Will Hanna, what is the "fingernail test"?

fingernail

Basically if it smears the material of the bearing enough that you can feel the ledge with your fingernail, chunk it.

I typically put used bearings back in where they come out of just as a means of reading the bearings, but I don't see why a bearing couldn't go in another cylinder.

It's real important to not only measure bearings at the bottom width where we traditionally measure them, but across the top of the bearing. With today's hard V bearings being the norm, it sometimes won't hit the bottom of the bearing (.840) but will start to close up the top of the bearing. The journal of the crank is 2.500 and rod bearings should measure more than that 'across the top'. Sometimes if a bearing goes down to say, 2.496 or so, I will press my thumbs in the middle of the bearing and GENTLY push out on the top edges to spread it out some. It doesn't take much to pick up a few thousandths to get it back within spec. If it is below 2.496, either the bearing has too many runs on it or you have a tuneup issue with that cylinder.

I speak this from experience, I've been with a team that was not checking the top measurement, just the bottom. The bottom showed no movement from run to run, so no changes were made to the tuneup to compensate. It kicked a rod a few runs later. Upon further examination, the cylinder in question was closing the top measurement up considerably, down to 2.47 or so. The rod failure could have been avoided had this measurement been taken.

Dished pistons, burned pistons, etc., all have the signature closed top measurement to the bearing. Sometimes it does whack the bottom of the bearing, but not always.

Hope this helps.

 

pistons

just realized I didn't completely answer your question.

Generally I put pistons back in the same hole, but believe me I think many of us have been in situations where your digging up any good pistons you have left to make a run or make a race, in that case, it doesn't matter.

 

chamfer

clarky,

That's a good practice, and I should have elaborated on that. I think you would have to really try hard to take too much material off the side of the bearing.

I've also saw people in the past that measured their bearings before they scraped the babbit off the sides after a run, which isn't a true measurement of what actually happened. I've seen them tell me, it hit the bearing .005, must have a problem, take the scraper, scrape the rolled babbit off, and it be back within .000 or .001.

To clarify the fingernail test, you will see 'shiny' spots in the bearing, which sometimes is smeared bearing material. If it has smeared it enough where there is a ledge on that smear that you can feel with your fingernail, that's when I chunk it. There are plenty of times where you see a shiny spot, but if there's not a ledge I can feel, well I run the bearing again, within reason.

Scotchbriting a bearing can get rid of a lot, but if it has enough material moved to make a ledge, chunk it. Bearings are relatively inexpensive compared to cranks and the other parts connected to that crank.