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Sticking Crankshaft During N54 Rebuild

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Hi,

I'm rebuilding a BMW N54 engine (my first engine rebuild) after a spun connecting rod bearing failure. I've replaced the crankshaft and bearings. I've double-checked all the clearances with a micrometer/bore gauge and plastigauge, and they within BMW's specifications (at the looser end). I also measured runout which is also within specs.

Now, I've hit a snag: after bolting down the bedplate to specs (Redone this a few times now with old and new bolts), the crankshaft becomes tough to turn initially. It requires about 20-25Nm of force to get started. After it starts moving, it spins fine without any sticking or unevenness. However, if I let it sit for a few minutes, it goes back to being sticky to initiate.

It's pretty cold in my garage, around 8°C, whereas it was 20°C when I did all the measurements last month. Back then, with the crankshaft torqued down, I didn't have this issue. This sticking problem seems to have started only after the temperature dropped. Could the colder temperature be causing the crankshaft to stick at first, or is it more likely to be an issue with the alignment of the journals or the crankshaft itself? (I'm leaning towards the latter, but I'm not certain.)

Any insights would be highly appreciated.

Thanks a lot!

Martin

It's possible the assembly lube is 'stickier' at the lower temperatures, and possible the block has very slightly changed shape over those few degrees, but while the former is possible the latter is most unlikely.

Same with the crankshaft having run-out/being bent - but that's the easiest to check. Take all the shells out except the end ones, and spin the crank slowly with a DTI on the other main journals in turn - you're usually looking for zero, but depending on the engine spec' tolerances, up to 1 thou', or even a little more, may be acceptable, but that does depend on other factors.

All in all, I'd suspect the lubricant used - have you tried spinning it with a less viscous oil?

Thanks for the answer. I already tried with different oils as I thought it could be the assembly lube that somehow was to sticky at that temperature. But there is only a minimal difference. This morning after the block had rested 10 hours fully torqued the break loss torque was 125nm with 5w40. I measured the runout at 0.045mm maximum (Was on the middle journal position) all others are about 0.01-0,02mm. But BMW specs is max 0.15mm runout at the middle journal (Seems crazy high considering max clearance of 0.046mm at the journals).

I will try to put the assembled block inside and see what happens, when it is back to about 22c.

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