Re: Engine rebuild, help!

Posted by Fish'n Jim On 2022/12/10 10:56:59
in answer;
{You should read up on engine building and not take my word for things.}
all the journals have to be on center axially and the OD's matching within tolerance for size and roundness. Otherwise, unwanted vibration will occur. A turning shaft has a natural vibration frequency and it has to be out of phase or it'll destruct due to harmonics. eg: if crank wasn't chucked properly, one end could be off more and machined out of round. (Assuming the crank wasn't already damaged, as noted. But they should have checked that before proceeding!) 10 thou is not alot of cutting, so more critical to get it right. Straight 8 cranks are longer so need more care in setup end to end and turning. Bearings are/should be 10 thou over(smaller hole size) now, if the crank was cut down 10 thou. If not, it'll not have proper clearance for oiling, too much friction(tight), or too loose and cause wear issues with movement. The bearings are also taking/dampening the load from the piston action.
General rule(heuristic) is one thousandth clearance for every inch of diameter. But it's usually specified for a particular engine service. eg, if racing engine different parameters based on WOT rpm range. Varies with the bearing material, oil type/pressure, lube method, etc. After the caps are torqued, the gap is measured to ensure clearance. End play etc. also checked.
They can "align hone" a block with caps torqued to make sure that the "holes" are all in alignment also so the bearings line up.

This Post was from: