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Bike Review - Posted 12th September 2014
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BMW K100 Buyers Guide, Part 1

Dave Simmons shares his thoughts on buying BMW's K100, occasionally known as the Flying Brick. This time; an intro, and engines...

The most useful opinion I can give to anyone thinking of buying a K100 is to buy a K75. The K100 is a four pot, the K75 a triple and all the better for it. The 75 is lighter than the 100 and has none of the vibes. All in all, the 750 is the better bike for most applications with the possible exception of lugging two-up with luggage across a continent where the extra power and torque of the bigger engine might prove their worth.

So why did I buy a K100?

Good question. Why do men marry beautiful women with no personality? Conversely why do beautiful women with amazing personalities marry rich men? OK, before I get too caught up in a tangent with myself what I am trying to say is that I became besotted with the RS fairing and in fact still am and that overrode any rational input to the purchase. This fairing is not available on the K75 and fitting one was too difficult. Still, almost all of what is said below equally applies to the K75 (although it was never available in 16 valve form). So, back to the K100.

BMW K100 Buyers Guide

The K100 was made in three guises, all with the same 998cc engine but wearing different skirts (or indeed no skirt at all). There was the naked K100, the entry level model, a purists machine with a trad round headlight. Then there was the LT/RT which was the older man's touring machine with comprehensive but ugly fairing, tall screen and cubby holes for your bratwurst. Then finally there was the RS which quite simply has the most lovely looking 1980's fairing going, complete with period rectangular headlight.

As is fitting for the different fairings, the handlebars are also different. The LT/RT has a sit-up position akin to a Massey Ferguson tractor, whereas the RS has tastefully dipped (but not in a yobbish fashion you understand) sports-touring bars. I don't know what bars the naked has but would hazard a wild guess at something flattish. There is no doubt that the LT/RT fairing is most suited to touring, it provides effective cover for the head and body as you nestle behind it's high screen. By implication the RS is made for a more dashing, nay perhaps spritely young whip who scoffs in the face of the elements, relishing with zest the zinging impacts of precipitation and wind. The truth is that the RS screen is a triumph of aesthetics over function, which for me is a price worth paying but worth knowing if you are not similarly enamoured with its looks.

BMW K100 Buyers Guide

I suppose this is not entirely fair. As a component of the fairing the screen does fulfil its primary function of removing windblast from the rider's trunk and maintaining the bike's stability up to the ton and beyond. However, rider feedback suggests that the standard screen is also at the wrong height for anyone between about 5-foot and 6'5". So unless your genetics have predetermined that your verticallness is outside of the normal distribution of Homo Sapiens then it will be a source of some irritation, as it will funnel a concentrated flow of wind in some of the wrong places.

Do not be fooled into thinking things can be improved by the judicious adjustment of the RS' screen spoiler. I found that at best it acted as a volume control in that I could have more or much more deafening screeching depending on its angle. The best solution, after much trial and error, was to remove it completely. Now, with earplugs and no aerofoil I can ride quite comfortably.

BMW K100 Buyers Guide

So, what of the mechanics of the whole thing? We have a one-litre lump on its side. It looks odd and is very heavy. It is odd, and that's why Bimmer did it - because no-one else did. A result of this is that most bikes smoke for a few seconds on start up after being left to stand as oil seeps past the rings - not a problem. I think, but don't quote me, that this was designed out on the 16-valve bikes by fitting different rings, certainly I have never noticed it.

The engine also provides half of the frame, being a stressed member and is undeniably rigid. It is, perhaps, one of the most belittled of lumps and urban myths abound about it being prised from the engine bay of a Peugeot car. The truth is much more BMW. It was developed and tested to within an inch of its life. It had to be, as it was designed as the successor to the long-lived flat twin and so had to be at least as resilient.

I really recommend that any prospective purchaser gets hold of a copy of Mick Walker and Peter Dobson's book 'BMW K-Series Motorcycles'. It has a good bit on the development of the K100 and some insightful period photos.

Back to the engine, in short it's bulletproof and capable of very high mileages. The original 8-valve K100 engine is probably best for this and some of these old kettles have done ridiculous mileages in the many hundreds of thousands.

BMW K100 Buyers Guide
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The last couple of years' production saw the introduction of a 16-valve head, which is also long-lived but with the head went Nikasil liners and these can occasionally let the side down. Not that this should put you off - it didn't me, mine has 90k on the clock and the compression is still very good. Also, by buying a 16-valve machine you benefit from all of the improvements made to the model over its lifetime such as Marzocchi forks, four-pot Brembo brakes, radial tyres and so forth. In fact the K100 16-valve is the same bike as the K1 (except for a gear ratios, seat and petticoats) so to my mind you're getting something of a bargain. A K1 is not likely to be had for less than 4000, and a 16-valve K100 can be yours for less than a grand. If that doesn't sway you, the 16-valver also has the most power and torque.

One oddity of the K is its combined oil and water pump which uses a single casting with two seals to keep the fluids apart. Usefully, BMW fitted a tell-tale to the base of the unit so a drip of water or (more often) oil from the bottom front of the engine means the seals are gone. If you own a K for any length of time then this will be a job you face. It's not difficult but it is easy to stuff it up. It took me three attempts to get it right, but then I am borderline incompetent on a good day and a hazard to mechanical health when jaded.

BMW K100 Buyers Guide

The main difficulty is inserting the new seals the correct way around and obtaining a good seal between casting and engine block with the minimum of (the correct) sealant. A drip from the hole midships beneath the transmission probably means the main engine seal is passing. This can be politely ignored in most cases but can be tackled by those with the will to spanner without too much trouble.


Next time: gearboxes, ABS, electrics, shaft drive and size. Matters

Photos: Dave Simmons / RC archive

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