In process of assembling my forged nb2 1.8.

The engine was purchased from a car part out and disassembled by me. All the bearings and the bearing surface on the crank were in good condition at disassembly.

I had some work done on the block and had my shop polish the crank bearing surfaces. They said everything was good.

while getting ready to check my bearing clearances with plasti gauge I was cleaning everything and noticed some discolouration on the crank near one of the rod bearings.

It looks like it got hot but as I said I saw no evidence of this in any of the crank or rod bearings. Everything seems fine during teardown.

Could this just be from the original machining by Mazda? Anyone seen this on a crank before, is this an indication of a crank issue?

 

Picture is from 6-7 years ago, so I don't remember specifics. But I do know when this motor knocked, I pulled it and saw this. I also initially thought it got hot from all the bearing material going through the motor. But iirc it's not uncommon to see bluing on the rod journals on miatas, I think the consensus was it's from factory, from heat treating.

 

Thanks, I have found a few other sources online suggesting this is from heat treatment.