreciated.

307 pistons are alot better choice IMO. .125 OVER FOR A STREET ENGINE IS WAY TO FAR......

I'm looking for info about the 327 pistons in a +.125 bore because I happen to already have a short block that's been bored .125 over and I don't plan on throwing it away. I'm wondering if the 327 pistons would give me a compression ratio similar to 307 flat top pistons in a standard bore. I'm just trying to see what my options are for what I've already got.

The 327 piston would be about the same compression? Not at .125 over.the compression would or should be a little higher then that of a .030 bore and the 307. With the standard 250 head you should be closer to a 10-1, over the like about 9.5-1 with the 307 and .030 bore.

If the original cylinder walls on a 250 were .160" thick (I don't know that, but that is the blueprint minimum on a Ford six and I would be surprised if the Chevy of the same era were any different) boring to 1/8 over would mean the thickness of the wall is now about 0.100 inches. That's less than 1/8 inch of unsupported cast iron cylinder wall thickness. And that assumes NO CORE SHIFT. If the core shifted by .030" then the wall is now as thin as 0.070" in some places. I've eaten kettle potatoe chips thicker than that.

Also, its bad machine shop practice to buy pistons to fit a block that has already been bored. The correct practice is to mic the pistons, add in the recommended skirt clearance, and bore and hone the block to fit the pistons.

I would find another block and turn that one into a weld fixture or donate it to your local school's shop and take a tax write-off on it. Or scrap it.

The conventional wisdom on these blocks is .060" over is dependable street max. More has been done and has worked well for some. Check Leo's book where he talks about core shift. Find some cheap 327 pistons (piston) and check clearance. It was not unusual in the past to knurl the piston skirt to get a closer fit. If you can play with it for little money it may be worth it. If the cost is more than other options it would be best to start with another block and build it other specs. Otherwise you will end up as an old man sitting in a pile of parts that fit nothing you ever wanted to build.

EVEN AT .060 OVER BORE YOU HAVE TO RUN A H/D RAD TO KEEP IT COOL. MY 250 IN MY PICKUP WITH A LOAD NEEDS A 4 CORE RAD. AND A 5 BLADE FAN IN THE TN. MOUNTAINS.

This is an error pattern we all fall into sooner or later: trying to buy the wrong parts to get use out of the wrong parts we already bought.
This is your chance to save money by not building a 4" 283 motor, which (if history is any guide) will have temperature and oil control problems.
Sell the block, 4" blocks are cheap.

Here is my experience with .125 overbore in a 250 block:

I built a stroker twice with .125 overbore. Both used 383 (common SBC stroker) pistons, 4.062 stroke crank, 5.7 inch inline rods. First time no block fill, second time filled about 1/2 way up except in front around the pump, same block both builds. I had trouble with overheating and clearly there was some block distortion based on bore wear patterns. I agree with above comments.

If you want to play with .125 over go for it. I recommend a sonic check on cylinder wall thickness. As a reference point my current stroker is at .060 over bore. There is one small dime-sized spot (back wall cylinder one down low) that sonic checked .070, no other spot thinner than about .125 or so. Thrust walls were all at least .200. It's been fine so far.

[quote=panic]This is an error pattern we all fall into sooner or later: trying to buy the wrong parts to get use out of the wrong parts we already bought. [/quote

Well put. Anyone interested in some wrong parts? Beater

Are cylinder sleeves available for the 250? If so, are they an option worth considering?

Hey strokersix,
Your real world measurements and experiences agree closely with the purely hypothetical example I put forth above - it made me chuckle on myself.

As Homer Simpson sang, "I am so smart - S-M-R-T."

If you cut out most of the cylinder wall to install sleeves you have lost some of the structural support that connects the block deck surface to the lower block. If you use thinwall sleeves to get back to standard bore then maybe OK. If you are trying to sleeve to 4 inch bore there won't be any original cylinder wall left.

This is my thought. I must qualify that I have not actually tried sleeving to 4 inches so your mileage may vary. I have sleeved single holes to .030 before but not all six nor to 4 inches.

I recently cleaned out some old stuff and threw away two sets of used L79 327 standard bore pistons. I had them for just this purpose and decided not to do it. I only mention this as a possible source of cheap 4 inch forged pistons.

Im wondering if the article in the 12 Port News might have been meaning the engine used 307 pistons instead of 327 pistons. Its well known by most in the Inliner community that a .060 overbore is the maximum safe acceptable limit for most street and race engines. And even in some race applications that are pushed out to 4" don't live long or yield the results they were expected to. Core shift will always be a factor and is present in all blocks. In my opinion, you are just throwing money away to go to a 4" bore. Like Stroker said, sleeving removes too much of the parent metal to securely support a sleeve at that bore size.

When I looked again at the article I believe it was a pontiac engine. I don't know if there's a difference between pontiac and chevrolet. But I do know that it said that he bored it out .125 and used 327 pistons. When I asked about sleeves, I meant sleeves to fit into the 4" bore with an ID of 3.875".

Thanks for all the good info here. Upon further review, I took another look at my shortblock and it's actually +.080 instead of .125. My memory is not what it used to be. The Leo book says anything over .100 should be magnafluxed so I'm assuming that even though .080 over is not the best situation it's probably safe enough for a street engine that won't be used to race or pull a load. I'll find out some day when my back gets well enough for me to put it together. Thanks again for the info.

I PROMISS YOU .080 WILL RUN HOT ON THE STREET

I've talked to Richard who has 4" pistons in a Pontiac engine. He said he has had a couple blocks he tried it in and failed on only a couple of them.

I have a standard bore pontiac 230 block that the thinnest part sonic checked at .238" (and a .030 over 250 block .060" thinner! Both engines use the same STD bore), so if the 230 was bored .125" over would still have .113" on the thinnest part, which was the front of cylinder #1 (on both blocks). Maybe Pontiac's had a few thick runs?

BTW, Richard has this 4" bore engine in a mid sixties El Camino and drives around Pahrump and Las Vegas. His cooling system didn't look fancy or excessive. IMO, tune and radiator size have much larger roles in cooling than cylinder wall thickness, especially when talking less than a tenth of an inch of metal difference.

Strange....my 250 is .060 over with 307 pistons and .040 off of head, 194/160 valves,ac,and Texas heat with 2 10" elec. fans and a Superior 2 core alum. rad.and heating problems is something I don't have...some of those "jap crap" radiator cores some people are using now want cool if you have 6 cores... Jerry

I've had good luck with Superior products and they are surprisingly inexpensive - even for a custom built unit to my specs. I always give them a to scale cardboard mock-up to work from.

Their 50 Chevy is a perfect fit...a true plug and play. I had him make the lower hose connection match a 250 Chevy water pump

A 2 CORE ALUMINUM RAD HAS 1" TUBES SOO ITS REALLY A 4 CORE BRASS-COPPER EQUAL

Plus its physically stronger and easily repaired w/ a TIG welder.

I run a 250 with 307 pistons and a good alum radiator which keeps the coolant temp under 200 even with the a/c on but my under hood temps are extremley high.

200 IS border line too hot for a chevy I-6 as the front 2 cylinders run alot hotter. i run a 160 stat to make everything live and it still goes up on a big hill loaded, 250 +.040 o/b, stock engine

Re: "the front 2 cylinders run alot hotter"

Isn't that the reverse of most L6 motors?

"under hood temps are extremley high"

Hi-Jacking this thread for a moment, Bill, while running with those high temps, do you experience any perculating/vapor locking problems? I fought mine all through August out here when outside temps were over 100. I replaced everything from gas cap to intake manifold with new including tank, filter, elec pump, fuel line, carb (Edel 600), 2" phonelic carb spacer and the problem finally left after the hottest weather left!
(292 with LPG pistons, AFCO alum rad, A/C, no hills or trailers!)

My temp gauge input comes from the front of the head so I guess i am OK !!!

Joe
I have been running it since 2003 with no problems. I use a stock fuel pump and a quadrajet carb on a home made intake. I tow my roadster with it. We have had it all over the eastern U.S. I have even considered installing a electric fan in the inner fender panel to pull some of the heat out. My main worry is the effect the extreme heat is having on electrical componets and hoses.

I should have started a new topic for for this so, belatedly, I'll do it now and maybe our more than capable Moderator can move a coupla posts to 'Extreme underhood temps'

Also remember that the article is talking about the Pontiac 230 OHC engine which is not the same engine or block as the Chevy 250 engine or block you mentioned in the beginning post. The Pontiac engine block is an entirely different casting altogether, and could be much thicker than the Chevy block which is normally thin once bored past .060 over. So your comparing and taliking about 2 completely different engines.

You actually are going to be in better shape than you think. When you bore the cylinders, only 1/2 of the .030,.060 or .125 comes out of each side of the cylinder walls. So at .125 over you will only be loosing .0625 from the wall thickness and be thicker than thought.

Related posted have appeared previously on this forum such as:

"boring out a 250" started by Al on 05/15/09 #49941
"Max Overbore for 250" started by JeffF on 06/25/10 #58727

Search using "Mexican" helps as the Mexican 250's had thicker cylinder walls and are frequently mentioned in the discussions on boring and cylinder wall thickness.