Simply means you can turn the crankshaft mains down as far as you want assuming you can get the bearings to fit both the oem main bores and the crankshaft main journals.  For the GM guys, it means you can potentially run bearing spacers or extra thick bearings made specifically for running small journal crankshafts in large main bore blocks.  While I wouldn’t go there myself, that rule leaves it open to run a 292 sized main in a 312 block assuming you can use a bearing spacer or extra thick bearing to make that happen.  Joe and I have discussed some of the possibilities this opens up.

Lorena, Texas (South of Waco)

Charlie, I'm not sure they'd know what the journal sizes are on a 312. You should be allowed to claim a stroked 292 + 10% and use the 312 crank in that. With the mains cut to 292 size.




Frank/Rebop

Bristol, In ( by Elkhart) 

The guys that tear down the winning engines and inspect them are die-hard racers with years of experience, and checking main bore size will be an issue, since it is mentioned in the rules.  I will not be a part of any build that I know beforehand is not within the rules.  Also, the stroke increase is only .015", not enough to make a great amount of difference when you do the math.  Some things are just much more simple to follow proven blueprinting techniques, and assemble the best Y  Block you can with the rules posted.  Everyone is subject to the same inspection upon tear-down, and to be found illegal would get you banned from participation and ruin your reputation.  I think it is more fun to build a so called "underdog" engine and win than it is to go there with the most popular magazine selling engine.(LS, sbc)  Joe-JDC

JDC

So conceivably, a 312 could be built with main bearing spacers and a 292 crank offset ground to 312 stroke to keep bearing friction down.


Lawrenceville, GA

I personally would not attempt that until the question was asked the EMC question line, and got a positive answer back in writing in form of e-mail.  As Ted mentioned, there are some sbc that do have spacers available.  Just an added place for a problem to bite you and cost a lot of money if it fails.  I do know there were problems with the sbc at one time losing bearings with the spacers.   Joe-JDC

JDC

Are custom bearings an option, thick enough to make up the difference without spacers?


Lawrenceville, GA

That’s a yes for that.
 
A 312 build would be on the order of 324 cubic inches.  The 292 build being looked at as a possible contender would be a claimed 303 cubic inches.  Both of these combinations use a 0.014” increase in stroke and 0.060” overbores.  Not really looking at the 312 combination for a number of reasons.  Cam tunnel bore alignment issues in many of the early 312 blocks comes to the forefront while the use of bearing spacers or a custom bearing on the mains is a not anywhere near the top of the “Let’s do this” list.
 
Some quick math calculates a reduction of 2” of main bearing travel for each crankshaft revolution for the 292 engines versus the 312 engines.  That’s a 5% reduction in bearing travel or speed for the 292 mains.  That simply equates to more power potential for the 292 sized mains vs 312 sized mains when all other factors are equal; dyno testing shows this being a six horsepower difference.  This corresponds to what one of the retired Ford engineers said when he mentioned “Four barrel carburetors were not installed on the 1957 292’s so that the 312’s would not be in a position to be outrun by a 292.  Lessons were learned from the 1956 model year where both the 292 and 312 engines had 4V carbs”.

Lorena, Texas (South of Waco)