I sent my 312 crank to be reground,  The block was zero decked, and when the crank came back, pistons 2 & 6 were up out of the hole about .006-.008 and 3 &7 were about .017-.019 out of the hole.  The crank was ground .030 on the rod journals.  We put the crank in a lathe and measured the height of the rod throws and found that 3 & 7 and 2 & 6 were higher than the others.  Could too much have been cut from one side of the journal to cause this?  Maybe the crank not fitted properly in the tool?  Operator lost center while grinding?
Is the crank completely ruined, or could it be still be offset ground to make a stroker?  Really hate to lose a 312 crank.



Butch Lawson

Manchester,  TN

Have you confirmed that all the rods are the same length? Mixed some 292 rods up in a 312 once and had similar issues.

Measured all rod and  piston pin heights.  Everything ok except the crank.  Put the crank in the lathe to maintain center line and rod throws are not the same height.  Have it narrowed down to the crank.  Don't know what to do with the crank now.  It could  be used in a block that is not "zero decked" or maybe could be reground as a stroker probably.

Butch Lawson

Manchester,  TN

You can cut the tops of the pistons to bring them back to 0.

A part of the problem is that you 0'd the deck before having all of the other parts machined and fitted - there is some tolerance for the crank grinding process so it is very unlikely that you would still have 0 when it came back.

What does the grinder have to say about it?  I think they messed up.  .017"-.019" out is just over 1/32" of stroke.

It should still be a good crank for a 312 or to stroke another.


Lawrenceville, GA

Before the block was decked or the crank ground, Charlie, the machinist says he measured the deck height and allowed for the pistons to be .005 down.  He then decked the block and ground the crank.  He did none of the assembly, I wanted to do that.  When all the parts came back and I began assembly, I found the pistons out of the cylinders at the top.
Cylinders 1 & 5 and 4 & 8, all four corners, are ok.  I know that I could cut the tops of the pistons to make it work, but I really don't want to do that.  Last night I took the crank from another 312 and installed it into my block and the pistons all come within .005 of the top.  I know what happened, guess I'm just venting and wondering what to do with the crank now.



Butch Lawson

Manchester,  TN

This likely happened because the machinist assumed the stroke to be 3.44” on all four journals.  Too many times the crank grinding person will not check the actual stroke on the crankshaft journals.  All four journals must be checked as some variance is normal.  Each time a crankshaft is turned, there is the possibility that the stroke variance will increase if the person doing the crankshaft grinding is simply dialing his gauge in off the existing journal diameters and not the stroke.  Your crankshaft can be reused in several different manners.  In the current engine it’s been suggested to just cut the tops off the pistons which would be suitable but will require looking at the balance.  The crankshaft could be used in another engine with the decks being cut accordingly or it could be offset ground to a new stroke.
 
Your machinist needs to be aware of what happened so he can address that on future builds.  My own preference is to do all deck cutting only after measuring everything during the engine mock up.

Lorena, Texas (South of Waco)

You can buy .040 rod bearings (.050 also) and have the crank reground on the rod throws.

Thanks Ted for the explanation.  The machinist is definitely aware that this went wrong and why.   I already have found another crank and fitted it into the block with pistons and rods installed.  It fits just fine with the pistons anywhere from .002 to .005 down.  I think I'll try and find someone who wants to build a stroker and could use the other crank.

Butch Lawson

Manchester,  TN