The Stovebolt Site Forums The Shop Area Engine Shop 250 Compression With 307 Pistons

Im working out a parts list for my 250 rebuild. Has anyone used 307 pistons in their 250? What would the compression be?

Thanks!

The 250 and the 307 use the same piston.

The 250 piston has a depression in the top that's almost 1/4" deep, and about 2" in diameter. The edge of the 250 piston also has a 45 degree chamfer. Most of the 307 pistons are flat top. The last 250 I built had a .030" overbore, 307 flattops, and .050" milled off the head. It still runs well on regular gas, with no detonation. I was probably on the ragged edge of needing to go to mid-grade gas, but the engine has been in service for several years with no problems. I built two others with 307 pistons, but did not mill the heads. The milling job adds a little low-end torque, but they all ran about the same at cruise speed.
Jerry

I was unaware of the dished piston. That would make a difference. I knew the bore was the same. Good info Jerry.

The piston diameter, weight, compression height, and wrist pin diameter are all the same, so the 307 is a direct swap with no modifications necessary. I discovered that about 20 years ago when I was ordering parts for a student's 250 engine rebuild. He had a lead foot, and those 307 pistons turned his little 2-door Nova into a real hotrod!
Jerry

Thanks for the information. Any idea what the compression ratio is when using 307 pistons and un-milled head/block? I want to keep it under 9.5:1. This engine is intended for a daily driven car so low end torque is a must.

Also thinking about screw in studs and guide plates. Any recommendations?

Just installing the pistons without a head-milling job should keep you under that figure, but it's been a long time since I did a CC test on that head.

If you go with the screw-in studs and guide plates, be sure to mill an equivalent thickness off the stud mounting boss to keep the top of the stud in the right place. DO NOT attempt to thread the bosses by hand- - - - -the studs will end up angled all over the place. I use a valve seat & guide machine to mill and thread the bosses. Unless you've got a well-equipped machine shop, better sublet that job to a good automotive machinist.

You'll need to remove about 3/8" of metal from the top of the stud boss to accomodate the thickness of the bolt head on the stud and the guide plate. While you're at it, use a set of rocker arm nuts with the Allen-screw positive locking feature. A lot of V-8 speed parts can be used, with just a few leftovers because you've got 6, not 8 cylinders.
Jerry

I can't verify it but I've read somewhere that it's around 9:1.

will the 307 flat top piston work on the 292 I'm going to be building one and want a little more than stock I have the a book but they never say any thing about 307 pistions.

No, the 307 pistons will not work in the 292. The only off the shelf piston that will raise compression are Federal Moguls LPG pistons. The LPG pistons will get you about 9:1. IIRC the part #980P.

Larry