Crankshaft swap

[Mcross]

Hello group

[Mcross]

New to the group. Looking for info about swapping a390 crank into a 365

[Roger Zimmermann]

I believe it's a no-go: the stroke is longer on a 390; the original pistons will be too tall! A friend of mine bought the wrong crankshaft for his '58 car; he had this issue. Had to buy the right crankshaft.

[The Tassie Devil(le)]

Swapping the crankshaft into anything that gives a different stroke, the main changes that MUST be completed is Connecting Rods and Pistons will need changing if everything else is the same, as in bearing sizes and possible clearance issues due to crank throws, etc.

With the pistons, the Gudgeon Pins position will change, as will the length of the Connecting Rods.

Plus, the reciprocating mass will require rebalancing.

Not saying it cannot be done, but it will be expensive.

Bruce.

[Mcross]

I took the 390 out of my 57 series 75 thinking it was a 365.  It had a bad cylinder. So I bought a 56 sedan parts car.  Machine shop informed me it had a bad thrush journal. Wonder if it would be possible to swap the guts out of the 390 into the 365 block  just looking for a way out with my back pocket still on my jeans. The project centers around my 56 series 62 hardtop.

[Roger Zimmermann]

Bore and stroke for 1956 and 57: 4" x 3 5/8" (3.625")
Bore and stroke for 1959: 4" x 3 7/8 (3.875")
Connecting rod length for 1956: 6.625"
Connecting rod length for 1959: 6.5"
Main bearing journals have the same diameter: 2.625
Crankpin journal diameter: 2.250", same for both types of engines.
In my opinion, the swap is not without issues.

[Mcross]

Would you think that the blocks are the same? Being that both have 4" bore.  I have every thing from the 390.  If I swap the entire rotating assy with the 390 balancer.  Might I be able to pull it off??   Just makes sense to me if the blocks are the same.

[dn010]

The engine from the 1957 should have been a 365, not a 390 unless it was replaced at some point in its lifetime. From other posts I've read on the subject, it "could" work if you used all parts from the 390 such as the heads, cam, crank, pistons, rods etc. but no one has ever commented if they were successful or not. If you want to find out, put the crank in the 365, install the rods and pistons and turn it slowly by hand to see if it will work. I'd be watching the internal portions of the block to see if anything is in the way of the longer stroke. Seems to me it would just be like installing a "stroker kit" in the engine as long as you use the 390 camshaft, heads, lifters, pushrods and all that other fun stuff.

[Mcross]

Came to my senses and just gonna pay the money to have the 365 crank welded up and repaired. The machine work is so high. And the 040 over 390 pistons wouldn't fit the 365 block without boring. And the way my luck has been going I'm gonna take the safest bet. 
But I would have liked to try it. Just to see.   Im looking forward to getting it back together. 

[Mcross]

And yes sir.  The 57 engine had been replaced. Strangely it had two different heads on it one had bigger valves.  It had bent some pushrods and broken one rocker.  Just wondering if that's what caused that.

[dn010]

"Why is there never enough time to do it right the first time, but enough time to do it over" - Happy to see you won't be living by this! Best to do it right the first time instead of having to do it over again a second time. Let us know how it works out.

[Lexi]

"Why is there never enough time to do it right the first time, but enough time to do it over"

One of my favorite, if not my favorite saying. Solid advice for Mcross. Clay/Lexi

[Poncholover]

2 different heads. Seen that before. Stupidity knows no bounds. Well, it ran for a while...

[The Tassie Devil(le)]

I agree.

I worked on a 1971 Eldorado, and no matter what we did, it was hard to get it to run nicely.   Turned out that it had a '71 head on one side, and a '75 head on the other.   Different Compressions amongst other things, including valve heights meaning that it was impossible to match up each side.

Bruce.

[Chopper1942]

One thing that wasn't mentioned about installing the "stroker" crank assembly in the earlier block. Is the deck height the same on both blocks? Often GM would change the deck height when they went to a longer stroke engine, but retained the same bore and bore spacing and block configuration. You may have got the "stroker" crank assembly installed and the rods hit the block or the pistons pop out of the cylinders. Just a thought.

[The Tassie Devil(le)]

The way the factory does it is to make pistons with the Gudgeon Pin sitting in a different position.   Plus, change Connecting Rods which could include making the replacements stronger where necessary.

Bruce.

[Chopper1942]

With many aftermarket stroker crank kits with an OE block, the wrist pin moves very close or into the oil ring grove of the piston. To prevent this the OE's would create a tall deck block. They would use the same basic block but increase the deck height. A good example of this is the big block Olds, Pontiac, and Buick engines of the late 60's & 70's. They also like to keep the connecting rod length ratio within a specific range. By raising the deck height, they can use longer connecting rods to maintain the best con rod ratio.

A classic example of not raising the deck height is the small block Chevy. The 265 cu.in and the 400 cu.in. blocks are basically identical except for the bore diameter. The stroke increased in the 265 from 3" to 3 3/4" for the 400. To keep the pistons from popping out of the cylinders, they had to shorten the con rod length from 5.700" (used by all other displacement small blocks) to 5.565". With this short of rod, the wrist pin hole is within a few thousandths of the oil control ring groove. The 350 piston has about 1/8" of material below the ring land. If they had raised the deck height, they could have used a longer rod with its advantages.