/2 differential timing

Correcting the timing difference between cylinders on the /2 BMW motorcycle twin.

by Duane Ausherman

Since 1951 the BMW twin has had a magneto ignition system.  It is made in such a way that can result in a different ignition timing for each cylinder.  some call this a double image, or split image.  One cylinder might be exactly correct and the other one might be many degrees off.  This error can most easily be seen with a timing light.  It is possible to correct this error and this is my attempt to describe it.  I have avoided doing this for the last two years for several reasons.

1.  My experience is from a highly specialized shop background.  The procedures that we used can’t easily be duplicated by an owner.  The perspective that I come from doesn’t even exist these days for these old bikes.  Our primary goal was to get the bike operating as well as possible in the shortest time possible.  The bikes were common, not collector’s items, as is the case today.  The mechanics were well trained and did these procedures often.  They developed a “feel” from doing it repeatedly.  They could recognize when something was different from normal.  They had this “normal feel” from the vast experience gathered in our shop.  I must add that we also had the legendary BMW mechanic, Bryan Hilton.

2.  The procedure that we used is problematic, at best.  This is the procedure taught in the advanced BMW service schools.  We could tell that it was with great trepidation that we were told about this procedure.  We were astonished at being told by the factory service person to do this.  Few certified mechanics of that day learned this part.  I abhor taking a hammer to a BMW, especially delicate ignition parts, but that is what we did.  The first few times we were terrified to try this procedure.  Later it became fairly common and much of our procedure was successful because of our experience.  The possibility exists of breaking something, as once happened on a /5, and it can be catastrophic, it was.

3.  I am describing something from 40 + years ago.  I have basically ignored BMW for most of that time.  I have always owned and ridden one, but haven’t been in the concentrated service environment since.  My memory is failing on many of these subjects.  I am amazed at how many things that were as easy as breathing, are now gone from my recall.  With tools in my hand I can still do some of them, but it is more auto pilot, than thought.

This is not for the faint hearted or the average mechanic, but only the skilled or very adventurous one.  Don’t even try to hold me responsible for the results.  This writing will bring a lot of criticism from “experts” and I understand that.  Were these experts there, doing it on a production basis? Feel free to ask politely, but as many of you have already discovered, I don’t suffer fools easily.

I will address the /2 (and earlier) and /5, /6 and /7 in a different article because, while the procedure is similar, they are really different.

The /2 magneto BMW (1951-69) ignition.

All timing inspection is done with a timing light.  It is a strobe light and it “senses” the spark and flashes a light.  You may direct this flash onto the flywheel.  The flywheel has 3 marks.  The first one to show up is the “F” mark for high rpm.  The next one is the “S” mark for idle speed.  The third is “OT” and is for adjusting the valves.

Not all timing lights will work on the magneto ignition of the /2.  Nearly all modern timing lights are inductive pickup and require 12 volts.  You may discover that your timing light that works great on cars just won’t give a consistent flash on your perfectly working /2 BMW motorcycle ignition system.  Try to find a light that works well at low rpm.  Then when you find an inconsistent flash, it has a good chance to indicate an error in the ignition.

In the first few years of production (roughly up to about 1959) the flywheels had the two timing marks made by a ball that was swaged into a hole in the flywheel.  The photo is almost at the bottom of that page.  It showed up well and was elegant.  As a cost cutting measure, the later flywheels just had a line stamped into the flywheel.  The lines were painted yellow and even when new they didn’t show up well with the timing light.  I suggest that you mark the flywheel timing marks with white paint.  I just barely touch a slot screwdriver into a small can of white paint and then touch the line on the flywheel.  Be very careful to not use too much paint, or it may run and be a real mess to clean up.  Let it dry before you crank it up.

How much error is too much?  That is up to you.  We found little, if any, improvement if the two marks are brought together from only 1/4″ apart.  We took the limit to be somewhere from 1/4″ to 3/8″ before we would think about fixing it.  Let me caution here.  You may see only one mark and think all is OK.  It could be that the other mark is so far off that you can’t even see it.  That is not uncommon.

Ideally, you will see one “S” mark at idle and at mid-range see one “F” mark and no more.  Often you will see two marks with some distance between them at idle and two marks at full advance (about mid-range) of about the same distance.  The error, or distance between marks, is more important at full advance than at idle.  The final indicator is the “F” mark.  If it is good and the “S” is a bit out, forget about it.  It is one of those things in life.

From an idle, slowly raise the rpm till the “S” mark disappears out of the window.  Just before mid-range rpm the “F” mark should show up in the window.  You should only see one.  Keep increasing the rpm another 1000 or so and see if the one mark, or two that are close together, stay there.  You may see one come and go and then another show up later.  That could mean that at idle, one of the “S” marks if so far retarded that you never saw it.

The first thing to try is a new advance unit.  In those days that seldom helped out at all.  I remember that fewer than 10-20% were due to a faulty unit, but we always tried that first.  Several times we found that a new bike would have the timing marks well out of any spec.  I certainly wasn’t going to tell BMW that the brand new advance unit, on a brand new machine, was “worn out” and expect much sympathy.

At first, I tried to find the error with a dial indicator to see what was “running out”.  A couple of times I found a bent camshaft nose.  I attacked the first one by hand dressing it down to hold the mag rotor straight, with the engine in the frame.  It is very hard to take out the wobble and still keep it trimmed up for a taper fit.  It is much easier to just replace the cam.

Usually, we were unable to find the specific part that was “out” of spec.  The BMW service person just told us that it “happens” and to do this procedure to correct it.

It is up to you to decide if you have an error or not.  After reading this, probably any error is acceptable, rather than go through all of this.  I have received many emails from owners who went to the trouble to correct the difference in timing.  All reported a large improvement in the smoothness of the engine.

Boring facts about the “what and why” of the ignition timing wear

The magnet is on the cam shaft, above the crankshaft.  The crank drives the cam with a set of gears, called timing gears.  One gear is small, steel and on the crankshaft.  The other is aluminum and on the camshaft.  The wear process and it’s implications must be discussed here because, by now, they are all worn.

As the piston goes out towards the head, on compression, it finds resistance.  This resistance tries to push the head off of the cylinder.  The cylinder is firmly attached to the block.  At the point of attachment the block is being pulled outwards.  The same thing is happening on the other side.  As the engine runs, the block is being pulled “wider”.  To get wider it also must get “shorter”.  As the block gets shorter the distance between the crankshaft and camshaft decreases.  This means that the gears get pulled closer together.  Another way of looking at it is that as the gears wear smaller they also get closer together.  They tend to self adjust.  If the only effect was that they wore smaller, then the gap between them would increase.  This increase would result in lots of valve clatter at low rpm.  It is common for an engine with 100 k on it to still have fairly quiet timing gears.  That is because the two shafts have also gotten closer together.

If we measure the play on slightly loose gears at different points of it’s rotation, we find that it is different.  At places where the cam has pressure, from the valve springs to go backwards, the space between gears is increased, or the gap is bigger.  The slack has gone back and forth so many times that it increases the existing gap.  At low idle speeds this can be heard as a clattering sound.  It goes away with an increase of only a few hundred rpm.  During this clatter, the timing suffers slightly and may be a partial cause of the difference between cylinders at low rpm.  It isn’t the major one though.  Nothing can be easily done about this gear wear and it turns out that it isn’t important anyway.  I just mention it to give an idea of where some of the timing error may come from.

A worn advance unit cam lobe, or lobes, can add to the timing error.  That can be easily seen with close inspection of the cam surface.  Replace it.  Try a “known” good unit and see if anything changes.  Sometimes the advance mechanism can be slightly sloppy on the magneto rotor snout.  It can get tightened down in slightly different places to change the timing error.  The locating pin up inside of the advance mechanism cam can damage the rotor when it has been installed incorrectly and tightened down.  After this happens many times the rotor tip is sloppy too.  One can see this by turning the advance towards advance or retard, while tightening the 6 mm bolt.  Then check the timing.  Change the advance by tightening it in a different position.  Check the timing again.

If the engine was perfect, with no gap in the timing gears, no slop in the perfect advance mechanism, it could still have a timing error between cylinders.  Fortunately for us in the repair end, it didn’t make any difference to us why, we just fixed it.  If you want to use hundreds of $$ of precision tools to measure things, dozens of hours proving and fixing it and hundreds of $$ for parts, be my guest.  I will try to describe what we did in the shop to correct the visible error in timing.

I won’t go through the magneto timing procedure here.  That is another big discussion, with some self-appointed “experts” having different opinions.  I think that Vech has it written up very well.  Don’t follow the factory info, as it is wrong.

In our normal service, on a bike that we had never serviced before, we would often discover that the magneto timing was off.  Not the ignition timing, but the magneto timing itself.  The Germans call it “Abriz” and the Brits call it “E gap.”  We would pop the rotor off of the camshaft and gently reset it with the 6mm bolt only slightly tight.  Use only hardened steel for this rotor removal tool, such as a cut-off drill bit.  We would usually just pop the rotor loose with the magneto in place.  That saved labor.  For the private owner, time isn’t as important.  You should remove the magneto so that you can remove the rotor fully and inspect it.  Take a good look at the taper fit on both the rotor and the camshaft.  They should both be clean and dry.  Do not use any lube on the taper. 

Now we come up with the obvious question, “How do we know which cylinder is advanced?” It doesn’t really matter to us.  It can be measured, but it isn’t important.  If you wanted to measure the points gap closely, you would find that one lobe of the advance opened them more and than the other.  This translates to opening the points earlier.  The fact that they open at different times is the whole problem.

Look at the advance closely.  It isn’t symmetrical, off to one side of the plate, it has the advance limit spring on it.  Just make sure that you can return the rotation of the advance to the same place every time.  You could paint a mark on it somewhere to make it more obvious.  Get the engine to the “S” mark.  That is where the advance is just about to open the points.  Think about moving the rotor a bit sideways to get it closer to the points rubbing block or farther from it.  If the rotor is moved closer to the rubbing block, then that cylinder will fire earlier.  If your magneto timing was perfect and you don’t have a slightly loose rotor, don’t worry, it may still work.  Decide which way you want to “move” the rotor.  Just pick a way and remember it.

I use a medium hammer and a brass drift.  Put the drift against the 6 mm bolt in the direction that will move it closer to or farther from the points block.  A point of theory here.  If you just happened to pick a direction at random, it could be 90 degrees to the points block and the rotor could be moved a lot and result in no timing change.  It would hurt nothing, but why hammer on your BMW for no result.  You could assume that nothing was being moved, when it might be moving a lot.  So pick a direction, towards or away from the rubbing block, for your first hit.

If the rotor isn’t very tight it will move much easier than if it is tight and has been tight for a few years.  The BMW service instructor told us that the taper is “reseated” to correct the error and this is how it is done.  We were never able to confirm that the taper does reseat, but that is as good of an explanation as any.  If the rotor isn’t very tightly seated on the cam shaft, it shouldn’t take very much of a hit.  How much is too much? This is where experience is very important.  The good news is that the /2 parts that you are hitting are so strong that they will take quite a bit of punishment.  I have never seen any damage to a /2 from this procedure.  The previous sentence doesn’t apply to the /5 and later. 

Maybe I can give an example of a “hit.”  If one is pounding a 16 penny nail into new pine at the rate of about 3 or 4 hits to do the job, that is a hard hit.  I can’t remember ever using that much force.  The amount of hit to “set the nail” in place so that it will stand by itself is a gentle hit.  The force that amateur carpenters use would probably take 10-12 hits and that is about the max that should be needed.  I always start with much less and proceed upwards.

You might legitimately ask “Why not remove the advance and hit the rotor shaft?”  Only because we don’t know what part is off and don’t really care.  If one hits the 6 mm rotor bolt, everything is “adjusted” that is in the system.

After a hit or two, start the engine and check it with the timing light.  Did the marks change or not.  Caution; you could have moved the rotor just enough from one side to the same amount of error on the other side.  You moved twice too far and the marks will make you think that nothing moved at all.  This isn’t likely, but it could happen.  Probably nothing happened because you were very cautious and didn’t hit it hard enough to do anything.  I don’t blame you.  Try again and check again with the light.

If nothing seems to change and the rotor wasn’t removed, then consider removing it and reinstalling it lightly.  If it is not fully tight and nothing changed, maybe you didn’t hit it hard enough.  In any case, even a mechanic with experience may need to hit it and check 6 to 8, or more, times to get it where he wants it.  I have had it in one hit, but more often it takes more than the 8 attempts.

You may find that the distance between marks has increased.  Return the advance to your previously “calibrated spot” and hit it the other way.  You may find that things are in the way for the “other” hit.  Rotate the engine 180 degrees and try again.  It is very important to be able to know which way you are hitting it so that you can change as need be.  I have had to “chase” the marks back and forth a few times to finally get it.  I have had a very few /2 where I couldn’t move them at all.  At least not with the “hit” I was willing to give it.

You have it where you are willing to live with it.  Now fully tighten down the 6 mm rotor bolt.  It doesn’t need much tightening as it is a taper fit.  In fact, you really don’t want the bolt to be very tight.  I have never seen one come loose while running.  More have been too tight.  Test it again and see if the marks stay once the rotor taper is fully seated.

Mounting the advance unit

The advance unit has a round peg that is supposed to align with a notch in the rotor shaft.  That is supposed to be a precise fit to keep the timing in place and be repeatable.  When all is new, the advance had a “slight” amount of slack on the rotor shaft.  The timing would vary slightly, depending upon whether one would hold the advance in the clockwise or counterclockwise position while tightening up the Allen bolt.  Sometimes the mechanic would only get it close and not really aligned.  Then the round peg would take a bit out of the end of the rotor and widen the notch.  That gives the advance even more slack and more possible error in timing.  My solution was to hold the advance with the play in the clockwise position of its slack.  That is the same direction of tightening the bolt.  If you always do that, then you will have repeatable timing when you remove and replace the advance mechanism.  I have seen many rotor shafts with several “new” notches from someone having no clue as to the alignment peg and its purpose.  Soon the peg is smashed and won’t fit the notch at the end of the shaft.

rotor.jpg (29847 bytes)

It is not uncommon for someone to over tighten the (6 mm Allen wrench) bolt that the advance smashes down on the rotor nose so hard that it is “mushroomed” outwards.  This mushroom effect serves to jam the advance unit and keep it from operating smoothly.  Correcting this error means removing the mag rotor and filing the mushroomed metal off.  You can test it in your hand by installing the advance unit and tightening the advance down to a normal amount.  Then check to see if the fly weights operate smoothly.

If your timing marks were 1/2,” or more apart and you correct it, you will feel a difference, at road speeds, in vibration and general smoothness.

Since writing this, I have received several emails from people who have been scared off from trying this procedure.  I don’t blame you.  I have received 2 or 3 emails telling me how much the bike improved by this correction.  That makes it worth writing it up.

Alternatives for failed coils

In recent years the magneto coils are failing.  A variety of rewound coils can be purchased.  One or more places are rewinding coils, so you must send yours in.  Some of the new and rewound coils are also failing.  A new modern system can replace the original system.  You may elect to go another direction and install a battery/coil back up system.

BMW Repair manual /2 ignition timing contradiction.

The subject has come up about where the timing curve should end on the /2 models.  Specifically, at what rpm should the “F” mark finish it’s advance.  Few of the /2 had a tachometer so it was kind of hard to have a number.  The bikes performed well with that timing curve.  We had no ping with the fuel available at that time.  Pinging from poor fuel was corrected by thicker base gaskets.  This was usually found only in third world countries.  Pierre Michaud, of Canada, quoted a 1966 workshop manual with the figure of full advance reached at 5800.  Here is the story.

1960.JPG (37974 bytes)        1963.JPG (64504 bytes)

                                                       1960                          1963

These are photos of the workshop manuals of two different years.  I only have these two years left.  Plainly, they are in contradiction or some change was made.  A check of service bulletins may show something.  If any change of this magnitude occurred I would remember it well.  We would have had to alter our timing specs and watch for it on each individual bike.  No such thing occurred.  Since the bikes do reach full advance mid-range I suspect that the 1963 and later specs are in error.  This surely wouldn’t be the first time that BMW specs have been found to be in error.

Would the bikes benefit with the later full advance?  I have no experience with that.  It works just fine as is.  It is reported that an electronic ignition system for the magneto, made by Mz-B, uses the later figure.  Are they aware of this or just going by the newer workshop manual?


I posted an email to the Yahoo slash 2 list and one of the respondents stated that I should include it on my site, so here it is.

I can tell you from a great amount of experience with this, just how we did it.  The F should come into the window in mid range, more rpm shouldn’t change it any, or much.  When it did keep advancing we would just examine and lube the advance mechanism.  The problem would be gone.  If the F comes too early it is because of weak springs.  If it comes too late or is jerky then it is due to lack of lube or some part is causing friction.  If the mark is double then it is due to some misalignment of something.  Double marks show up better with the strobe on the advance than on the flywheel in cases where the marks are way out.  Not good and I have a page on this issue.  I always put the strobe light on the advance and watched the weights during advance and retard.  That is because the flywheel doesn’t have enough marks to “watch” it there.  The weights can make a big jump that isn’t visible at the flywheel.  The advance has a peg to fit into a notch in the rotor.  That “locates” it in one place.  Idiots tighten them down a bit out of alignment, sometimes way out and it won’t run.  The notch in the rotor gets “hogged” out a lot.  As I tighten the bolt I rotate the advance against the direction of rotation.  I think it was clockwise.  That is where it would go anyway if the bolt were a bit loose.  Just a small bit of insurance, but mostly it allowed me to put it back the same way every time.  The way that the advance is located while bolt is tightened can change the timing a lot.  The idea of it needing to get to 5800 for the full advance is shocking.

That engine would be screaming with no cooling in my enclosed shop.  We just didn’t do that.  We would take it up just past mid-range to see that the F stayed in one place, but that is all.  All of the tune up can be done so quickly that a fan isn’t needed.

In my opinion, the main reason for changing the advance limit spring is because the old one is broken.  The new ones didn’t break during my time.  I never found a source for them.  That was about late 1968 when I first saw them, not back in 64.  The old spring had almost no flex to it.  The newer one had considerably more flex and probably would change the curve somewhat.  I didn’t care about that part as it mattered none to me.  I just tuned them.  Often, when the old spring broke the timing went too far advanced and the engine would over heat and seize and/or hole a piston.  That was fairly common (especially on the R60) and so I watched for a broken spring carefully.  I even made up a few parts that weren’t springs, but just metal stops and they didn’t break.  I saw no problems result from using them.  I hated to throw away a perfectly good advance, just because of a broken spring.  I kept the old ones and by 1970, had several dozen in a box.  Many had broken springs.  The ones with worn out fly weight pivots got robbed of the spring and I installed on another advance.


Due to my request for contradictory evidence below, I received info from Klaus Wolter.  I have visited his home on two occasions and respect him highly.  Basically, he did a real test of the advance curve.  He found that the advance curve changed slightly at the upper end of the rpm range.

I will entertain any reasonable ideas or evidence on this issue.