Friday, 4 August 2017

Checking clearances on a plain bearing crank

Now my mate Carl didn't only buy one XS750, but two. Unfortunately he got taken pretty well from behind with the "valuable" spares that were included with the second bike, so an afternoon of spreading the chaff from the wheat was due. Or in other words see if he did in the end get enough parts with the bike to build another engine.

Part of the deal was this crank-case-combo, which obviously never ran together as all the markings on the cases and the crank (look it up in the Haynes, it's quite well explained) were way out of spec.

Rather unsurprisingly, even with the cases just nipped down a bit the crank was stuck.

Luckily there was a second (partially f*cked) crank included. The bearing shells on the middle rod were pretty much gone, but luckily Carl had received a box full of old conrods, which meant that we shuffled them around until we found a good one to go in the center-spot.

A bit of plastigauge later and it is confirmed: Slightly over spec, but well within tolerance for farm-equipment! 😉 Btw. as a rule of thumb: If the bearing clearance is around 1/2000 of the crank-pin diameter, you can't be too far off. (Always consult a manual of course, but if you do and check the specs you'll find out that you usually land somewhere in the middle of the tolerances.)

And lastly: You don't always need plastigauge to confirm (roughly), whether a crank is in the right ballpark or not. A bit of oil and a bit of spinning can you give a pretty good indicator, if it's going to be alright or not.

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