I didn't even think to put this question out there, but after seeing the other 489 thread I figured it can't hurt.
I'm having trouble getting a hold of the guy that built my engine. Having to go through a third party. I should probably just call him direct, but...
I don't know my stroke. Is there a way to back into the length? Also, is the HP/Tq number that I was given realistic? Here are the specs I know:

489 ci 10.2" deck. 2015 GM crate block.
+.020 pistons. It's supposed to be a total of 502ci. What stroke would get it there?
502 oval port heads shaved .010.
Edelbrock Performer 2p oval port intake on spacers.
It's supposed to be between 10.5 and 11:1 compression. Thoughts are closer to 11:1.
The cam specs I have are only that it's 565/304.
I’m told 540hp and 520lbft. Sound right? High? Low?

Should be 4.25 stroke with longer rods to make up deck height difference. Hp number is realistic. A friend bought the same type engine out of a pulling tractor. Made 700 hp with big roller cam. Tall deck. Your engine will not be 502 at .020 over. I come up with 486 and change. 4.27÷2=2.135× 2.135=4.558×3.14=14.3128×4.25=60.8295×8=486.6361

If it started at 489ci and then was .020 over, how would you come up with 486.6? Do you mean 496?

Ok. Thought you meant it was supposed to be a 489. 493.4981. A 496 is .060 over. 489 is .030 over. You are .030 +.020 for .050

No. It's a 489 block originaly that's then bored +.020. Then it's supposedly stroked to get to the 502ci.

To many inconsistencies. A 502 is a 4.5 bore 4.0 stroke. I had a 505 which was a 4.35 bore, 4.25 stroke. And actual displacement was 504 point 8or9. I think there is one oddball size crank between 4.25 and 4.5 stroke, think it is 4.35 maybe. You need to run a piston to the bottom and measure from the deck. It will get you a close number on stroke.

The reason to build a tall deck over a passenger block is less side load on the cylinder walls at high rpm.

Dave: only way to know for sure is to either open it up (not sure if that is worth it just to satisfy your curiosity) or get specs from the builder.

Not sure if all these are correct (got them off the net)

396 = 4.096" x 3.76"
402 = 4.125" x 3.76"
427 = 4.250" x 3.76"

454 = 4.250" x 4.00"
489 = 4.280 x 4.250
496 = 4.250" x 4.37" (2001 Vortec 8100, 8.1 liter... my brother had one of these in a GMC pickup)
502 = 4.466" x 4.00"
572 = 4.560" x 4.375"

I am of the opinion that the only reason to run a tall deck block is to be able to stuff a longer stroke crank in the block than will fit into a standard deck height.

I built a 582 out of a standard deck height block (admittedly it was a Dart Big M block with a 4.600" bore) that used a Callies 4.375" stroke. I could have built a 632 for the same money, but those 402 decals I put on it wouldn't look right on a tall deck.

Almost every aftermarket part has an identifying number on it. You might have to drop the oil pan to read the crank, and while the pan is off yank the front timing cover to look at the nose of the cam. Heads have casting numbers (ZZ502 heads where Edelbrock Performer RPM castings to start with but actually have GM casting numbers and a Bow Tie cast over the exhaust port). Those ZZ502 heads where CNC ported into a 290 cc "Roval" port (actually almost square) instead of a round port; so I hope your spacer plate and intake where gasket matched.

As to power level once you find out your displacement figure on one horsepower per cube unless you are running a solid cam and reving it above 6,000; then you might make 1.2 horse per cube. Anything beyond this goal probably won't be running on pump gas or has a power adder of some kind due to the VE of the BBC design.

Big Dave

It's suppose to be a 7,500 rpm motor. Not that I'll be pushing it there. I have part numbers for the heads and block. The heads aren't the ZZ502 heads. No bowtie. I pulled the intake and the heads are c&c'd and definitely standard oval ports which I'm happy about. I agree that it wouldn't be smart to pull the heads and timing cover for my own curiosity. If I don't get a good answer in a couple days I'll call the builder. Assuming the bore of a 502 is 4.28 like Lynn found, I backed into the stroke with some simple algebra. If someone could check my calculations, I'd appreciate it. It wasn't easy typing it on my cell phone...
Cubic inches = bore x bore x stroke x # of cylinders x .7854
502=(((4.28+.030)(4.28+.030)X)8).7854
502=(((4.31)(4.31)X)8).7854
502=((18.5761X)8).7854
502/.7854=(18.5761X)8
639.1647568118/8=18.5761X
79.8955946015=18.5761X
X=79.8955946015/18.5761
X= 4.3009886145

502ci=4.31x4.30


*

Bah, just realized I used .030 over in the calc instead of .020.

Bore of a 502 is 0.30 inches under 4.500 inches so that you can get one rebuild out of the block before it is scrapped. So the stock bow tie block used for a 502 is 4.470 inches, not 4.28 inch as stated.

Big Dave

This started as a 489 block, not a 502. I misquoted when I said assuming a 502. I should have said assuming a 489.

Displacement =half the bore x half the bore x pie x stroke x number of cylinders

Different formulas can arrive at the same result. Are you saying the formula I used isn't accurate?
I just ran the numbers using your formula and came up with 4.321 for the stroke. Using my formula and correcting the over bore to .020 instead of .030 like I initially used, I came up with 4.321 also.
So both formulas confirm to me that my motor should be:
502ci = Bore 4.3 x Stroke 4.321