MM Z crankshaft question

[Alexander1997]

When I removed the old jugs/pistons for a ring job (will be putting replacement jugs on), I noticed on one of the journals a scratch. Doesn't appear deep and when I run my finger across it, it doesn't catch my finger nail.. So just from that, it appears not very deep. Is a small scratch anything to worry over before reassembly? The rod bearings look fine and the crank isn't rusty, so I had planned to just put it all back together until I noticed that scratch then wondered if I needed to think about getting the crankshaft polished?

 

[JOCCO]

From what you are saying probably not. Polish with fine emery cloth and mic the crank to look for taper or out of round. Rod bearings were $25-30 each rod for one I did this winter. Also look at the old bearing it should tell you if it's been turned like .020. Last check the rods at crankpin and piston pin for issues.

 

[T.R.K.]

I recommend mike the crank or at least Plastigauge the fit between the rod bearings and the crank. (There should be a Youtube video showing how to use Plastigauge.)
I recall one RTU engine Plastigauge could not even give a 'read' on. Measuring the crank revealed the crank journals were worn .019.
The question you are trying to answer is do the bearings and crank fit close enough to run and not knock. Knocking is hard on connecting rods and causes them to break prematurely.
While not ideal, one scratch on the crank-especially one that goes around the crank and not across the journal-is probably not going to affect things much. A scratch does indicate that some debris of some kind went through the oil system at some point in time. Lots can happen to an engine in the last 70 years.

 

[JOCCO]

I recommend mike the crank or at least Plastigauge the fit between the rod bearings and the crank. (There should be a Youtube video showing how to use Plastigauge.)
I recall one RTU engine Plastigauge could not even give a 'read' on. Measuring the crank revealed the crank journals were worn .019.
The question you are trying to answer is do the bearings and crank fit close enough to run and not knock. Knocking is hard on connecting rods and causes them to break prematurely.
While not ideal, one scratch on the crank-especially one that goes around the crank and not across the journal-is probably not going to affect things much. A scratch does indicate that some debris of some kind went through the oil system at some point in time. Lots can happen to an engine in the last 70 years.

Very good advice.