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I bought these heads probably 12 years ago, and the guy I bought them from said they came off his 1972 LT1 motor. I just got off the phone with Summit Racing, they told me that these heads casting # HH 12 31 80, are low power, 268 HP and were probably off a chevy truck. Any insight will be greatly appreciated.
That isn't a factory number that you've listed, look on the top of the head btw the springs to find the casting number and date cast numbers.
Thanks for the information 68post, now lets try this again. I found the following information on those interwebs. It looks like these are 2.02/1.60 86cc's???
Again, these are supposedly from a 1972 LT1 Corvette.
3973487X 350 68-73 1.94/1.50 2.02/1.60 76cc X cast 10cc's
Thanks for posting this, much appreciated! From what I've read it looks like these heads (with the X casting) have larger intake ports, which would help to explain the 20 in. of vacuum my engine has. These heads also have screw in studs and push-rod guide plates, I wonder if these were factory, or aftermarket????
I just remembered that I had this tag in the garage, it was on one of the heads when I got them back from the machine shop. As I don't know what it all means, I'm hopeful that one of you guys can decipher it for me.
Do those heads have angled plugs? I have a set of those that came off a CE LT1 engine that was bought in 1971. It's my understanding (could be wrong) that the over the counter LT1 heads came with angled plugs and the production heads came with straight plugs.
Do those heads have angled plugs? I have a set of those that came off a CE LT1 engine that was bought in 1971. It's my understanding (could be wrong) that the over the counter LT1 heads came with angled plugs and the production heads came with straight plugs.
It has screw in studs and guide plates. 2.02" / 1.6" valves. They are correct 72 LT-1 heads. It had 16 new cast iron guides installed. Springs were checked.
Nothing about the "X" or the port size has anything to do with vacuum.
Probably the best GM small block production head......the larger chamber and intake port makes it flow better than any other small block production head. In fact, the 71-72 LT-1 made the same exact power than the high compression 70 model due to this. GM claimed 11 to 1 on the 1970 LT-1 but none ever saw that.....more like 10.4 to 1......the 71-72 head was 9 to 1 with a -4cc flattop piston.....
These heads can function decent with a nice blending of the bowls and port match......but they max out at about 390-400 horsepower no matter what.
Probably the best GM small block production head......the larger chamber and intake port makes it flow better than any other small block production head. In fact, the 71-72 LT-1 made the same exact power than the high compression 70 model due to this. GM claimed 11 to 1 on the 1970 LT-1 but none ever saw that.....more like 10.4 to 1......the 71-72 head was 9 to 1 with a -4cc flattop piston.....
These heads can function decent with a nice blending of the bowls and port match......but they max out at about 390-400 horsepower no matter what.
Jebby
Thanks Jebby, that's encouraging information. I'm on a short budget and have had to pull this engine out of the car, due to reasons I really don't care to discuss. I've been digging for all the specs on these heads because, since I have to tare the entire motor apart, I may as well put a better set of pistons in it to boost performance a little.This engine has 48,000 miles on it and has never been bored, nor does it need to be. the bottom end is completely stock, original everything. I'm running a Comp-Cams CS 268H-10 cam. I've looked at these from Summit, but I would very much appreciate and value your input on the matter. KB 9902HC-STD - Keith Black KB Performance Claimer Pistons Part Number: UEM-9902HC-STD
I just remembered that I had this tag in the garage, it was on one of the heads when I got them back from the machine shop. As I don't know what it all means, I'm hopeful that one of you guys can decipher it for me.
OE closed height was 1.700" ... someone may've installed +0.100" longer springs (? perhaps elgin or pioneer RV1943) ... well-proven and accepted practice
instead of stock, or machining guides for posi-type seals ... they took a "belt and suspenders" approach by installing both an OE square O ring plus an umbrella ... well-proven and accepted practice
They also machined a backcut angle on both intake & exhaust valves. ... usually improves flow ... well-proven and accepted practice
Springs' Closed (1.800") seat pressures measured 75 psi.
Springs' Open (0.500") pressures measured 225 psi
I speculate those heads may've come with smaller 1.94"/1.50" valves but shop installed 2.020"/1.60" ... common practice, typically improve flow
keep in mind, I'm looking at tag only; without benefit of seeing heads
** add
seems some X casting had even Larger chambers; also those old heads may have been milled a time or two and chamber size diminished
Suggest it'll pay you to positively determine chamber volume before choosing any piston; either you pour 'em or get someone to do it.
Last edited by ebbnflow; Jan 30, 2022 at 06:10 PM.
Thank you for taking the time to explain all of that, it gives me a better understanding of what I have. I will ask the machine shop to determine the volume for me.
Need Varification
💯%.....trade them for cash, and get some better heads. You'll be so much happier 
