Question about BMW K 75 S Motorcycles

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What causes premature wear in the rings and bore of BMW K75s?

What could cause sudden premature wear in the rings and bore of a BMW K75s with 80,000Kms? The bike has done mostly touring K's, was going well (no smoke, normal power), and then after sitting in the garage for 2 months, started roughly with lots of white smoke (oil) then spluttered and died. The BMW mechanic says 2 of the cylinders had: half normal compression, substantial wear, blow-by and the oil rings had stuck in the piston grooves. I was astounded as the bike was going well. Nobody seems to know what has caused it in 2 not 3 cylinders! After new pistons, rings and rebore, I hope it wont happen again. Any thoughts on the cause would be GREATLY appreciated!

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The K75 has very reliable engine .. its unusual for it to fail in this way .. the frozen rings would suggest overheating and thats possible if the radiator fan fails (that happens often before your 80,000) .. many of the pre 1990 k75s would smoke after storing for awhile .. that was a ring gap positioning problem .. that was corrected in newer models .. the smoke however would not have damaged the cylinders .. maybe in the extreme could foul a plug .. is it possible that someone put something in your gas tank ??.. or could it have been run without oil/water or perhaps the wrong type oil (like transmission fluid) ... that engine should have lasted longer than most cars .. nice smooth bike ..

Posted on Jul 25, 2009

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I would recommend a leak down test to distinguish if the low compression was due to ring wear or an issue with the valves/seats...cylinders bores dont just wear out from sitting.
It's very possible that the valve seals were worn allowing crankcase oil to seep into the combustion chambers, thus causeing it to smoke. Being a 3 cylinder water cooled engine it should not have an overheating issue to cause the rings to stick...unless it was neglected, and not regularly serviced, debris in the oil filter, low oil pressure.. sludge from worn out oil. A good clean air cleaner is important...bad filtration will cause premature engine wear. Install a K & N high flow filter... and your good for the life of the bike.
Hope this answers your question...good luck with your bike.

Posted on Jul 25, 2009

  • philsbmw Dec 15, 2010

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Stay away from the K&N filter. Unless you're truly religious about keeping it clean and oiled properly, it'll pass a lot more particulate **** than the stock paper element; a poorly maintained K&N is one of the things that can cause premature wear in any engine.
All the K-series engines have 'Nikasil' coated cylinders- the coating is harder than the hones used for cylinder re-sizing, hence, no re-bore is possible, and no oversize parts are available from BMW. If you paid for a re-bore, you've been had!
Most likely, your bike was put away and stored improperly; water sitting in the engine can quickly corrode and freeze rings, etc, and then when you try to start her up, these parts can be broken.
When you store your bike- or anything else with a motor- pull the spark plugs and pour or spray a little bit of oil or 'engine stor' into each plug hole, then, with the plugs still out, turn the engine over a few turns to distribute the oil to the rings, etc. Put the plugs back in just enough to keep out dust & moisture.
When you're ready to go riding again, be sure to pull the plugs and turn the motor enough to clear out most of the oil- if you try to start an engine with too much liquid in the chamber, you can bend things!

Posted on Dec 18, 2010

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