Rider, your engine builder made a very common mistake. Ford lowered the compression in the 390 in the 70s by using leftover 410 mercury pistons. It was a quick and dirty way to get it done. What they should have done was to design a new D cup piston. But it was expedient to just replace the ~1.75 tall pistons with the ~1.65 tall pistons from the 410 mercury. At first they used actual leftovers, then, when those ran out, they renumbered the mold and kept making the 1.65 tall pistons. So most of the later 390s come from the factory with just over 8 to 1 compression. If a rebuilder choses a piston that matches the truck piston, and then uses composition head gaskets -.041 thick, instead of shims like the factory, -.017 the result will be 7.6 to 1 compression. The cure is to change pistons. But getting the pistons you need can be difficult. There were thousands of the low compression pistons made. When you order pistons for a 390, the low compression slug is almost always what you get, regaurdless of what you ask for. I've had wharehouses offer to give me the low compression slugs for free if I would buy my gaskets and other stuff from them. I declined. The pistons you want are 390 car pistons. There are several part numbers you can use, but you usually have to know what number to ask for. You can also use the 360 piston.
The other option, if you don't want to replace the pistons, is to replace the thick composition gaskets with shims, and to do a heavy .060 milling on the decks and intake surface of the heads. Milling the decks of the block would be better, but the engine has to come out for that.
DinosaurFan
Last edited by dinosaurfan; 10-02-2009 at 08:15 PM.
Reason: spelling, fat fingers
Buy a set of stock replacement piston for a mid 60's engine.
How do I attatch a picture? Grizzle33 has a picture of his 360 rebuild that has the exact same issue (top of Piston below deck) I copied it, I hope he doesnt mind. Heck, this may help him as well.
There is no help for the 360. The piston heights and rod lengths don't work to correct the down-the-hole piston. See the link in post 18. Some good comments on that thread from Blue Oval Performance.
Thanks guy's! That is the weak point of my engine,,the 390 with the low compresion pistons. I'll replace the block (.060 over) and pistons next year. Can you provide a P/N for about .030 9.5:1 pistons. To me that's a little low, but hey, I'll take what I can get. 10:1? anybody... Better than what I have.
Just wanted to say this is a great thread. Good info. Shop for bolt on goodies sounds less attractive every day unless you build the engine to take advantage of it. Worth paying attention to what you guys are saying.
Thanks.
Just wanted to say this is a great thread. Good info. Shop for bolt on goodies sounds less attractive every day unless you build the engine to take advantage of it. Worth paying attention to what you guys are saying.
Thanks.
GREAT thread if one understand's the info given...I agree with ya.
I, well, err, um started this thread Thanks redmanbob for the P/N's. I'm trying to look up the specs now. If you have them already, that would be awsome!
Egge will make you custom pistons to your specs, but these are identical to the ones in my 390 4V Mercury. I got a set of pistons through ebay and noticed that when aligned the wrist pin would not pass true. I got some help here and found these L2291 factory replacement items. But as mentioned Egge will be happy to put the pin wherever you would like.
Riderman I have been looking at these silvolite pistons and here is some nice 390 choices. This is page 50. click next page and there are more. Sometimes these are for sale on EBay for decent. I don't know if this helps.
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common mistake
Better than what I have.
Thanks redmanbob for the P/N's. I'm trying to look up the specs now. If you have them already, that would be awsome!
