this may be a little premature since i haven't pulled the piston out to mic it. I was wondering what the max overbore size a 312 block can handle. i've pulled apart a motor that has been sitting for years put turns over vary easy. It read x 100 on the piston the wear looks good on the cylinder walls. I mic'd a few cylinders they read 3.904 avg basically a 100 over with some wear of .007. So i'm hoping i can just polish and ball hone and ring and bearing the motor but the clearance to me seems like to much. So any helpful advice would be appreciated. If i had to bore the block it might be junk i don't know

your block should be at its limit.i never saw a .100 piston.must have been a special order.are you measuring the cylinder at the bottom.if you are .007 is a lot of clearance.

ya i was surprised to see the 100 stamped into the piston, well if its at its max maybe i could knurl the pistons or just go with it and use it on another project. I need to tear it down and measure everything. The motor came out of a 57 ranchero with a thrown rear universal joint. The big y block must of had some power lol i'll find a use for it

The blocks vary and because of this, sonic testing would help to verify exactly what you have for cylinder wall thickness.  Core shift within the blocks is the major deterrent to performing major over-bores which is why you need to sonic test the cylinders for thickness.  By knowing where the core shifts are prior to boring, the cylinders can be offset bored based on the sonic test results.

 

I’ve come across a pair of Y’s that would make a 4” bore without sleeves and both of these were 272 blocks to begin with so that gives you an idea exactly how much cylinder wall were in those particular blocks.  There are a couple of 312’s running around in this part of the country bored 0.125” over and running successfully.  One of those I sonic tested after it had been run over 100K miles and was surprised actually how good the cylinder wall thickness was at that much overbore.  That engine was rebuilt and is back on the road today.  I’ve heard of other 312’s bored as far out as 3.938” but cannot make any comments on those.

 

Keep in mind that the run of the mill head gaskets for the Y only handle up to a 3.860” bore so anything larger than this will require a special 'big bore' head gasket.  For this reason, I keep composition head gaskets for the Y on hand that will accommodate either the 3.937” bores or the 4.00” bores for those that push the bores out to the limit.

Lorena, Texas (South of Waco)

finished tearing done the motor piston measured all real close smallest was 3.897 and largest 3.898. Mic'd the rest of the cylinders smallest 3.904 largest 3.906. Tightest clearance .0055 and largest .009 so it needs sonic tested and bored. pistons might be reusable in another block, and the crank and rods looks good at .010-.010. when i was checking everything out i notice a round circular arc weld on the starter side of the block, it looks like someone made a nice repair to the block where a rod must of been thrown this old y block been through the ringer. If i was able to go bigger on the bore where could i get some pistons that would work they would have to be .120 over to straighten out the bores.

thanks for any help or ideas Josh

283 chev.030   .040 should get you in the ballpark. A little resizing the rods and you nshould be good to go. I believe the .040 over is 3.905 and there cheaper than custom pistons.

-Gary Burnette-

Gary, thats what i used in my engine.

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

The 283 pistons are not a drop in replacement.  The compression height is too tall, the tops need machined.  The wrist pin is bigger than the 312, the 312 rods have to be honed to chev size.  The chev used pressed in pins, so the pistons have to be machined for retainer lock rings, and the pins shortened accordingly.  The Ford rods are wider than the space between the pin bosses in the piston, and either the bosses have to be machined or the rods narrowed.  I did this conversion once, never again.

John - "The Hoosier Hurricane"

These pistons are common for non strokers and a larger bore. And I believe we rebushed the rods.

-Gary Burnette-

Aussiebill, Just built a 312 using .030 over 283s which took my bore out to 3.905 and bushinged the pins from I believe 912 to a 927 floating pin.

-Gary Burnette-