Inliners International Forums General Discussion Engines 250 chevy replacement pistons

I'm a new guy on here. I hope you'll overlook my errors. Having never built a 250 Chevy before, I have a lot of questions. I've been following this sight for some time and never posted anything. Now for one question--Has anyone on here ever tried to adapt 318 Mopar pistons to a 250? Their bore is 3.90, which would be a .015 over bore for a 250. The comp height is 1.658 to 1.741,depending on the 318 bore. The pin height is the manufacturers method of compensating for a compression increase when over boreing. These pistons are available in a flat top with no valve reliefs, and no extreme chamfer around the top. The only drawback I can see is the pin dia. is .057" larger than the 250. I am going to have the rods resized and shot peened any way, so my other questions are would the enlarging of the small end of the rod hole for the larger pin weaken them too much, and how much room is there to play with on the piston height?

I think the main drawback is the S/E of the rod on the 250 engine. The pin bore usually isn't centered very well and boring it that much oversize will compromise its strength if it is off center much. The compression height is right in the range of it being something to consider if it weren't for the pin issue. I've bored hundreds of SBC rods out for full floating on the S/E to install a bushing(but still not as big as the Mopar pin), and this is basically the same thing but for use with much lighter pistons and wrist pins, so not as much inertia trying to pull the rod apart. But you often have to sort through several sets to get enough that are more equally bored on the S/E to start with.

some place around here I've got some bushed pink rods (they are forged rods from GM), and I thought I would measure them and see what the rod was bored for to make room for the bushing. As I remember the bushings were about .060" wall with standard size pins. maybe the outside of the small end was larger.

Yeah, I was referring to "pink rods". I used a .975" OD bushing on all the one's I did. It was the smallest OD bushing I could get in a good quality race bushing material.

I don't know, but would it be possible or practical to put bushings on the pistons? Might have to bore them out to fit the bushings. Would the cost be too high? Is this something that is "not done"? Jay6155

I've seen bushings in the pin bores of pistons in large industrial engines before but not in any car engines. I'm sure with enough patience and time researching bushing catalogs one could come up with something that would fit.

cnc-dude---There used to be a rod shop that was reworking some either Buick or Olds deisel rods rods, they were forged rods and some of my friends were running them in sbc engines. Do you know what they were and if they might have a rod withmore meat on them than the 250s.

Actually, the 350 Olds V8 gas engines have 6" rods in them. You can bush them easily and have a set of budget 6" inch rods for the 350 Chevy. I've done a few of those before for some racers.

That is what I am used to seeing, engines for class8 trucks. That's what made me wonder about bushings. Those 318 pistons sure look tempting! Jay 6155

I'm not going to knock someones ambitions if they want to try something like this. It has been done in other engines before and could work out OK in this application as well. This could be one of those hidden speed trick gems that could be beneficial both from the budget standpoint as well as the gain of having a tall compression height piston without a huge chamfer. Just buy one piston and a couple of bushings and see how easy it is to put them together and them not come out.

I wonder what material choice would be good for bushing pin bores in pistons? High strength 7075 aluminum to match thermal expansion with enough wall thickness to maintain press fit? Seems conceivable. I expect you would have to turn these bushings from bar yourself.

You may have to hone the pin bores in the piston after fitting the bushings. I would also include pin oiling grooves or holes in the bushings.

Or, how about this: Float the pin in the rod eye, press fit into the piston pin bores. This might make the fitment a little easier. Backward from normal but could work? Maybe a bad idea... If I tried this I think I would use pin buttons for belt and suspenders approach.

Some OHC Pontiac rods are more beefy than Chevy 250 rods and might take bushings. I'm told the OHC rods are cast armasteel or some such junk but not sure I believe it. They look like nice forgings to me.

There are several pin bushing OD/ID combos available that can be used to get you close to the Mopar OD and Chevy ID in a quality Bronze material, such as from Crower, Carrillo and others like that. The ID will still need to be pin fit once installed. The ID of the piston may need to be honed to allow the correct press fit for the OD of the bushing also, but not an issue.

As for the aluminum bushing idea, it is an option but would likely be considered a custom job, and then you start creeping back up on having a lot of money invested in a $9 or $10 piston quickly turning into 3 or 4 times that once it has to be handled that many times.

What about using a Mopar rod? Checkout some of the 4 cyl offerings. I think with a little work, they fit.

CNC Dude, how big do those sbc rods have to be bored to make them bushed and full floating? I am wondering how close to the 318 pin do they get? I would think that boring the rod SE would be the easiest and cheapest way unless some standard size bushings for the pistons could be found. After all the point of using 318 pistons is to avoid the cost of custom pistons from company's like Ross. If standard size bushings can't be found IMHO the price would spiral quickly out of control. Jay6155

I used bushings that Carrillo has for their H-beam rods. They were .975 on the OD, and were the smallest I was able to find. This allowed to keep the boring of the S/E's to a minimum for making them full floating with a bushing. But, we bought the GM pink rods in bulk quantities of a hundred or so at a time, and I often had to select them by hand to get a set that the pin bore was as close to being centered as possible. Never had a failure in one in the many dirt track engines we built. They would turn 8500-8800 RPM 50 laps at a time for a season or two with no problems. But they only had a 400 gram piston hanging on the end of them also.

CNC: Yes, cu$tom work for sure. I forget sometimes that not everyone is as bold (foolish?) as I when it comes to trying things on my own.

Often more learning in mistakes than successes if you can afford them...

I'd like for someone to try this and see if it is a viable alternative just so we will know. I might buy just one piston and experiment myself and see how the bushing idea in the piston will work out.

Don't forget that compression height of the piston can be adjusted slightly when boring for bushings!

The OHC Pontiac I-6 is a real mystery motor. I've personally torn down a half-a-dozen cammers and found only one with a set of the "good rods". But they are good. My machinist says they are modest-good quality forged steel, SAE 1030 - 1050?

ARMASTEEL was used because it could be cast not forged and had enough strength to pass muster as cast. Look at a bunch of rods for narrow parting lines or wide lines. Then check out which rods went into LowPerf or HiPerf engines.

Remember the SD455 Poncho V-8 with the HD rods? Rumor had it that they were ARMASTEEL also. But I doubt it. If Pontiac went to the trouble to make a stronger 455 rod why would they ask for trouble by using a LoPerf design.

The Poncho 421 SD's had forged rods from the factory.

1963 era approx.?

That's why when my friend would want to go to Pontiac race days1/4 mile) I would say OK, but let me find my catchers glove, he would ask why a catchers glove, I would say so I can catch some rods flying out of the blocks!!!!
What a shame on so many nice Ponchos had cast rods.

IIRC, SD 455 & 455 HO's had forged rods.

MBHD

The forged Pontiac rods from the early 60's were not very good. The later 455 SD were much better. We did a 455 SD NHRA Super Stock engine for a guy that used them in his engine. Back in the late 70's when he raced them, you could only get the rods from Pontiac, and only if you provided a VIN of an actually Super Duty car. Top secret I guess.

They probably were not that good, but they were better than all the 326, 350 389 & 400 cast rods.

I just know this because I used to try & find some 421 rods for my friend.

It used to be slim Pickens when it came to finding a good Pontiac rod that seems not so long ago.

Now there are aftermarket rods available & much better aftermarket cylinder heads.


MBHD