Okay guys, I got my 390 motor back from the machine shop and he bored it 0.30 over. I have my camshaft/ lifter kit, seal kit, oil pump and all that ordered and it'll be here shortly. I waited to order the pistons until I knew what Ito was bored out to. So that being that, I've heard about 360 pistons being used in a 390. Can that actually be done? Asking cause the 360 flat top pistons are literally half the price compared to thelp 390 dish top. And I would also like to ever so slightly raise my compression.

 

Welcome to FTE.

Usually, you figure out what you need to clean up the bore. Say it is .030. Then you order .030 pistons. The guy boring the engine bores it to .030 and does a finish hone to the actual diameter of the piston plus the specified clearance.

So when you get your pistons have them fitted.

OK, the key spec for a 390 is the approximate compression height of 1.77". When they built pickup 390s, they used a piston with a 1.66" compression height. (from the Mercury 410)

Flat or dished? That depends on how much compression you want. So you need to figure out your compression ratio. What heads? What is the combustion chamber volume? Was the deck or head surfaced? How much?

In cars as OEM, the flat top was 10.5 and the dish was 9.5 "advertised".

Stockish or RV cam? The dish jobs might be the better choice. Lumpy cam? Try the flat tops.

There are dozens of build threads here so look around and ask as many questions as you like.

 

I should've stated it's a truck motor. Everything is stock been in the family since 73 ( it's a 71 f250). I sent the heads and crankshaft to the same guy that did the motor. He told me he " smoothed the heads and rebalanced the crank" this is my first rebuild and I'm trying to do it as cheap as possible and off of what mechanicall knowledge I have. And as the crank goes I ordered the enginetech es 643. Stock replacement with a 199 intake 192 exhaust durations

 

You certainly don't want to use flat-top 360 pistons with a stock cam, or at least I don't think so.

Your choices should be: dished 9.5 Car 390 pistons or dished truck 360 pistons. And even then, check compression height, combustion chamber volume in the head + dish volume in the piston, and figure out the static compression ratio before you do anything.

Don't take my word for it though, wait for more responses. It's early morning, and I'm writing this way too fast

 

I second the ~9.5 pistons at a minimum with that mild of a cam (-19 cc pistons, 1130 p/n).

I would have also kicked up at least a couple levels on cam, something in the 205*-210* .050 duration, 110* lsa range.

 

Previously on FTE:

https://www.ford-trucks.com/forums/1...r-a-390-a.html

Actual cam specs

https://www.rockauto.com/en/moreinfo...=5576025&jsn=3

 

If you end up getting a new flat tappet cam, make sure to use the correct break in oil and break in procedure.

 

This past summer I put together a 390 with 360 pistons (third one I've built like this since 1998) and it makes for a great-running, premium-only engine. This is the first one I've done with a stock replacement cam though (similar specifications as a "GT390").

This one is in a worn-out, dented up pickup that does fencing in the summer and performs plow duties in the winter.

I wouldn't hesitate to do it again. The best part is that if I want to make a hot rod engine out of it with a big cam and replace the 500 CFM 2V with something a little more rev-friendly, I don't lose too much dynamic compression.

 

Flat top or dish? The came in both flavors in 360s. So far no one has reported finding both in the same engine.

 

I would use 410 pistons then. But I would be leery of that machinist if he said he rebalanced the crank. That's not needed at all here And you can't balance it without the rods, rings, bearings and pistons in hand.

 

Flat tops with valve reliefs. I have never seen the taller compression height 360 pistons with a dish and I have taken apart literally (not figuratively) a couple dozen 360s.

Just guessing here but since even the "4V" 390 pistons with a similar compression height and the dish along with the valve reliefs give 10.5:1 per literature, the 360 pistons would be even higher. Thank goodness Ford gave the combustion chambers some quench to help stave off detonation.

I guess since Ford was the master of mixing and matching pistons in the FE family, it is possible to have them put 390 4V pistons in a 360 and end up with even less compression.

 

He doesn't want 10.5 to 1 flat tops in that truck motor with the stock cam.

 

He asked about the plausability of cheaper 360 pistons. I gave my experience (he asked about slightly raising the compression) with my current one with a stock cam. Yes, nearly 11:1 is too high for anyone not wanting to run 91-93 octane gas which I do in everything just because the stuff I get is ethanol-free and I refuse to put that **** through any fuel system of any of my vehicles/equipment. Also, I'm running a C6 and not pushing it hard.

360 pistons do-able in a 390? Absolutely. Is the recipe for everyone? No.

 

That said, the different compression ratios of the 390 are pretty steadfast when it comes to off-the-shelf cast pistons.