I was told by a BMW Technician yesterday that at 120,000km as routine maintenance I should check the torque on the Crank Main Bearings. Is this correct ????

i would suggest if your doing that (and its worth doing )....at least get some plasitgauge and check the gap of the main bearings as well

check the references and confirm that its still in spec .......

loose bearing caps and bottom ends have a decided affect on oil presure and longetivity of the whole deal ..

cheers

Hi Charlie 99 Thanks for the promt reply. Does this also include the big end bearings ?

if you have the side cover off ...why not ?

i think there is a download of "the workshop "manual available in the forum ....if you dont already have it ....or a good reference at least .

a word of caution though ....some manufacturers list the bolts and nuts as one use only ...i guess they suffer from "stretch " under torque conditions ....weird i know ...but i did notice it somewhere

charlie99 wrote: a word of caution though ....some manufacturers list the bolts and nuts as one use only ...i guess they suffer from "stretch " under torque conditions ....weird i know ...but i did notice it somewhere

Good tip, Charlie. Common in the big ends on any engine.
The bolts is designed to stretch, when they`re properly torqued.

Hopefully I can add something on the technical side of re-tensioning the bolts. It isn't possible to just place a torque wrench on the bolts, set to the correct torque value and pull on the nut. This doesn't work because the "starting torque" is greater than the original torque setting the nut was pulled up to. The nut must be backed off first by some amount of a turn, maybe half a turn, then retensioned to the correct value. If you use ne big end bolts, they need to be tensioned either in stages, or by the initial tension then degrees of rottion method. FWIW, some high performance engine builders actually tension the bolts to around 80% of the final value, 3 times, completely releasing the nut each time, then final tension on the fourth time. This is done to match the threads to each other under load, which is said to prevent the nut bedding into the bolt under engine load, so reducing the big end bolt tension slightly. Hope this helps, Dennis.

aye! dennis ....always good to read the recommended tecnique

there are manys recommended methods ...but always good to check with the manufacturers recomendatons ...they usually know the limits and design spec of the part they use .....but again there is some degree of unwanted confusion about what is safe...and marketing ....i read many times that "replacement with new parts "should be performed at every instance of proceedure "...but we all know that its "b/s" ...and it is quite safe to go again well for a few times at least ....not counting those oops events ...that many unfarmilliar to methods commonly make .. so the balence is ... "replace all parts, just in case " ..

i guess has to be a big plus to reliability ....but i call it "risk management" and it sucks if your a poor soul just trying to get the "beast" back together.

charlie99 wrote:i guess has to be a big plus to reliability ....but i call it "risk management" and it sucks if your a poor soul just trying to get the "beast" back together.

Poor or not.....a stretch bolt (identified by a smaller diameter at a small part of the length) is quite important to change everytime.

If you stretch it another time, you`re in the risk that it snap when in service..........
then you really get that poor feeling.

yes inge,,, for sure if it is stretched ..i was having a wimper at the "must do " mentality .... sall good mate

This is a difficult one, as we don't know how close the bolts used are to their mechanical maximum. I have seen a fair share of damages of larger diesel engines due to issues with the big end bolts. These are often hydraulically 'torqued', by stretching the bolts with a specified amount of hydraulic pressure using a hydraulic nut. Still some of them got loose/broke/etc and resulted in some hefty expensive damages (in more than 50% the engines were scrap). Checking other bolts at times proved them elongated ('overstretched'/plastic deformed) and/or with little cracks, mostly in the first few turns of the thread, with various extend of 'fatigue fractures'. The cyclic loading of these bolts can be very high, so renewing is a risk-minimising action, where the alternative (a substantial damage) is not an acceptable option.
The difficulty with our bikes is that we don't necessarily know who did what before; having the bolts oiled/greased before applying torque that is specified for a dry fit can already overtighten bolts, resulting in early fatigue and cracks/fractures.
Hope it makes a bit of sense. Basically going through the loosening and re-tightening of the old bolts could become an issue. For the cost I think its a risk you wouldn't want to take. However you mind or wallet might tell you different.

I dont know how to take this one. I have a Clymer manual, there is nothing aboutt his in it.

I have regularly gone over 150k miles in cars and never heard of doing this. My K100 has almost 96k miles on it and I would be going on basis of leave well alone if its fine, which it is. I am no expert on these bolts but the bit I do know is that they stretch and so it would seem one would actualy have to replace the cap bolts. I think if I was going that far I would leave alone or else replace the shells as well. My own decision is not to do this item.