360 to 390 conversion

[bluemoose]

I remember see some great info on converting a 360 to a 390 somewhere on this site. I did a few searches, but they were a bit too broad. Anyone know what I'm talking about?
I am considering converting and need a few answers. Most specifically what I need for new parts. Is it the rods only? Will I need to have it bored because of the increased travel? Can anyone suggest some kits that can be used for this? I just don't know what I don't know.

Thanks

[forrestbump]

Assuming you already have a 360 FE, all you'll need to convert it to a 390 is a 390 crank, rods and pistons...That's IT!!

[bluemoose]

Forrest,

But do I need to bore the cylinders? I assume if the pistons have been traveling the distance a 360 does for 150,000 miles and now are going to travel the distance a 390 does (sorry, I don't have the terminology here) won't there be a compression or fitment issue in that additional travel?

[Negative]

From what I have understood, the FE engines (360/390) are the exact same. The difference is the amount of travel in the valves and pistons. The 390 crank moves the pistons further down to allow for the extra 30cu of fuel, and the rods push the valves open longer to allow full filling/exhaust. I haven't heard that you need new pistons, but I could be wrong. I will check on that for you as well.

<edit>
From what I just researched in some of my old bookmarks, the need for 390 pistons is dependent on your block casting numbers. You will have to get those numbers and see (is all based on the block and piston deck height). This part I can't help you with, except you will need to maintain a .010 clearance.

[forrestbump]

So your 360 is together and running...OK, Got It!

When you're ready to do your 360 to 390 conversion, you'll need to start by dissembling your 360. YES, you'll probably need to over bore your block, but then again, you just MAY get lucky and find the bores will only require a honing.

So after the condition of the bores are determined THEN you'll simply get the correct over size pistons for your new 390.

If it were me, I'd start looking for a good 390 crank and set of 390 rods as you'll probably end up getting new pistons anyway due to having to over bore your FE block.

[forrestbump]

Continuing on with "What Is The Difference Between a 360 and a 390", 360's are "Long Rod" motors, whereas a 390 has a shorter rod in it. Confused? Don't be...it's wrist pin placement in the piston, connecting rods and crank shaft that makes all the difference between them.

[fireguywtc]

The only reason you will need to bore it is if you need to bore it during a rebuild to clean up the cylinder walls. A stock 360 and 390 have the same cylinder bore size. The large displacement only comes from the longer stroke.

[bluemoose]

So your 360 is together and running...OK, Got It!

No it is torn apart. Well the top is off and everything else except the pistons and cam. I haven't have the opportunity to get them out as I still need to flip the engine on the stand. The cylinders do not have the hone marks, but are in really good shape. No deep scratches and the lip at the top will catch a fingernail in only one cylinder. I would love to get away with just honing. My concern is that if there is wear there, there is not going to be wear where the the additional stroke of the 390 will be. Am I not understanding this correctly?

[forrestbump]

A dial bore gauge will be required to determine the concentricity (roundness) of each cylinder bore and a micrometer to determine bore wear. These measurements will let you know whether or not you'll need to bore your block. Compression rings are what wear out the bore and where they ride in the cylinder is the same for 360 or 390 pistons.

...you're getting it and NEVER be bashful asking questions, it's how everyone of us learned.

[70shortbox]

Ideally you would want to get it bored over, but plenty of people get away without it. It's best to measure the bores with a dial bore gauge or some inside mics. A used piston and a set of feeler gauges can tell you roughly the taper and out of round. Good and consistent piston to bore clearance is one of those things that make some engines run much better and longer than others. On the other hand it's not like a poor rod angle pos like some brand c stuff in the same application type. 5 or 6 thou is about the most I like to see.

[gregamorton]

HOTRODFEGUY so I can use a set of 360 pistons to up the compression on my 390? I guess this is because the piston has a higher up lip on the top of the piston? I wonder if this would be a issue on using higher lift cams than stock?

[onetuford]

So your 360 is together and running...OK, Got It!

No it is torn apart. Well the top is off and everything else except the pistons and cam. I haven't have the opportunity to get them out as I still need to flip the engine on the stand. The cylinders do not have the hone marks, but are in really good shape. No deep scratches and the lip at the top will catch a fingernail in only one cylinder. I would love to get away with just honing. My concern is that if there is wear there, there is not going to be wear where the the additional stroke of the 390 will be. Am I not understanding this correctly?

The shorter rod length compensates for the height of the piston. Oh, and to dispell another myth I've just read, the wrist pin locations are the same for both engines. I've seen this same question asked a bunch since I've joined some good answers, some not so good. Do you have the donor rotating ass'y you plan to use? The easiest route would be to to get 390 core. But then your gonna probably need a rebuild anyway, so look for a running 390 to just swap in and keep your 360 block for a spare.

*edit* I just re-read the entire thread. I think Forrestbump was refering to compression distance, which would be different based on application, so in that case the info is correct. As far as a kit goes I've only seen 428 or larger crank kits. Very pricey. Check out Eagle machine for a stroker kit.

[onetuford]

The site to check for the kit is coasthigh.com. Once a while back I posted that 360 truck slugs could be used in a 390 and I got a verbal spank and the thred was locked. I have since pulled apart both engines and saw NO difference in the skirts. My older brother tells me stories from the '80's of doing the truck piston swap with much success.

[billbates40]

So as far as 360/390's go, I'm guessing that headers on a 2WD 360 will also fit fine on a 2WD 390?

[forrestbump]

YES!