Thread: Turning A Ford 360 INTO A 390

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I have a Ford 360 in a 1973 Ford F100. My plans are to make it a 390. Can you install a 390 crank and rods to achieve this? Also will 360 pistons work for this set up or will I need 390 pistons as well?

Just need the crank and rods, the pistons will work. Most 360 pistons say 390 on them.

How would it do if I used a 390 crank with the 360 rods and pistons?

Originally Posted by hphillips

How would it do if I used a 390 crank with the 360 rods and pistons?

Let me try to help you out with your education......

Half the stroke plus the rod length plus the piston compression height equals the "stack" of parts you will bolt into the block and must equal or come very close to equalling the block deck height.

BLOCK DECK HEIGHT: distance from the centerline of the main bearing bore in the bottom of the block to the flat surface of the block where the heads bolt on. There may have been other block deck heights on FE motors through the years, but the one I'm familiar with is 10.170". Therefore, the stack has to come somewhere close to 10.170". If the stack is greater than the block deck height, then the piston will hit the bottom of the head before it ever gets to top dead center (unless you use a VERY thick head gasket). If the stack is much less than the block deck height, then there will be too much space between the piston and the underside of the cylinder head to promote a good squish to quell detonation.

SQUISH: the distance from the crown of the piston to the underside of the cylinder head with the piston at top dead center and with the head gasket in place.

PISTON DECK HEIGHT: distance from the crown of the piston to the block deck where the heads bolt on with the piston at top dead center and without the head gasket in place.

COMPRESSION HEIGHT: distance from the centerline of the wrist pin bore in the piston to the crown of the piston.

STROKE RADIUS: 1/2 the stroke.

ROD LENGTH: distance from the centerline of the rod bearing bore in the big end of the rod to the centerline of the wrist pin bore in the small end of the rod.

360 pistons have a nominal compression height of 1.776". 360 rods have a nominal length of 6.540". Stroke radius of a 360 crank is 1.750". Add these together and we find that the stock 360 stack is around 10.066" Deducting 10.066" from 10.170" (block deck height) leaves the piston down in the bore by 0.104" with the piston at top dead center.

Swapping to a 390 crank that has a radius of 1.890", using 360 rods at 6.540" length and 360 pistons with a 1.776" compression height will make a stack measuring 10.207", which is 0.037" taller than the block deck height. You could do this OK if you used a head gasket that is about 0.080" compressed thickness. That would leave a 0.043" squish clearance between the piston crown and the underside of the cylinder head with the piston at top dead center. This is not a combination that I might put together, I'm just showing you that it is possible to do with the right head gasket.

Now, here's the fly in the ointment. With a used block, there will be a ridge at the top of the bore where the top piston ring traveled almost to the top of the bore, but not quite. So, now that you have a longer stroke radius, the top piston ring will travel farther up in the bore. You'd have to ridge ream each cylinder to remove the ridge to prevent breaking the ring.

If you read everything I've written here and don't understand it after considerable study, then trust me, you have no business swapping parts around in the first place.

I'm certainly no FE expert, so I'm sure one of the sharper Ford guys will correct me if this is incorrect. I'm just the math guy and enjoy crunching numbers.

I understand fine. I just didnt have access to the specs of the internal parts. Thank you for the info.

I won't say all but a lot of the 360's were built with 390 pistons and both 360 and 390's use the same rods.
You can build a 390 from a 360 by just swapping the crank but you do have to deal with the ring ridge at the top of the cyls.

Originally Posted by REM

both 360 and 390's use the same rods.

Not true. Nominal length on 360 rods is 6.540", for 390's its 6.490".

I guess I'll never understand why people would want to do that involved swap - there are enough rebuildable 390's(or even some 428's) available, even 335Hp GT motors that have even better heads and internals then the original 360 gas guzzler, no cojones motor!!

Originally Posted by IC2

I guess I'll never understand why people would want to do that involved swap - there are enough rebuildable 390's(or even some 428's) available, even 335Hp GT motors that have even better heads and internals then the original 360 gas guzzler, no cojones motor!!

Probably the same reason why there are still guys who want to put a 400 stroke crank in a 305.

Originally Posted by IC2

I guess I'll never understand why people would want to do that involved swap - there are enough rebuildable 390's(or even some 428's) available, even 335Hp GT motors that have even better heads and internals then the original 360 gas guzzler, no cojones motor!!

Just keep in mind the GT 390's used the same heads that were on the milder 390, 360 and 352's. C6AE-R, C7AE-A and C8AE-H.