Fly wheel marks?

[Andrew]

Hi , I,m a little confused , on my 1951 R67 the flywheel has an arrow with two letters stamped "S & Z , then little further along is OT [top dead centre} when I look in the Barrington book they say the S & Z marks and in different places on the f/wheel ! Or am I missing something , what's the correct set up for markings .

[schrader7032]

Do you have a picture of the S and Z marks on the edge of the flywheel? I'm aware of four different marks that are typical on BMW motorcycles. They are OT, S, F and Z. Typically F and Z are used for the same thing as marking full advance. The Z comes up more on later Airheads in the 1980s though. So, it does seem confusing that you have an S and Z right next to each other. Do you also have a full advance marking?

[Seek]

Maybe SZ means Spätzündung? Spät means late and zündung means ignition.

How about a picture? I can't remember to have seen this ever before.

[Andrew]

Thanks for your help, I've never downloaded a picture on here before ....please can you give me an idiot guide on how to do that , much obliged . Andrew

[schrader7032]

To add pictures, check out this thread:

viewtopic.php?f=11&t=16324

[Andrew]

Sorry for late reply , but I’ve taken photos of the flywheel marks for you to see (hopefully)!

Flywheel odd marks!

[therealkennyboy]

From the shop manual in the library, a good drawing which shows both the OT and SZ marks:

The SZ mark is used when installing the magneto:

Didn't see anything in the Owner's Manual, but I skimmed it pretty quickly.

[Andrew]

Hi, I know that a lot of time has passed but does anyone know how I set my points with my rather weird two separate marks OT/ SZ and no other marks on the flywheel . I never got a definitive answer last time regarding this problem , should I have the points just opening at SZ ?

[vechorik1373]

Andrew,
OT as you should know by now, is TOP DEAD center, and is marked OT so you can set the valves. It has no other function.
Remember, you can set the valve clearance only on the cylinder that is at OT on the compression stroke.
You must turn the engine one revolution (360 degrees) to set the valves on the other cylinder.

The SZ mark (later bikes are just marked with an "S") is the idle timing mark, which is where one static times the bike.
To do so, if all you have is an ohm meter (which will do the job) is to temporary disconnect the wire from the coil from the points, and attach one lead of the ohm meter to the little nut and bolt on the points base, and the other lead to ground, anywhere.
The coil fires, when the points crack open. The Ohm meter will tell you EXACTLY when this happens. You must turn the engine VERY slowly and have the meter where you can watch the flywheel and the meter at the same time. You must get it to break the points when the SZ mark comes up dead even with the stationary mark cut into the front of the viewing window of the crankcase. To check for differential timing (one cylinder firing ahead or behind the other) turn the engine 360 degrees, and see where the ohm meter indicates that the points opened. Ideally it should show the points opening exactly at the SZ mark the second time. If it does not the ignition has differential timing. The severity of this condition will be indicated by HOW MUCH distance on the flywheel from opening the points the first time at SZ and where it opened the points the second time around. BMW allowed only 3 degrees difference. That is roughly 1/8 of an inch.
The more differential timing you have, the more poorly it will run, and the cylinder that is most advanced will run hotter, and you will notice that the header pipe is blue a lot further down on one side than the other.

[Andrew]

Thanks Vetch, that is exactly how I have always set my timing , where I recently got a little confused was watching a video and because my machine does not have any other marks other than OT and SZ , I started to wonder if the SZ was for setting the timing with the balance weights out as if under acceleration. So now I’m sure I’ve been doing things right. I obviously cannot use a strobe because of no mark (on the flywheel) for fast engine revs ! Thanks again Andrew