pistons

[djburton]

Making a 390 out of a 360. Upon disassembly of the 360 (99% sure it's never been touched before),we found these pistons...6 marked 360,1 marked 3904V, 1 marked the elusive 3604V. Pistons all the same. Thoughts?  Anyway, it's a budget deal,block really decent standard,nice reconditioned 390 crank and rods. The 360 pistons are about .035 in the hole. Will that work with D2 heads?  Thanks!

[My427stang]

If the dishes and valve relief are the same, flip them upside down and check pin height.  Easy to do on a hard surface, line up pins and see if they are the same from one to the next, building a bridge

If it's a 1.76 compression height piston (for all eight) I would expect it to be 10.138 or so total stack height, which would leave you right about what you see with a 390 crank.  .035 in the hole is pretty deep, many have run there but compression will be low and it needs to be because quench will be poor. 

If it's a budget engine, which I assume with used pistons, and you want to get around, it'll do OK assuming you don't put a cam too early or small in it and watch the timing curve.  I did one like that as a temp motor and it ran very nice, not fast but a good runner.

If you wanted to get a little fancier, cut the deck to 10.150 and run a Felpro 1020 and you'd be closer to .057 quench, that's about the top of good quench, but not bad quench either.  Then cam accordingly

[DuckRyder]

What Ross said.

The 3604V is a new one, and is there something slightly different under the pin boss area? Casting flash or something?

Anyway, been a while  (10-15 years?) since i messed with this but IRRC 360 piston ARE 390 car pistons, 360's used a slightly longer rod but they were still way down in the hole which is how they got the dismal 8.2:1 ish compression ratio. Truck guys have used the 360 rods and pistons with a 390 crank to get higher compression. Incidentally 390 TRUCK pistons are 410 pistons. again way down in the hole to get low compression.

Somewhere i have picture of some of the various speed pro/sealed power offerings with the pins interlocked and it's pretty obvious.

[djburton]

Ross, pin height and crowns are the same on all 8. It is a temp motor till we can come with something more exotic. I'll try to talk the owner into cutting the deck. Duck...Yeah,there is a slight difference in that area. 360 pistons AND rods on a 390 crank?

[DuckRyder]

,,,360 pistons AND rods on a 390 crank?

yup, I've never done it and don't advocate it, but a lot of the truck guys are cheap... allegedly puts it slightly out of the hole on an uncut block which works with thick rebuilder gaskets and low RPM and small cams and valves.

If you search the old forums you probably find more than one and most of them running the old Crane 901 or 941 cams... It was an oft repeated recipe... note oft repeated and good idea are not the same thing.

Anyway...

[My427stang]

If you can stay at or below deck with the 6.54 rod, may as well in a low power build.  However that's .050 difference, I doubt you can. 

Whenever I did that, I used 390 rods and crank with the truck piston.

[frnkeore]

The difference is actually .052. 6.488 vs 6.540.

It can be done but, you have to be careful and mock the assy up, in the block. The blocks can vary on their deck ht. My Edsel block was 10.172 at the lowest corner and went .020 over that at the highest corner.

The math goes this way:
1/2, 3.784 = 1.892 + 6.54 + 1.760 = 10.192. .022 out of the hole. Add a FT 9061 (.061 thick) head gasket and you get .039 Squish.

Most rebuilder 390 pistons are 1.760 but, some sell the 410, 1.660 piston as a 390 too.

[djburton]

"A lot of the truck guys are cheap." LOL! Kinda sums this whole deal up. I guess I'll stick it together as planned and see what happens.
Thanks,gentlemen!