Cadillac Jahns piston help

Maybe somebody could help. I have a early cadillac 331 was built in the early sixties and never run it has a set of jahns pistons. Does anybody know what Jahns recommended for piston to wall clearance. Thanks Jon

 

Jon,
If the pistons are a 'cast' material, they would be .0025-.003"

If they are a 'forged' material, they would be .005"

 

That's on the new stuff I wasn't sure if these would be the same. I have a set of early Mickey Thompson's in my nailhead that came with the spec card and those require .010.

 

Wow- .010 ? That will collapse the piston skirts on forged pistons today, and break them off on cast pistons.

 

Yup and if I ran them at.005 they would probably seize when they got hot.

 

You know, I was with understanding that all the Jahns pistons at that time frame were a cast piston.
Do you have a ball-miche that you can use to measure the piston in a few specific areas ?
Maybe we can figure out what the tolerance should be !

 

I've got an old Jahns catalog at home that had a section on proper clearance. If no one comes up with an answer before this evening, I'll give it a look.

 

That would help thanks

 

This is some info that I'm in search of so let's bring it back to the top after years of hibernation!!!

 

i am going to have to go with the .010-.012. Don't forget these pistons have no expansion slots in them and will expand way more than OEM pistons. I had Jhans pistons in my first 270 and in the Howard head 302 GMC engines. They came with instructions to run 10 to 12 cold. Which i did. Nothing broke except a few **/Alt records on the salt.