I'm rebuilding my 390 since I had a dead cylinder (broken/melted ring land) and the other cylinders had pretty low compression. The engine looks like it was rebuilt recently, but likely didn't set the gaps right. The heads are in good shape with a nice valve job still present, I might go ahead and lap the valves and install some new stem seals. Basically this is a truck engine, just for towing and possibly the highest fuel efficiency as possible. It has just a basic Comp HE 252 cam and edelbrock carb, so nothing out major. I figured I would condense all my questions into one thread instead of starting many of them:

-Right now it has 0.060 over pistons, the bores are just within the taper and out-of-round limits so I could just hone it. The trouble is the pistons I have I don't know the brand so I can get a new one. They have a weird groove pattern on the skirts I've never seen before, but no brand name.
Any ideas on where I could get another one just like it?

-Also if I do decide to bore it out more it seems like the only option is the std size 4.130" 428 pistons, which would give me different compression ratio. The largest 390 pistons I could find are these 0.060 over, couldn't find any 0.070 or 0.080 over pistons. Should the 428 pistons work or is there other options out there?


-On the heads and intake manifold there is a port that looks like an EGR port, but where the carb bolts to there is no opening for it? Where does this port go to in the intake manifold, seems like it's just a dead end?

 

On most 360/390 blocks, +.060" is about the max you can safely bore it without having the block sonic checked first. Personally I like +.040" or less.
I would replace all eight pistons and use the higher compression passenger car pistons. These have the wrist pin location 0.010" lower which raises the piston in the cylinder and will bring the compression up to around 9.5:1
The passage you see in the heads and intake is the exhaust crossover passage. If your running headers, the passage serves no purpose, but if you still have the exhaust manifolds, the drivers side has a heat riser valve sandwiched between the exhaust manifold outlet and exhaust pipe. When the engine is cold, this valve is shut and forces the exhaust from the driver's side through the cross over passage under the intake manifold to the passenger side. This warms up the carburetor quicker and improves cold weather driveability.

 

Thanks for the quick reply. So the passenger pistons and the truck pistons are the same besides the piston pin location? So 9.5:1 would be safe for just towing, was the stock truck CR 8.5:1? What is the compression height of the passenger pistons?

I have headers installed, so I guess that clears it up.

Here are the piston skirts, any idea on the brand? I would rather just buy one piston than a whole new set.

 

The original pistons had a dish and a compression height of 1.660" the static compression was 8.2:1 more or less, some say as low as 7.8:1. The passenger car pistons were flat tops with four valve reliefs and a compression height of 1.760". The books say 10.0:1 compression but in reality it's more like 9.5:1
In the Silv-O-Lite brand, the low compression versions are #1139 and the high compression are #1131.
I haven't seen pistons with skirts like you have before.

 

Thanks for the information. Do you think 9.5-10:1 CR is good for a truck engine that will tow 4000 lbs+? I'm used to Honda engines where as high as 11:1 CR is stock, so I don't know limitations for these engines (as far as fuel and ignition timing). I'm going to CC the combustion chamber today to see what compression will actually be to make sure.

 

Running premium fuel you'll be okay. You might be able to get by with mid-grade.

 

The combustion chamber was about 70cc, using the flat top pistons I get a compression ratio a little higher than 10:1 depending on the dish volume for the valve reliefs on the flat top pistons. I plan on getting just a stock replacement Fel-pro head gasket so I figured compressed thickness to be about 0.038, does that sound right?

 

That's probably close. As a rule of thumb, without measuring, I use .040" for the compressed thickness.
I was looking in the Silv-O-Lite catalog and they do list an +.080" oversize for the 390. It's # is 1130-80 and lists a compression ratio of 9.5:1. It has a 1.760 compression height and a dished head. The dish is 3.00" in diameter and .147" deep. It also includes four valve reliefs.
Are you including the deck height in your calculations?

 

I did a few calculations using different information, some with details like compression height, deck height, and rod length. They all ended up at about the same number.

I will have to check out those pistons.

 

I have a 360 that I'm rebuilding and puttin the 390 crank and rods in and the machinist that I go to said to put 360 pistons in that it will bring the compression up to around 9.3-9.5 to 1. I don't know if y'all done somthin like that .