An old mistake.
I have owned my BMW R75/6 for many, many years now and have always had something of a love/ hate relationship with it. Get it going well on a long run and all is right with the world, but despite the image that BMW are keen to portray, it has been less than reliable over the years. I must point out that not all of this unreliability has been BMW's fault. For example, I have been through two Boyer electronic ignitions over the years, and as mentioned in the story link below, had an oiling fault caused a previous owners attempt to fit an oil pressure gauge (more on this particular fault shortly). There has been one nagging issue that has been in this bike throughout though, and it bugs the hell out of me.
The bike's engine has always made more noise than a bucket load of angry rattle snakes! I have written about this elsewhere, including here: Living with a 1974 BMW R75/6 (oldandireland.blogspot.com) in a story first published in Real Classic magazine. Eventually. a few years ago, I gave up on my own ability to fix the rattles, and entrusted the bike to a mechanic who used to work for our local BMW main dealer. He kept the bike for months, charged a lot of money, and whilst the bike initially came back quieter, it soon started rattling again, and it leaked oil, which it never did before. Then, after another fault that I have written about here when the starter motor failed, and because I was increasingly reluctant to use the bike in the state it had got into, I retired it to the garage, where it has sat, unloved, ever since.
BMW made a number of updates over the years to deal with the noisy engine . Most notably, they changed the design of the valve gear twice after my bike was built, but I have already uprated my valve gear (see the picture). They also changed to a larger diameter of pushrod tube to stop the rods rattling off the sides of the earlier, narrower ones. These wider tubes should be relatively simple to fit to older engines and I began to consider this as a last resort for my rattily bike. Then, while reading through the excellent Motorworks site (https://www.motorworks.co.uk/), to see what options were available to me to fit these wider pushrod tubes, I discovered something new.
For the series seven bikes, BMW dropped all the smaller capacity bikes (500 and 600cc). They also dropped the 900cc bikes and introduced 1000cc ones in their place. To allow for this capacity increase on the 1000cc bikes, they changed the outside diameter of the cylinder barrels by 2mm. I presume this was to ensure that the barrels were still strong enough to deal with the enlarged capacity. Now comes my discovery. BMW did not make this change on the introduction of the series seven bikes, instead they introduced it during series six production. I had always believed that all series 6 bikes have 97mm crankcase openings, but the change actually happened in September 1975, so later series 6 parts are not compatible with earlier ones, like mine, even though they are from the same model! Damn!!
This affects my bike, because when the oil pressure gauge failure mentioned above happened, I was hundred of miles away. To get home, I had to ride carefully, stopping often to top up the leaking oil. Despite this, one of the cam followers broke and scored the surface of the crankcase that it runs in. Because of that, when I rebuilt the engine, I bought a second hand series 6 crankcase from Motorworks, and built my engine parts into it. As I now know, this was a bad mistake. The crankcase supplied by Motorworks is from a very late R600/6, and thus, as I have recently found out has the wider crankcase openings. This has meant that the barrels from my old bike have a degree of movement on the crankcase and this makes the alignment of the valve gear incorrect, and is the likely cause of the rattles that have plagued me for years. Damn, yet again!