As I recall the Jahns pistons from years ago, they probably weren't that high in quality & the front edge of the domes were not clearanced at all near the spark plug & in fact would hit long reach plugs in a Y-blk. The current ones that are sold by J. Mummert appear to be a much better design.

Paul

Jahns pistons are not the best, the ones I have had experience with where heavy, not very well cast, and the weights were way off between pistons from a supposedly matched set.

Next;  +.125" over is an awful lot to bore a 312 block, they can take it in some cases but check your cylinder walls first. Then you will need a set of "Big Bore" head gaskets, Mummert dose carry them for up to 3.935" bore.

Not trying to rain on your parade but I don't think it is the way to go. Just one mans opinion.

   God Bless.   Tim                           http://yblockguy.com/

350ci Y-Block FED "Elwood", 301ci Y-Block Unibody LSR "Jake", 312ci Y-Block '58 F-100, 338ci Y-Block powered Model A Tudor

tim@yblockguy.com  Visalia, California    Just west of the Sequoias

The Jahns domed pistons I briefly owned had very thick skirts resulting in a heavy slug.  Required clearance as I remember was .007 or .008, apparently required due to all that aluminum expanding at running temp.  Guys who actually ran them say they produced quite the racket from piston slap until everything warmed up.

The forged dome pistons now offered by JM definitely makes the Jahns obsolete.  I would say bad buy at any price if you were actually planning on using them. 

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

A good buy or bad buy? Anyone have any experience reducing piston dome height? Maybe I will use that pitted bird block after all! Thanks G.S. 

56 VIC  Gene Stoehr Sr.