Hello,

I have just begun a project to put together one good 390 out of a complete 360 and a 390 short block I bought for $150 with the understanding that it had just been rebuilt. This is for my old '73 F250 which I haul my camper on.

The 360 was leaking oil into the prestone, not vice versa, so I was thinking about doing the gallery "bypass" surgery until I saw the 390 short block. It has high compression pistons which I figured on swapping out anyway, my plan being to use the rods, crank, and block from the 390 block, and the pistons and heads from the 360. BTW, the pistons in the 360 are the slipper-skirted kind which Steve Christ's book shows to be the 390 type.

I thought I had a pretty good deal going until I got into it. The crank on the short block looks good and the bearings do look new. All but one cylinder look good as well. Apparently the oil ring on #5 was not seated properly and left a noticable groove in the cylinder wall in the short time it was in use--500 miles I was given to believe. The groove is about as wide as my finger and maybe about .0010 or .0015 deep if I can trust my gauging ability. I started honing but stopped before getting too out of hand. The shop manual says that if I need to I should be able to bore out just the one cylinder and get say a .040 or .060 piston. The others are all .030. Another procedure described in one of the books is knurling. Then there is the sleeve idea. Any advice on when to do which?

Then there is the cam. The short block 390 supposedly has a radical cam suitable for mudding but not necessarily for towing and hauling. The cam in the 360 worked ok in most situations, low rpm pulling power being what I am after, so I'm inclined to use it in my Frankenstein engine. But how do you tell what kind of cam you have? There are numbers of course, but I haven't located any cross-reference tables for cams. Any tips would be greatly appreciated.

 

First off, welcome abord!

Secondly, let's check a few things. You have no intentions to bring the block to a machine shop? Why do you not want to use the 390 pistons? Is the 390 block bored at .030 to accept your +.030 360 pistons? Using the 360 pistons may work, but you will want to check your clearances and moderate your compression ratio as necessary, they are usually close tot he same specs as the high compression 390 pistons. If the cam has some numbers, post them up. Some of the crew have those embedded in their noggins and can identify a copmany for your to search.

What I would do. Put a list together of what you want to use and work from there. Take the block to a machine shop and have them do a "polish" for a few $$, then order up the necessary parts needed to get everything back together. Or, take your 390 block and have it bored another .010 and get new rings to compensate.

Just a few $.01's --Mike

 

Here's what I would do:

Use the 390 crank and rods.

Use the 390 pistons in the 360 block, but have the machine shop bore the 360 block .030" over and have 'em fit the pistons to the bores. This will make for a nice quiet engine. Dont forget the oil leak repair.

Dont use the 360 cam. In a high compression 390 it will ping like crazy. There are plenty of better cams out there, your 390 may have one in it. Post numbers. You will need new lifters. When you put a cam in a different block, the lifters will move slightly in relation to the cam.

Use the 360 heads.

 

Hello again.

Thank you Mike and Rusty for your replies to my first post. Briefly, I took both blocks to the machine shop. The 390 is still there having a sleeve put in. The heads from the 360 are also at the machine shop.

I looked over the two cams, both of which appear to be in good shape, and have the following numbers:

390-- F25 SF63 C1 D1 CC CMC
360-- A[upside down U] 4

I assume the 360 cam is stock since it also appears to have FMC logo cast into it.

Also, I am trying to identify the pistons I still intend to use, which were in the 360. The number on the inside is 1131 92.

I have been looking elsewhere on the forum for more information on cams. I gather that there may be an optimum cam/piston combination. If I have to get a completely new cam, what might be the better ones to get to use with the pistons mentioned above?

TIA, Paco