Are there any other problems in stroking a 312 crank to 3.700 other than the rod to cam clearance. Will most cranks go that far at 2.00 rod journals?

Gary Ryan San Antonio.TX.

3.62 is about it for a standard 2.000. 1.98 will gain you a bit.

Yep. The problems are pretty significant. You will need about a 1.93 rod journal for a 3.7" stroke. And if you are able to resolve that, you may run into problems with the oil passages when the crank is machined.



You're probably going to need a custom crank, or just settle for about 3.62"


Lawrenceville, GA

Don and Charlie have brought up some of the stroke limitations for the 312 cranks so I’ll throw some more detail at it.  The maximum stroke that offset grinding will get you on a ‘like new’ 312 crankshaft will be 3.6580” if targeting for a 0.030” under 2.000” journal (1.970”).  If targeting for a 2.000” standard sized journal, then the maximum stroke will be 3.628” stroke.  And these values are theoretical bests as you really want to be able to catch the outside area of the journal sufficiently at the conclusion of grinding on the journal.  That means targeting for at least 0.005” to 0.008” less stroke than what you think it will make to insure some cleanup room on the topside or outside area of the journal when finishing the journal to the final size.  Not going for the maximum possible amount of stroke also leaves some room to properly index the journals to insure that they are all 90° apart from each other.

 

And Charlie brings up the oil galley issue in that turning the journals down and widening them can expose the oil gallery at the rod journal filets.  When that happens, the crank is basically scrap.  Being conservative on the rod journal widths will help to keep from exposing the oil gallery at the rod journal edges when targeting for 2.000” journals.  I maximize rod journal widths to 1.810” on the offset ground cranks and then work on connecting rod big end widths and bearing widths to get the required rod side clearance.

 

And if you have several different cranks in which to choose from, pick the one that has the oil holes in the journals closer to the center of the journal.  As you machine the journals to a smaller dimension, the already exposed oil holes will creep closer to the filet or edge of the journal.  Starting off with a crank with oil hole locations that are biased closer to the center of journals will help to maintain the oil supply more in the middle of the connecting rod bearing rather than forcing it to the outside edge of the bearing.

 

And here are some past threads discussing offset grinding.

http://www.y-blocksforever.com/forums/Topic2615-3-1.aspx

http://www.y-blocksforever.com/forums/Topic9269-3-1.aspx

http://www.y-blocksforever.com/forums/Topic18275-6-1.aspx

http://www.y-blocksforever.com/forums/Topic33348-5-1.aspx

http://www.y-blocksforever.com/forums/Topic55593-3-1.aspx

 

Lorena, Texas (South of Waco)

Thanks all, that clears up a lot of questions.

Gary Ryan San Antonio.TX.