Need help identifying pistons (Jahns)

-
Plik

Plik

Ragin Cajun
Joined
Aug 8, 2009
Messages
999
Reaction score
15
Location
Houston, Tx
Hey guys,

I'm trying to identify the dome volume on a set of pistons i recently acquired. They are a set of 383 .060 over JAHNS forged pistons. I can't find any info on them online other than the company no longer exists. Maybe one of you "old schoolers" have an old catalog lying around? ;) Or you can give me your best guess? I'm thinking +9 CC

Here are a few pics..

1279687202.jpg


1279687203.jpg


1279687205.jpg


1279687207.jpg
 
Domed pistons are rated in negative cc's. Well......i hate comparing anything to Keith Black pistons......BUT, they have a similar piston that's -6 cc's. reason I say that is cause they make their pistons really strange. It's sorta a step head with a dome, so while it's similar, it's not as similar as you really want for a good comparison.
 
I figured I'd dig this up to get some other opinions? I'm in the processes of putting the short block back together and I'm still not certain what my static compression ratio will be with these.. might get some play dough and try to measure it.
 
Take some clay or play doe, squish it into a round disk a little bigger than the piston diameter and about 1" thick. Then press the piston into it 1/2" so the dome and valve reliefs' shapes are transferred, then fill it with water from a large animal syrringe. I think the domes are about 17ccs, and the valve reliefs around 7ccs, so a "net" volume of 10ccs. But that's a guess on my part.
 
I can't tell the difference between a cast or forged piston, but I ran a set of Jahns cast 273 pistons many years ago and they called for .0075 piston to wall clearance. Yes, they were a little noisy, but that little 273 ran.......... They were domed similarly to yours and were rated at 10:1.
 
I'll pick up some play dough this weekend and have some fun lol.. 9 to 10cc should give me around 10.5 - 10.6 Static CR and with my cam (XE274H) an 8.4 to 8.5 Dynamic CR. sounds good to me!
 
Make sure the block's decks are where they should be. If the piston is way down in the bore (like most end up without extra care) you may not have that high.
 
They sit at zero deck, that thin raised surface on the top of the piston sits about .005" above deck..
 
Ok so the piston volume measured at around 12 to 13 CCs.. which puts me around 11:1 with my setup. Looks like I'll have to step the cam up to an XE284H (240/246 duration) to get my Dynamic down to around 8.5
 
Jahns was one of the old school piston makers from when I was a young man. The only draw back to the old pistons is they were heavy compared to the newer piston makers. Nothing wrong with them. If your going to drive it on the street you could have some of the dome milled off.
 
You are running aluminum heads correct? The 11:1 static might be a good thing. I think the XE284H would make a very strong running engine.
 
I don't know if you caught my mention of the piston-wall clearance that I had to run with my old Jahns pistons. If you don't have the specs for these I wouldn't run them.
 
You must log in or register to reply here.
-

Back
Top