390 pistons for 360 rods????

[snake]

Is anyone aware of aftermarket pistons that area availabe for a 390 using the 360 rods? (I should say to convert a 360 to 390 without replacing the rods)
Maybe I'm missing something but bore and stroke determine displacement so I'm not sure why the rods need to be changed unless pistons aren't available with the correct pin height to get proper combustion ratio.......

[70_F100]

Yep, you're missing something.

Stroke is determined by the length of the throw on the crankshaft. Displacement is determined by the bore, the stroke and the number of cylinders.

360 and 390 have the same bore and the same number of cylinders. Therefore, the displacement of the 360 cannot be changed to 390 without changing the crankshaft and connecting rods to increase the stroke.

[snake]

Didn't I say that??? displacement is determined by bore AND stroke(and obviously the number of cylinder but I was leaving that out here since I'm specifically talking about two 8 cylinder engines). and yes, stroke is determined by the crank- not the rods.......

But the change in stroke will change the location of the piston in the cylinder unless you also change the pin height in the piston. So that's what i'm asking about....

[woods]

I don't recall an aftermarket piston for such a thing. You might get away with a 360 rod and a 390 truck piston (410 piston) with a 390 crank, using a thick head gasket.
You don't see this done because the 360 rods are pretty "spindly" compared to the 390 rods. However, if the 360 rods were a bit more beefy, this would be something that engine builders (the ones who know what they are doing) would be really into. A longer rod is considered a "good thing".
So yeah, it can be done, but it's going to take some fiddling around. Also, don't forget that if you are not going to bore the block, the cylinder at the top (around 100k of it) will be smaller than the rest due to wear, so that is something else to consider.

[woods]

As I am working along here, I had a recollection. When I was young...you know, back before the internet came out with an almost never ending list of stuff that can't be done. I distinctly remember more than one person putting a 390 crank in a 360 and not changing the rods. One of them was a friend of mine "Cliff".
This made me do some sort of rough "head math", which is not really my strong point, so I may be a little off. Now, Ford changed a lot of things on the same engines, so this is more of a "most common found" kinda deal.
The 360 connecting rods are 6.538 to 6.542 Yes, the vary that much, not from a book but from a guy who has measured a lot of them. The general consensus is that they all measure 6.54, so we will stick with that number for this.
The 390 crankshaft is 3.78 stroke and the centerline of the crank to the top of the hole is 10.17.

Now, if you used a 360 piston or a 390 car piston (same pin height) pin height of 1.759 You will end up with this...
6.54 360 rod+1.89 (390 stroke divided in half)+1.795 piston pin height. You get a total of 10.225. Since the center of the crank to the top measures 10.17, that puts your piston out of the top of the cylinder .055. There is nothing wrong with the piston coming out of the top of the block a little, but you would need a head gasket that measured around .090 to have an acceptable quench area.

Okay, so lets say you use the 390 truck piston with the 360 rods and 390 crank. The truck piston has a 1.66 pin height. So 6.54 rod+ 1.89 stroke+1.66 pin = 10.09. This will put you 8 down in the hole. So yes, with the right parts, it most certainly can be done.
The 360 rods are weak, but how weak? Are you planning on building a nitrous fogged 8000 rpm thrasher or are you going to build an engine that rarely sees 4500 rpm? The last one would deal with a good set of 360 rods in my opinion.

[snake]

Woods - thanks for the rundown on all that. That was basically what I was wondering but I didn't have all the right numbers to do the math. However, I didn't realize the 360 rods were not as beefy as the 390 rods. (I thought the length was really the only difference)

For my specific potential application, I'm certainly not doing anything super hi performance but just a good solid truck engine for towing a car trailer. But to put another twist on it - I've got a 428 crank that I was thinking I'd put in my 360 and ultimately end up with a 410. But I don't have the 390/428 rods - thus the question if I could do it with the 360 rod.

Any other thoughts/comments?

[woods]

If you used the 1.66 pistons with a 428 crank and 360 rods, I belive that would put your piston .022 out of the top of the cylinder. So a 60k head gasket would be what you needed in that case.

[basketcase0302]

snake,
You might be interested in this thread:
http://www.fordification.com/forum/view ... 28#p407953

[DuckRyder]

As Woods has pointed out it CAN be done (in addition people have shaved the tops off pistons to make it work).

In my opinion it is not the right way to do it and 390 rods are relatively easy to find, not expensive and much beefier than 360 rods so in this case CAN does not necessarily equal SHOULD.

Also, as I've posted in the past, if I were to go down this road again, I would go ahead and make a 410, that goes triple if I already had a 428 crank.

[snake]

Thanks guys - i found a reasonable set of 390 rods so its a non-issue now. But I appreciate all the thought and comments. I have the 428 crank so (assuming it will clean up ok) i'm about to head down the 410 path. I should start a separate thread on that to get feedback on other folks experiences - both good and bad!

Thanks again.

[woods]

Follow Robroy's engine build up.

[Manny]

Thanks guys - i found a reasonable set of 390 rods so its a non-issue now. But I appreciate all the thought and comments. I have the 428 crank so (assuming it will clean up ok) i'm about to head down the 410 path. I should start a separate thread on that to get feedback on other folks experiences - both good and bad!

Thanks again.

That is a good path and one where you need the 390 four barrel short height piston. The second one in woods "Trigonomfealgefordthunderbra" equations
That was something else my head hurt reading that......

[My427stang]

This is an extremely old post, but building a neat little 396 right now just like the OP asked way back when with all leftover parts

1.73 compression height Diamond moly-coated skirts, gas ported, forged pistons were accidentally ordered by a guy I met through another forum and he ran for a few months then just wanted to get the correct ones. Worked great for this shelf build

1.73 CH puts it at 396 inches with a 4.08 bore, zero deck with a 360 rod, 390 crank, and a 10.160 deck (.010 cut) The neat thing is the piston/pin combo is close to 650 grams, factory 390 is about 800 grams. That equals reducing every piston and pin by 3 tenths of a pound! Going to be 10.25:1, reconditioned 6.54 connecting rods with ARP bolts, balanced, hyd cam, 280 adv intake/284 adv exhaust, 230/236 @ .050, .530 lift, 112 LSA on 106. Streetmaster intake with a 650 to start but likely will want a 750.

I haven't got a home for this one yet, but it should be a stout and very streetable 396, depending on whether I do it as a spare motor for the truck, or a car engine to sell depends on the heads. It'll either get C8AE-H truck heads with porting and CJ valves, or a set of out of the box Edelbrocks.

Nothing like building a motor that doesn't have a home yet, but likely will dyno it behind a 445 in the works and see what the two do in comparison