F, OT, S, Z, marks on BMW motorcycles

What about those marks F, OT, S, Z that are on your Flywheel (clutch carrier) of your BMW Airhead Motorcycle?
What do they mean?
How to mark a flywheel that has been lightened, and the marks lost. ...and other niceties...you can even use this information to degree-out camshafts!

F,OT,S,Z.htm
39A

OT means Top Dead Center: that point, exactly, when your motorcycle's pistons are fully outwards. OT, in German, is Oberer Totpunkt, more correctly translated as the top dead point. The most often used reason to use OT is when setting the valves. NOTE that the pistons in airheads are two strokes away from each other. Proper procedure is to check/set the valve clearances with the particular cylinder being adjusted at the OT mark ...AND!!....THAT cylinder must be on ITS COMPRESSION stroke.

The OT mark is also used for setting the ignition timing on some dual-plugged Airhead motorcycle conversions; other such motorcycle conversions use the S mark; refer to my dual-plugging article. Another use of the OT mark is when installing the flywheel (from 1981 called a Clutch Carrier); timing chain or crankshaft or camshaft sprocket.

The S mark is the static timing point, that is, no rpm or low rpm, and the S stands for Späetzüendung, which means minimum advance; late or retarded. The S mark is used with single plug ignitions and also with some dual-plugged conversions.

The F and Z markings are basically the same. The F (the depression dot) and the Z marks are for the maximum ignition timing advance position. The F stands for Früehzüendung (early ignition, that is, spark advanced). The F dot (or, Z marking, ~1980+ bikes) is seen (via spark triggered strobe light) by raising the rpm until the timing no longer continues to advance, this point is ~2000 rpm on the early STOCK ATU /5 models, and ~3000 rpm on models after that. There are a number of mechanical advance versions, so if your /5 maximum occurs at 3000, don't be alarmed.

The following is specific to determine OT (Top Dead Center), but can be used to place S and F and Z marks; and, if you know the details, you CAN determine if you have an early or a late duplex camshaft sprocket (they are 3° cam, or 6° crankshaft different); and you can use the method to degree-out camshafts, ETC.

Here is it, for OT (TDC):
The very best way to determine things and mark the flywheel for OT is this way:
1. Obtain or make a piston stop   They are not expensive, but if handy you can make one from an old spark plug body. You need to remove the insulator, and tap the body for a fully radius'd bolt's threads.
2. Obtain or make a degree wheel and fasten it to the alternator bolt with washers and a pointer fashioned nearby. Hot rod shops have them. There are two types. 360° and 180°-0-180° types. Either is usable. You should make or get one that has no hole
      in the center, or a small hole. If the center hole is large, you will have to make a washer or stepped washer, to use under the alternator bolt.
3. Remove both spark plugs.
4. Install the piston stop into either spark plug hole.
5. Adjust the piston stop so that it contacts a piston when the piston is near, but NOT at its outermost movement, when GENTLY rotating the flywheel to the contact point. If your piston stop has a locking nut, engage it.
     I usually set the piston stop about 1/2 inch in from what is would have been top dead center on the piston. This is not at all critical, but better accuracy is liekly had with the piston within 3/4 inch of fully outwards.
     In other words, the piston will hit the piston stop before the piston can come fully outwards.
6. Rotate the engine GENTLY in ONE direction. When the piston reaches the piston stop, rotate the degree wheel to approximately zero.
7. Rotate the engine backwards a bit, then rotate it forwards again, until it contacts the piston stop; and keep a bit of pressure on the flywheel (or, alternator bolt). WRITE DOWN the degree wheel reading while you have that bit of pressure going on.
8. Rotate the engine the other way, all the way around, until the piston stop again contacts the piston. Be GENTLE during the rotation. Again hold a bit of pressure, and again WRITE DOWN the degree wheel reading.
9. Remove the piston stop.
10. Rotate the engine until the degree wheel indicates exactly half-way between the two readings. At this position the pistons are 100.00% fully outward. It is relatively easy to get an accuracy of 1/2 of one degree if you are careful.
         Look at the pistons via the spark plug holes if you want a double check on your work,... rotate the engine back and forth a wee bit, you should see that your work is correct by looking at the pistons move down and back to what your eyeball says
         is top dead center.

The method of using a degree wheel as described is a MUCH better method than trying to measure the piston movement with a dial-gauge, or, the worse job is with your eyeball on the pistons ONLY....never do that!
The degree wheel and piston stop method removes all bearing play and other small inconsistencies. You would find that the piston does NOT MOVE as the crankshaft moves, for several degrees, when the piston is at TDC, hence the need to do things with a piston stop. It is not only various bearing slops that affect this; but think about how the circular rod small end moves in the piston pin, a hair before the piston reaches the top, to the same position after.   Yes, several degrees of problems, SO USE MY METHOD with degree wheel and piston stop.

11. When you are done and are SURE you are at the half-way point on the readings, then you can mark the flywheel through the inspection hole. This is OT, and that should be dead-centered in the hole.
12. In the same way, you can mark the flywheel for anything else you want, since the degree wheel can be moved. Why? How?
Simply have the engine at the determined OT point. NOW...MOVE the degree wheel so your pointer points to ZERO.
Now you can mark anything you wish on the flywheel, at the number of degrees either side of OT that you want.

You can degree camshafts this way (at the proper lift, etc.)

Rev.:
01/18/2008: from article 40; to a new page here; and edit for clarity
05/30/2011: fix hidden coding; fix copyright notice; make dual plugging
a hyperlink; edit wording slightly; clean up article and add
emphasis where appropriate.
08/13/2011: Add the information on identifying degree positions of various sorts

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