390 piston

[elvis67]

HI guys ordered 390 rebuilt kit the piston they send me are silvolite 1139 .it say on the box 390 pin to top is 1.66 . but I measure my old piston pin to top is 1.75 and im pretty sure that my 390 was never open it is the original piston .please help me understand is it possible silvolite is not correct . IIsaw 1131 as a .1.75 height

[elvis67]

or is it because my 390 came out of a car and they send piston for 390 truck engine is there difference between a car and a truck 390 engine ??? and that bring me to a other question if the piston height is lower 1.66 the ratio is not the same right? why would the truck 390 as less power? than the car engine excuse my ignorance hope you understand my post Im new in this

[two-bit]

"The 390 truck engines used the 410 piston and the 410 is prominently stamped on them. Passenger car 390 pistons, 2V or 4V, are 1.76 or thereabouts compression height (the 390 4Vs being also used in the truck 360s), 410s and truck 390s are 1.66. That's how they lowered compression in trucks without stocking new pistons."


I stole this info from somewhere else.

Yup.... they used different pistons car vs truck. When you ordered the pistons, did you tell them car, or truck ?
Wrist pins are the same, and yes, the truck engines ran lower compression than the car engines.
What you have will work, you just won't have the compression ratio that you are planning on.

"The devil is in the details."
Good luck with it.
Two-bit

[elvis67]

thank you very much two-bit for your reply
I order a new set of piston 1.75 height for more compression probably less mpg but more power . just hope that the camshaft will still be good for it melling sby-23 cs-643

be running c6 trany 3.25 axe
69 small box f 100 sorry for my writing Im frenche frog

[two-bit]

No worries about the writing.
I met a incredible German woman, at least she spoke German, when i was at Rammstein AFB in 1993. I was enroute between a rather ugly situation in the middle East and getting to Walter Reed Medical Center in the states. Her english wasn't that good, and my German was even worse. Couldn't speak much anyway, so i didn't really matter. The intention of what she was trying to convey was more than enough. To this day i have no idea her name or where she is today. What i do know, is that even broken language, can convey thanks, and compassion.
Got a little sentimental there....

Good luck with your build. Be careful of more than .500 lift on your cam with stock valve springs. They don't like it.
Glad i could help.

Two-bit

[Lee]

Hi,

Back in 2003 I took my 390 to a shop to have it rebuilt and they installed 1139's in it.

I then researched these pistons and found them to only have a CR around 7.7:1 with composite head gaskets!!!!!! This is ridiculous!!!!!!!!!! No power!!!!

I re-pistoned it with 1130's to get closer to 9.0:1 CR and can still run low octane pump gas while pulling a 7klb trailer without detonation.

Lee

[Kirilo_68]

First time posting and first rebuild so bear with me. Sorry if this should be a new post/thread; I have a similar question regarding new 390 pistons.

I'm rebuilding my 360 and converting it to a 390. I have the crank and rods are on the way. My goal is to increase the compression ratio some, and not exactly sure what pistons to get. The cylinders will be overboard 0.030". I've researched mainly Silvolite and have a question on the compression height of the piston. If I keep close to stock height of 1.66" (for 390 truck) I'm calculating a compression ratio in the high 7s (7.96:1) or low 8 (8.2:1). I would like to be in the low 9s and to get there would need a piston with a 1.76" compression height (Silvolite 1131H), which is close to the 360 pistons. However, I'm calculating a piston to deck height of 0.029" using this piston. Is that okay? I'll be running the stock cam with rebuilt heads. What octane can I use with this CR and deck height?

FWIW the pistons I pulled out were all stamped 390 and had slipper skirts. I'm going to have the machine shop measure their heights as I didn't do it before hand.