I bought a set of these really nice pistons for inline 6 250, there are more. they are well built and don't have the bevel on the edge, nice and flat and D is shallow
http://www.ebay.com/itm/250-CHEVY-GMC-PI...=item25a7cdf5f8

Those are just standard 8.5:1 250 cast pistons, correct?

These would be a little bit better ones to get to bump up the compression a bit.
http://www.ebay.com/itm/307-CHEVY-PISTON...870&vxp=mtr

Flat tops, w/4 valve reliefs.

Or these, if pic is correct. http://www.ebay.com/itm/Federal-Mogul-TR...39f&vxp=mtr

MBHD

Those are .030 over according to discription and they look to have a shallower dish to.

To my knowledge, there are 8:5.1 pistons or 8:0.1 pistons, they did not make a 8:75.1 or a 9:0.1 piston.

With production tolerances being not so great, if you had 8:5.1 pistons, you would be lucky to have an 8:0.1 compression ratio, pistons being down the bore, chambers sometimes bigger etc.
Plus todays head gaskets are not thin as they used to be.
.038" & thicker as compared to the old style ones of what, .015-.020"?


This is all I found about the description, other than .030" over.
"Product Details
Sub Type: Engine Piston
Notes: Description--Engine Piston "



MBHD

When I was looking for pistons for my 153 I ended with a few sets. None had the advertised pin height. It seems that in order to control compression in bored engines the manufacturer reduces pin height. So the 307s should have been zero deck for me but weren't. I did get a set of 307s with only 2 valve cuts.

The ones Vman got are better than most because they don't have the chamfer around the top and the D does look shallower. The price was right. So if he is looking for some low buck improvements for a mild engine all he needs is a 194 head.

A man who thinks as I do.

MBHD

Now you are scaring me.

Yeah I got some of the 307 pistons also and used them on my last rebuild.
I'm thinking of checking the actual volume of the Depression on the 250 pistons and comparing that to the 4 valve cutouts on the 307 pistons. Alot of the 250 pistons on ebay have the big beveled edges and deep D shaped depressions on the top. these 250 ones I found have a very shallow depression. I dunno

On my engine I had the deck of the block shaved down until the 307 pistons stuck out a wee bit. Then with a .040 gasket I ended up with about .035 clearance on the head. seems to work fine , been running well over a year. Just street driving.

I bought a couple of mercruiser 250 inline heads on ebay and they have 72. something chambers on them, real nice one of them was barely used, still had tight valve guides, I ground out the valves to fit 1.88 intakes. worked really nice. did it with a vintage valve grinding set by hand. Did a three angle cut on seats and added stainless vales, read that the stainless valves work will with cast iron seats.

Vman, can you measure the depth Of the d shaped dish and the pin height of those e bay pistons please? Jay 6155

OK sure I will measure those tomorrow and let you know.

OK here are the measurements.
From top of piston to top of pin 1.173
Compression height from center of pin to top of piston would then be 1.636

Depth of the D depression in top is .077

Thanks for taking the time to give us the measurements! Jay6155

Just as a comparison. The flat top pistons I sell for 250 have a 1.67 CH and allow decking of .015 from block for zero deck.

The piston with 1.636 CH will be way down in the hole for sure.

Chevy and or GM has been using stainless exhaust valves from the factory since the late 1930's