I have a 239 that we bored +.125. I knew little about y blocks, so I assumed you could turn a 239 into a 256. Worked out ok for me, it’s been 15 years. I’ve since put a 312 crank in it.

  Dennis in Lititz PA

Bore and sleeve it...

As brought up earlier, the link Charlie Brown posted pretty much lays out why stroke is more important than bore size.  Here’s the link again. 
http://forums.y-blocksforever.com/FindPost136569.aspx




Sleeving a Y can prove to be problematic and especially if doing it for the purpose of creating a big bore Y.  Some if not all the cylinders will break into the coolant passages when boring for the sleeves and that in turn allows a coolant seepage point between the original cylinder (what’s left of it) and the sleeve.  Because the head gasket fire ring is irregular in shape, it doesn’t seal on the edge of the cylinder and in this case the sleeve; this gives the coolant a place to enter the combustion chamber.  Torquing the head bolts simply exasperates the problem in that the decks are no longer fully supported to the core of the block which allows the decks to be pulled loose from the sleeves.  I’ve had the decks crack between the cylinders as a result.  Here are pictures showing just one of the six cracks in a 4" bore sleeved block.

 

 

Sonic test.  Sonic test.  Sonic test.  And make sure the sonic tester is calibrated with a piece of Ford cylinder wall material and not with the steel calibration button located on the front of the tester.  You’ll get artificially larger cylinder wall thicknesses if the tester is calibrated with steel rather than cast iron.  Enough larger in fact that you’ll think that you have a 4” bore capable block.  Measuring the clearance between the cylinder walls at the core plugs will help to validate the sonic test cylinder wall thicknesses.  I have cylinder wall material from the various blocks from the Big Three and they all vary in cylinder wall thickness readings from actual measured thicknesses if the tester is not calibrated accordingly for each type of material.
 
Haven’t come across a 292 block of any year or casting number that will make a 4” bore with a cylinder wall thickness in all eight cylinders that will handle much in the way of compression ratio increase and/or higher rpms.  A number of 292 blocks are maxed out at 3.860-3.870” for normally aspirated performance applications and that’s only after sonic testing to verify where the core shift lies.  Many blocks simply do not pass the sonic test for max bore due to core shift.  If the block had already been previously bored, then the opportunity to offset bore the cylinders into the thick sides of the cylinders is diminished.
 
Some of the early 272 engines are 4.000” bore capable but finding one of these is getting more infrequent.  I've only come across a handful of these thick walled 272's over the years as most of them are only able to bore out to a standard 292 size and that's it.  Too many of these ‘extra thick cylinders’ 272 blocks have gone to the scrap yard simply because they were 272’s instead of 292’s and 312’s.  The 272’s just don’t get any respect.  There’s a 4” bore 272 running around here locally for the last number of years but it was built as an experiment without performance in mind.  It was selected for a 4” bore based on the sonic test numbers being very good.  It was originally going to be bored out to be a standard bore 292 but the extra thick cylinder walls made boring to 4” possible.  At 332 cubic inches, it’s a big Y with nothing but a massive over-bore.  The cylinders were marginally on the thin side based on the sonic test results after the boring operation but it’s held up well to this point.  Keep in mind that this engine has not been hot rodded and is simply a driver.
 
If you do go the 4” bore route and find it doable, I do have Best Gasket 4.030” bore head gaskets available for the Y in the event you’re having trouble finding them from the normal suppliers.  These particular gaskets have revised coolant passages and also accommodate ½” head bolts if going that route.

Lorena, Texas (South of Waco)

i never knew any block made 4 in.the 272 truck block is thicker than the car block according to an old y magazine article.the trucks are dif blocks and the cars are cf.the 272 blocks i bore measure about .290 to .300 on average between cylinders.292 blocks a good one goes around .250.that means my 272 will have the same thickness at 292 that a 292 will have at 3.8.i have done 3 of these blocks now.all are ceby.

Charlie, first I must apologize for relying entirely on my 70 year old memory to recall facts about the 292 engine I rebuilt 28 years ago. I remember that it does have standard 312 pistons and rings in it, so that means that it is a 292 bored .050" over. I don't know where the idea of the .120" over came into my memory from! Anyway, I do appreciate the correction and I really like the vast amounts of Ford Y-block engine knowledge available here.[/quote]

Thanks for all of the replies. The intent is to have a Y-block engine at the 312 ci that will rev faster with all components staying OEM including engine components, transmissions,
 clutches or torque converters; thus maintaining that the engine is a 312 y-block.