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.  

Lovely CNC machined valve gear from Motoren Israel in Germany.  It is very rigid, and can be shimmed before assembly.

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!

Imagine the possibilities for misalignment if the barrel is mismatched by 2mm to the crankcase opening!

In my defence, this mismatch is not entirely due to only me.  Motorworks supplied the crankcase knowing both the age and engine number of the bike, and the mechanic I more recently gave the bike to did have the barrels off but also did not notice this.  The information available now on the internet makes exact aging of components so much easier too.  Without the internet dating of the replacement crankcase I would never have known how late in series 6 production it came, and this information was not around 20 odd years ago when the engine was rebuilt.  I suppose that this shows the difference between an amateur like me and a good engineer?  I accept too much as given, especially when that given comes from a person or organisation who are supposed to be experts.  A good engineer should be cynical enough to accept nothing and check everything.

Oh well, no use crying over old mistakes.  James Sherlock provided a set of early series 7, barrels (of 800cc capacity) and pistons at a decent price.  The picture above shows one of them being fitted after a light honing to deglaze them.  An engine assembly lube was of course used to keep things going during that important first minute or so until the oil circulates properly.  The misalignment theory was confirmed when disassembling the old barrels.  Just have a look at the pushrods pictured below.  The scoring equates to the area where they pass through the cylinder head.  The mechanic had evidentally managed to assemble the engine with much more misalignment than I ever had, becaule I nevermanaged to score the pushrods at all!  They are later model pushrods, which fortunately left the originals for me to refit.

So at least the barrels now fit correctly, but the heads too had problems.  One of the exhaust threads on my bike had been repaired previously, with a  phosphor bronze threaded ring shrunk down onto the machined down port.  Unfortunately, the mechanic (yes, him again!  The guy who seems to have caused leaks by reusing gaskets and done very little else for all the time he had the bike), had screwed the exhaust ring on so tightly that I could not get it off.  And if you have seen the exhaust spanners for these bikes you will know the substantial amount of pressure they can apply.  The ring moved on the head slightly too, so would need a proper repair.  I carefully cut the exhaust ring off to enable disassembly of the engine, but could not use these heads again for the meantime.  Fortunately, in my hoard of parts bought cheaply on Ebay before BMW Airhead parts rocketed in price, was a set of decent looking series 7 heads for a 750cc bike.  It has long been my intention to rebuild an engine in the original crankcase to make a bike with matching numbers (old BM's being highly sought after these days), hence my stock of parts.  Since the fins on series seven bikes are slightly different, using these heads on this mongrel engine leaves the originals free for the rebuild of the matching engine some time in the future.

Of course there are other issues here too.  My bike had been converted to use 2 spark plugs per cylinder, but the replacement heads are standard, with only one spark plug.  The ignition advance curve is different for single plug heads, but fortunately, I had the required ignition module, and the change only took a few minutes.  Of course, the coils needed changed too.  The bike started fairly easily after all this work and ran well on low revs at least.  A few brief runs up and down our road shows that it is not picking up cleanly, so the carbs will need to be cleaned even though they were drained before the bike was laid up.  To this end, I have bought an ultrasonic cleaning tank which has just arrived today.  Another old problem which hasn't improved during the bikes lay up is the clutch, which despite any amount of careful adjustment is really sharp at its engagement point.  I am fairly certain that this is due to wear in the clutch thrust bearing, and since I have this in my stock of parts, the repair will hopefully be both quick and fairly easy from here.  It could do with a front tyre too, and that I have not got.

There were problems with the replacement cylinder heads too during this top end build, but I would like to get something posted here now, so that story will follow when I get time to write it.  Watch this space.