The Stovebolt Site Forums The Shop Area Engine Shop 283 or 307 pistions in 250 engine questions?

Who can i hook up with that can tell me about this mod? I have searched here and on inliners with no joy. My questions are 1) How do you determine how much of the skirt to cut off. (I read somewhere that you had to cut them off) 2) which of the two choices ride higher in the cylinder? (compression) I maean which pistion has the higher wrist pin height? 3)What does this raise the compression ratio to with a stock height 250 head? 4) Do the pistons need to be releif cut to clear the valves? Thanks you guys Have a good evening.
Slider

Higher wrist pin height gives lower compression, not higher compression. Maybe you need to buy Leo Santucci's book on hopping up Chevy sixes.

Hy sliderinwa, The 250 has a 3 7/8" bore like the 283 and 307, but it has a 3.53" stroke, the 283 has a 3" stroke and the 307 has a 3.250" stroke so niether piston would be appropriate, hope that helps.

The 307 piston has exactly the same size, weight, wrist pin diameter and compression height as the 250 piston. The only difference is that the 307 is a flattop, not dished, and raises the compression. I've built 3 250 engines using them, and they work great. They're a direct swap, with NO modifications necessary.
Jerry

HRL - good to see you back! Your knowledge base was missed.

Hot rod Lincoln. Thanks a bunch. I will buy the book as well. Please bare with me on these questions. I have the need to know, and if i can`t find it in the achrives i then ask. You fella`s are the best.

Hey,, cut Slider some husk.. he's from BENTON City by golly!! :p

Yeah, and we ain`t got any dad-burn red lights either! I love it here. Dry and hot part of Washington State. We`ll not to hot in winter, but dry.
I know the sbc rods are off set, and i don`t think i need any thing but press fit wrist pins. I will try and find out how much the compression is increased by going to the 283/307 pistons.After having worked on Chev v8, vw`s and Harley engines all my life i am totall excited about the inline six. Way cool to me. something new to learn. i guess is the word.Have a fun day. I have order the chevrolet inline six power building manual .

I don't recall the exact numbers, but the last 250 I built got 307 pistons, AND a .050" head mill, and it still runs on regular gas, with a stock cam, and doesn't detonate. I figure that's about the ragged edge of too much, though!
Jerry

Edit: The 250 rod length is WAY different than a V-8, so you'll need to use the 6-cylinder rods, not the smallblock V-8's.
Jerry

The way I understand it, the 307 is the piston to use because almost same as 250 except flat top and just a few thousands short of deck height. The 283 is not used because it has a shorter distance from pin to top of piston, therefore would be too short in cyl..

Am I correct?

Stock 250 piston is dished about .085" also.

What cam for street driving? Stock or a RV grind?

Thanks,

You've got the picture, Walt! The actual compression pressure will be highest with the stock cam, as it closes the intake valve sooner on the compression stroke and there's more volume to compress. An RV cam usually makes only a slight increase in duration, but it has a higher lift to allow for longer useful open time of the valves. RV cams generally develop lots more low-end torque than stock cams, without making much of a change in the top-end horsepower. If you want a stump-pulling off road engine, or you're hauling heavy loads on the highway, the RV cam is the way to go.
Jerry