Inliners International Forums General Discussion Hi Performance Thoughts on my new pistons

I should have posted this earlier but time didn't really permit.

I just picked up a set of pistons off of ebay. They're out of a chevy 307, 30 over. The buyer said (and backed up with pics) that the engine had had a rod let lose and two of the pistons were damaged. Basically I have 6 good pistons from a 307. Lightened, forged, racing pistons basically.

My engine is a 1979 250 L6 from a camaro. Mom's old car, I know how it's been treated. So I know it's stock bore. When it gets a little more tired I'm thinking I'd bore it to 30 over and use these pistons. I know what the 307 had the same bore as the 250 and in theory should work.

Has anyone ever done this? I believe I've read about it being done but I'm not sure I've seen any examples of it. Are there any special things I need to worry about? Good/bad? Is it really as easy as simply putting these in? (no special tricks from the normal). And since they're from ebay are there things I can do to get them checked? Do machine shops have ways to test pistons? This is my first engine and even though it could be a bit before I do it I wanna know everything there is to know about it. Thanks!

Greg,

Both 307 ans 250 shares around the same compression height. As much as the 283/230/153.
The only deal is that the gasoline 250 normally have dished pistons, and the 307 ones are flattop. You mentioned forged race pistons, how about their head? Is it a flattop? If so, go for it. If domed, pay close attention to the dome interfering with the head. If domed, You can bore the block and make a mock up assembly to check out what should be done in order to avoid piston/head contact. And remember one more detail: Flattop and zero deck height should provide more than required compression ratio. Don't go wild on compression or You'll be tied to a engine requiring high octane gasoline. Or useless in normal use.

The link to the auction is http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2467726443

For the money I couldn't really resist grabbing them. Time will tell if they're actually worth it. This would be my first engine buildup. I figure with the L6 combined with forged lightened pistons I'll have a smooth running engine that will be both tough and capable of good RPM. I hope to keep the compression in the 8:1 range. Eventually I hope to run a few pounds of boost with the engine (long term goal) and these pistons will work good with that.

I'm a newbie too and I figure I'll be safer with an assembly that's harder to over rev. That and these should resist detonation so I'm safer that way too. I'm gonna build a 6 to last for a long, long time.

I'm not that sure of any of this so I'm open to suggestions. Do the pistons seem light to anybody. I know lighter is better but 438.1 grams.............

Greg,

No good news, man! The pistons are ultra short 1.175" comp ratio. You'll need 6.125 rod - occasionally the same rod used on Small block Mopars. The small rod end should also be bushed for them, but that's an easy one. The only drawback is that the big end is too large to be used on a 250 crank. The rod pin is 2.000 std, the sbm rod pin is 2.125, so no bearing to fill the gap. Since the 250 crank is nodular iron, no welding for stroking or filling gap. But may sound interesting to make a weird combo on a smaller V6 chevy. Or go big, turn down a 292 crank to fit the smaller 250 block, use some proper rod - the 5.700 rod out of the 250 if you care to turn down the rod pin from 2.100 to 2.000, and You're thru. There is a lot of clearencing into the block, as well as some counterweights turning down also. Guess that a cam for a 292 is required because the fuel pump exccentric interferes with the crank somehow. It's a bunch of work to make it happen, the r/l is lousy at 1.38, but the looks of an innocent low deck 250 and the big big 292 stroke looks somewhat interesting.
But if I were You, I'd also have got them. The price is just ridiculous. Great deal, Dude! Take some time and You'll end up realizing a decent use for them!

the 307s can be used in the 250 blocks same as for the 283 pistons in the 230 blocks. As for turning the Pin Don't If the pistons have a snap ring groove in them Just have the rods floated. Now depending on the skirts on the 307s in some cases the skirts need to be trimmed to clear the crank. I have ran TRW 307s for 17 yrs now on the same set of pop ups. I had to have the tops cut for valve clearance and the skirts needed crank clearance. Those 307s should bring you about the same compression as listed on that site if you match the same head CCs And have the block decked. and I think you'd be a happy camper. hope this helps.}[oooooo]

ps i know of a few guys that run the stock type 4valve releaf 307s and have been very happy with their settups.

Thanks for all the info guys! Even those tid bits have helped me a whole lot. I've already spent a good deal of time on the net but there's so much here too!

It sounds like my first intuition was right anyway, it was a good deal to grab those. It's kinda annoying there isn't more I can do with them already but then it's a challenge too. In the end it will be worth it.

How hard is it to swap rods around? I was surprised to see a mopar rod listed as an alternative. I guess I should talk to my uncle. Is it really as easy as seeing to it that the sizes are right at each end? I'm gonna have to do some more research on this. But I'd love anything to guys have to say.

I've been busy doing some research. I've found a number of interesting things.

http://www.racewindsor.com/performancepartsplus/ratio_table.html

That list doesn't list any rods that are exactly 6.125" like Alexandre Garcia mentioned. However, there are a number of alternatives. I don't know about how the rods swap around yet, but I figure if I stick to chevy parts I have a better chance of them working together.

CID Bore Stroke Rod Length
I4-151 4.002 3.010 6.050
350 LS1 3.900 3.625 6.100
366 3.935 3.766 6.135
396 4.094 3.766 6.135
402 4.125 3.766 6.135
348 4.125 3.25 6.135
409 4.313 3.5 6.010
427 Z11 4.313 3.65 6.136
427 4.25 3.766 6.135
454 4.25 4 6.135
502 4.466 4 6.135

I don't know if any of these would work but it's a start. Also if they do chosing the rods will allow me to build in what kind of compression I have. I've already modeled some of this in Excel to compare compressions with different values for stuff. I need a few values though.

Gasket Thickness 0.060"
Piston dish 7cc
Head 72cc
deck height ?

The last one is what I need, but feel free to correct any of them. With those values and 0 deck height I get a compression value of 8.4:1. Which I think must be in the right area. In return I can post the spreadsheet for anyone who would like to have it once I have it done out nicely.

Greg,

The 151 6.000" rod is a direct swap, no need of reworking anything, as long as the lenght fits your needs.
The other rods all have bigger bearing ends, they all use a thicker crankpin, the 350 goes 2.100", the Mopar has 2.125", and the bigblock chevies all goes 2.200" std. This means that there is no bearing to fill the gap, as long as your crank has 2.000" crankpin. Also you should consider the rod width, all V8 rods are normally narrower than an inline rod. So, even if You were able to fit a 350 6.100" rod into your engine, for exemple, the rod foot is too narrow for the crank and will have excessive side play.

Well, I've been looking around and I've found a few interesting things. I think I found my pistons at Flat Land Racing. The same model I mean. The specs all seem to match. They're listed under the LS1 chevy smallblock. Interesting. They're a little pricey too (I think).

http://www.flatlanderracing.com/jepistonchevyssbLS1.html

Exhibit B is a set of rods that I think would be suitable for my engine. I have the site here.

http://www.flatlanderracing.com/crhbeamscat.html

I'm not sure what typically comprises a rotating assemblyas far as costs go) but if these pistons check out alright I wouldn't cry too hard if I had to buy the rods.

The last big issue I'm worried about is the end clearance. I've heard here and I wonder myself about using a SBC rod on an inline engine. What can I do about the play at the crank end? Do I even need to worry.

Thanks to everyone! This is my first engine and I appreciate all the help!

Greg,

I see that it's not wise to use small block rods in an inline engine. There will be a lot of side play, in a race engine maybe, in a street engine no way. I do not have the difference here at hand. But it's not a small one. For sure bigger than 0.040". Not to mention it will put a lot more oil flying around the crankcase and it will make the rings life harder. I'd not use a V rod on an inline engine.

I've been doing research and I found that:
L6 Chevy Crank pin width = 1.029"
V8 Chevy Crank pin width = 0.94"
So the difference is 0.089"

I'm not sure I understand the problem with the rings. Would the extra motion at the bottom of the crank put more stress on them (cause they'd have to keep the rod straight)? And the extra oil I don't know about either. Is that the result of the extra clearance again? I'm doing a lot of reading and searching online but it's taking time to learn.

Greg,

Everything is quite closely related. The extra large sideplay makes it easy for the oil to escape and go flying around, the crank turning fast makes it looks like a cyclone, a lot of oil splash around, a big part of this oil goes to the cylinders, much more in fact than it would normally goes, the piston is much shorter than the stock one, so easier for the oil to get to the rings, and the rings assembled in a piston that is less stable in the bore for some reasons, it's shoter, it's tied to a rod with a lot of endplay in the crank, and the rings itself are prolly thinner than the stock ones etc... and gather it all togheter and You have a sure answer to why it should not be any good to the engine!
But anyway, have in mind that nowadays is much easier to learn, there is the wonderful www, a bunch of cool books like Santucci's one etc... some years ago it was much harder.