Hello,

as mentioned some threads before, I'm restoring a F100 from 65, powered by a FE 390 from 68.
Measuring the pistons and checking what is available on the market, made my wonder.
Now what is in the engine:

• crank shaft marked: "2U"
• pistons marked as: "3.90 // 2V//FoMoCo // made in US//Sa 36//C8 AE 10"
• connecting rod marked as: "C7AE-b"
• heads marked as: "C8AE-H"

I found some pistons at rockauto.com from Sealed Power (SEALED POWER 381NP) with 1.660" CD. The old pistons have 1.729" CD (measured from middle of the connection rod bolt to the top of the piston. So it seams, there is something wrong.

Can someone give me a hint, what pistion might be the right one?

Thanks and regards,
Joachim


Old piston and connection rod

 

That piston, does it have a slight dish to it with valve reliefs?

Your piston picture shows "390" and "2V". That would seem to indicate "390 with a 2 venturi carb", a very popular car engine in 1968, I say car engine because the compression height you list is approximately what is listed for car pistons.

In pickups, as OEM, starting with the 1968 model year, a 390 was also available. However, those engines used a different piston (from the '66-7 Mercury 410) that had a compression height of approximately 1.66".

Ford did that to lower compression due to the anticipated harder usage a pickup engine would see vs. a car engine.

Unless you are going tow maximum loads, you can go with the car spec piston at 1.77" compression height. The slightly dished version would keep compression in the 9ish area, the flat tops 10ish, all depending on your combustion chambers, gasket, and the exact assembled dimensions of the rest of your engine.

Almost every thread in this forum dealing with rebuilding a 360 or 390 goes into this discussion at some point, (The 360 uses the 1.77 compression height, again to result in a sunken piston and lower compression) So check out a few threads with those rebuild titles for more info.

 

Thank you very much. This makes sense, as the oil filter is mounted the way, it is in car engines.
Here is a pic from the piston top view.

 

I went though the same questions and found a nice set of forged Speed-Pro flat tops. Have your rods reconditioned, put some ARP rod bolts on them and pick a set of 1.77 compression height pistons for your final bore. I have them same heads as yours too. We put in some stock size stainless steel valves from SI and cleaned up the runners with a set of cartridge rolls and a die grinder. With the 8554 Fel-pro head gasket, you should be right at 10.1:1 static compression. Now your next concern will be cam selection to deal with the raised compression and getting the proper quench.

 

I've run this combo with Crane's 272* Energizer and it was fine on 89 octane fuel. Also in a GT390 with an older Cam Dynamics 290-something degree cam with .516 lift and it too was good with 87-89 fuel.

 

I'm running the Comp XE274 in my truck. I figured on running some of "the good stuff" at the pump just to be safe. It's not going to be a daily driver anyway.