just got a core motor from wrecking yard, block casting # 12561358 and heads 12561358.

chevy shows them as 502 crate motor, fuel pump boss and flat tappet cam with provision for roller tappet holder.

The problem is: the bore size is 4.250- 454 size.

Heads have enormous ports, 118 CC chamber, forged steel crank,the exhaust manifolds are almost twice the size of a normal 454.

Don't know what to make of it !!!

Are you sure the block isnt 12561357 ? That is a 454. Then again GM has certainly made mistakes in casting numbers. Have seen plenty of them over the years. It would be very unusual to find a 502 engine in a wrecking yard. What is the engine code stamped on the front pad of the block ? That would give you another clue.

Either way if its 4.250 bore then its 4.250 bore. What else is there to say.

yes...clearly cast 12561358 and the heads 12562932 (502 head casting), cast crank 10114188, 7.4 liter on side of block, looked in freeze plug and almost siamesed cylinders. I actually did not want a 502, 454 is even too big for the 70 chevelle that's getting restored. Just confused by the casting numbers and size of the ports of the heads.
Could GM use same casting cores and mark 7.4 for the 454 and 8.2 for the other?
oh well...I got what my customer wanted so everybody is happy...just confused!!!!

Not normally but humans do make mistakes so if someone was putting the numbers in the cores for casting and used an 8 instead of a 7 . . . there you go. Like I said before I have seen tons of mistakes like that over the years

In my engine rebuilders software that head comes up as a 454 truck head used from 1987-1997 The ports are not really that big, typical oval port heads.

thanks for the info, it should make a nice smooth motor for the chevelle, would like little more compression but at +.030 over flat top pistons .040 gasket, .020 deck is 8 to one!!! would like 9 to 1.

I don't understand much of this thread. But I'm quite dumb and don't get much.

502 casting block but no 8.2L markings on outside of block and 4.25" bore ?

'Huge ports in heads' that are oval ports ?

'Exhaust manifolds twice the size of normal 454 exhaust manifolds' ?

Blocks with incorrect casting numbers are a lot more common than guys realize. When you see them all day every day you stop worrying about it. As far as the manifolds go they are probably just truck manifolds which can be larger than what guys are used to seeing on a 71 Chevelle. I bet the ports are no larger than any other 454 oval port head. Pics would be nice

If you are buying pistons anyway get a set of the Sealed Power H693CP hypereutectics. They will give you around 9 1/4 to 1 depending on exactly what cc the heads are. That would be perfect for a street engine in a Chevelle.

the ports on the 12562932 heads are bigger than the big oval heads Gen IV motor, have them side by side.

MkIV oval port heads are not "large oval ports". Most of the pre 1990 ovals were "peanut port" heads. The 932 heads are large ovals.

Now,, you mention a Chevelle. You will REALLY be doing the owner a favor by finding some mid '90s GM Vortec heads w 100cc closed chambers. ZERO deck the block for a .040" quench clearance with the flat tops. You might even find somebody willing to swap heads even with you. The closed chamber and tight squish will give you a lot more compression AND detonation resistance. The throttle response on the street will be MUCH more satisfying than where you are currently headed. Cam it around 220@050 duration. Initial timing 16-18 degrees, 36 total, all in by 2500rpm. Run a vacuum advance - if an auto use ported vacuum. If a stick shift, run the vacuum direct to the plenum.