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Discussion and questions related to the course Practical Engine Building
King bearings, rb26, crank straigthened and grinded, polished
assembled it, using motor oil 20-50, spun the crank 10-20 times, saw a slight increase of resistance on one angle, spins by hand easily but definitely higher resistance on one angle
dissasembled and found 2-3 bearings with long thin scratches on the bearing getting the crank weight
i definitely cleaned it well, but it definitely looks like debris, the scratches just took the black color of the bearing, im just wondering if its normal to get scratches when spinning on motor oil and no pressure. if scratched, when does it become an issue?
Definitely not normal and always down to cleanliness issues - it may not even be the engine side, it could have been in the oil, or the oil can, or have been dust (windborn stuff gets everywhere) on the tip that was carried from the can to the bearing - or the crank being nicked somehow, but that's unlikely.
As a friend who had an engine re-building service would say, there are three rules to engine building 1/ cleanliness, 2/ cleanliness, 3/ cleanliness!
Depending where it is, the bearings may still be useable with everything cleaned up again, and the oil drillings for the crankshaft checked for nicks.
So, the crank has a hairline crack horizontal to the spin around the oil hole in the middle journal. Seems the crack had something in it which scratched the bearing. Cleaned the crack and ran it against another bearing and no scratches then.
Sadly that only means the crank is subject to catching debris there (or breaking in half) machinist told me they put the crank to a lot of pressure to straighten it, so if it didnt snap then it wont now. But im not playing any chances on it snapping or catching debris.
Im now using 25 Neo rotating assembly.
Crank's shouldn't be "straightened" by using a press, but by skilled use of a hammer and shaped drift - as far as I'm aware.
That "machinist" seems something of a cowboy, cracks propagate to the point where the mechanical strength is too compromised and it breaks - he/she should have at least checked for cracks after the "straightening" to make sure there weren't any issues from the process, and notified you of the risk beforehand and the crack after, so YOU could make the calls.
It's an expensive PITA but the best call you can make is a new, or checked used, crankshaft - if the one you have is cracked, it WILL fail, and usually at peak rpm and load and potentially destroy everything else if you're unlucky