Hello everyone. I have a ‘40ish UL project and I have two sets of heads, 8.2 & 9.3. I’m having a hard time finding information on these to determine which ones to run. My parts book says 8.2’s are high comp and the 9.3’s are mid. I have another book that says high compression were only made in aluminum and the numbers on the cast iron heads related to combustion chamber volume. This bike is going to be rode, not too hard and I just want reliable power. Thanks! Happy new year! Once my cases get back from the machine shop I’ll start a thread on it.

 

The chamber side of the head will have one or two bars on it just outside of the gasket surface. Those denote compression ratio.

 

I understand. The 8.2’s have two stroke marks and 9.3’s have one.

 

What is your concern? Neither is high compression, just one is higher than the other. I would select the higher of the two and run that. To do it properly you should cc the head and piston / valve area to know what you actually have, you might find its not what any literature says it is. I've found that to be the case in a variety of different engines over the years. As a machine shop owner used to tell me, if you dont check, how do you know?

 

I don’t really have a concern I’m just looking for is one better than the other? Like the 8.2’s have a smaller chamber which would equal a little more compression. Or should I just go the aluminum head route? Once I start putting everything together I will certainly measure my CR. It’s the first time I’ve ever touched one of these engines and there really isn’t much info on them. Thanks

 

I would run the aluminum heads if I had a set sitting there, my 80 came with them which is currently in pieces. One has 1 bar and the other 2 so I'll be cc'ing them and matching the chamber sizes.

 

both of my sets are cast iron. I’ve been looking for aluminum. The repops are $700ish.

 

They run hot and you live in a warm place, aluminum might be the best way.

 

that’s really where I’m leaning. Sell the cast and go aluminum. It makes more sense and I’m just finding excuses to be cheap.

 

We all draw the line somewhere. I know I have but not so much being cheap, more like stopping the bleeding.