502 block cast.# 12561358 has 454 bore size
 

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.

I do have a set of 98 vortec with the 100CC chamber. He has a set of H426 CP Speedpro pistons that will give him 8.9 CR, heads were shaved .020, he wants to go that way....It's his money.....we may be using 224 cam at .050. I like the Vortec heads...best of the GM family. Hard to find here, mine are not for sale .
I measured the ports of the 932 heads and about the same as the vortec, only the chamber is what makes the Vortec better. I have now on my engine dyno a replica of the 350 69 Camarro Z28 with a 30/30 cam and he like the sound, heavy lope.So I may use a much longer duration cam, With a 4 speed he should do OK, Here in Southern CA, its more of a bragging rights thing that actual speed ...no place to go fast, too many cops!!! If they sound and look good everybody happy.
Your spec for the 454 are good, for a logical person it would be ideal but here we dealing a different breed, not racers like us, I call them yoyos, my son calls them worse. But they have money

If he's dead set on that crappy low compression non-squish setup, then you may want to look thru the Howard cam profiles to find a 225-ish cam with a tighter lobe separation. He will end up with more "chop", and a hard pulling midrange. It will feel faster than it really is.

M

i was thinking a 225 to 230 duration at .050, getting H426CP speed pro pistons that with .020 off the heads will put it close to 9 to 1. Thats all I can do !!! I am limited to what he can spend
thanks for the info
jim

454ci,oval port heads,cleaned up nicely,9.3-1 compr.,230,236,ls110 mid .500's lift flat tappet,victor jr w/ a I believe bolaws 930cfm carb,made 524hp in 1 engine and 527hp in the other w/ 517tq in both w/ 2 peaks at 4100rpm and 5100rpm.motors ripped.ran them for years in my boat then sold them to hot rodders.they loved them.:D

we know compression make the motor run tops, but I am afraid to build a HP motor for some of these kids. They don't know how to treat them, they abuse them, rev them up in neutral just to hear the noise, They are happy when I tell them a BS amount of horsepower, just bragging rights.I make a buck and keep them happy. Most of my customers are nice young guys and I help them as much as I can, mainly by keep the motors mild, they last a long time. ( I remember when I was that age...wanting to go fast...but no money !!!)

trust me,,put in good oil pumps,plenty of cooling and proper bearing clearances and they can be beat on.
I ran the 1000 island poker run,75% of it betw 4800-5200rpm and the last 25 % at 6200+ rpm.build it right and it can take it.