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Flywheel F, OT, S, and Z marks;
Clutch Carrier. Installing correctly.
You can even use some of this information to determine camshaft and/or ignition timing!

© Copyright, 2014, R. Fleischer,OT,S,Z.htm

The OT mark means Top Dead Center (TDC):   That point, exactly, when your motorcycle pistons are fully outwards.  OT, in German, is Oberer Totpunkt, more correctly translated as the top dead point.  OT is used when setting the valves.  Pistons in Airheads are two strokes away from each other in timing & valve operation. Both pistons are at the SAME position, in or out of the cylinders, at all times.  Proper procedure is to check/set the valve clearances at the OT mark.  That piston must ALSO be on its compression stroke; where both valves of THAT cylinder are closed.  BOTH cylinders can NOT be put on the compression stroke at the same time.

The OT mark is also used for setting the ignition timing on some dual-plugged Airhead motorcycle conversions; other such conversions use the S mark.  Refer

Another use of the OT mark is when installing the flywheel (from 1981 called a Clutch Carrier) or installing a timing chain or crankshaft-camshaft sprockets.  The S mark is the static timing point, that is, NO RPM, but can also be used on Airheads at VERY SLOW rpm.  S stands for Späetzüendung, which means minimum advance; late or retarded.   The S mark is used with single plug ignitions & also with some dual-plugged conversions.  In stock Airheads, the F or Z marking is used to set the fully advanced ignition timing & hopefully the S mark timing will then be 'reasonably close'.

F & Z markings are basically the same, except for appearance and a bit of interpretation. The F depression dot & the proper Z mark are both for maximum ignition timing advance.  The F stands for Früehzüendung (early ignition, that is, spark advanced).  The F dot (or, Z marking, ~1980+ bikes) is seen PROPERLY (via a spark triggered strobe light) by raising the rpm to some point above where the timing no longer continues to advance.  That point is above  ~2000 rpm on the early STOCK ATU /5 models, & above ~3000 rpm on models after that.   There are a number of mechanical advance versions, so if your /5 maximum occurs at ~3000 rpm, don't be alarmed, your bike may have a later version installed. 

DWELL:   This is covered in depth in:

Once in a while there is some confusion over the direction of flywheel (or clutch carrier) rotation direction, & as to which markings appear in the timing window as you mechanically rotate the flywheel (or clutch carrier), whether from the alternator bolt or the rear wheel; VERSUS, how the markings move when a strobe light is used at the timing hole, with the engine operating. When facing the engine from the front, the rotation is CLOCKWISE. 

Specifics to determine OT (Top Dead Center), but can be used for S & F & Z marks.

You can even determine if you have an early or a late duplex camshaft sprocket (they are 3° cam-timing different, this means 6° crankshaft); & you can use the method to degree-out camshafts, ETC. 

For OT (TDC):
Perhaps you only want to locate the proper flywheel holes to use (Airhead flywheels and clutch carriers do NOT have 'indexed' holes where they fit the crankshaft) want to do cam or ignition degreeing work.  ALL ...and much more ...can be rather easily done! 

A simple method to install the flywheel or clutch carrier, to be sure it is in the correct position, assuming the flywheel or clutch carrier still has the OT mark:
For just positioning a flywheel, you do NOT need a degree plate, nor a piston stop, nor any other special tool.  There are FIVE holes on the flywheel (clutch carrier) which means there are 72° between holes on ALL Airheads.  That is equivalent to a large difference on the circumference of the flywheel, a bit over 5-1/2 inches at the rim of the flywheel on all but the R65 & R45 where it is a bit over 4 inches. 

You need only to reasonably closely position the pistons to totally outwards by eyeball & flashlight into the spark plug hole.  Then place the flywheel onto the crankshaft so the OT mark seems to be sort-of close to the timing hole, as you install the flywheel.  You don't have to have the OT mark actually showing in the timing hole, just close to it, & you can see that as you assemble the flywheel to the engine; you can even mark the back of the starter ring gear on the flywheel with white paint or chalk, making this all easy.  If the OT mark is missing, but other timing marks are available, you can use them, with consideration for where they are located (see any literature on ignition timing marks for Airheads).  You may well wish to read the rest of this article!

Into more depth:

As original & stock, the flywheel or clutch carrier is already marked for OT and ignition timing by BMW. 

Perhaps your OT mark is gone? ...from flywheel lightening?   Need those markings?   Maybe special markings?

Here are the BASICS on how to do what you need:

Obtain or make a piston stop   They are not expensive.  You could also make one from an old spark plug body.  You need to remove the insulator & drill, tap & thread the body for a bolt.  The bolt inner end must be quite rounded ... so grind it to be be such, & sand it ROUND & SMOOTH.  Piston stops are also available at hot rod 'speed' shops, some outboard motor repair shops, etc.  Be sure to get yours with 14 mm threads.   I suggest you get a commercially made one from a speed-shop; or outboard motor shop.  See 9, next section, below.

Obtain or make a degree wheel & fasten it to the alternator bolt with washers & a pointer fashioned nearby.  Hot rod shops have degree wheels. There are two types. 360° & 180°-0-180° types. Some have both markings. Either type is OK.  One with a small hole in the center is better, as it is a bit easier to use. If the center hole is quite large, you will have to find or make washers or stepped washers, to use under the alternator bolt.   IF you have markings on your flywheel or clutch carrier, and know it was installed correctly, I suggest you rotate the engine to the OT mark, and then rotate the degree wheel to the ZERO mark. 

Remove both spark plugs.  You will be rotating the crankshaft, by however safe method.  If the transmission and driveshaft and rear wheel are still in the motorcycle, use the highest gear number in the transmission & jerk the rear tire forward or backwards, until the piston is visible with a flashlight shining down the spark plug threaded hole in the cylinder head.   If the transmission and clutch is already removed, then rotate the engine using the flywheel or clutch carrier using two or three bolts in the flywheel ...they need NOT be torqued tightly ...just barely and lightly tight is usually enough.  BE SURE the crankshaft is FIRST blocked from moving forward BEFORE the flywheel or clutch carrier is removed.  See:

Understand the Basics, above?
Want to find EXACT position of OT on the flywheel?
No marks at all?
Need to otherwise mark the flywheel or clutch carrier?
Doing camshaft or ignition degreeing work, ETC?
Want to do precise measurements, so as to mark the flywheel or clutch carrier ...or for determining other things? 

The following information & procedures are for these and anything more you might think of.  Just where you start from MAY be a matter of what marks, if any, are on the flywheel.  If any marks are in existence, you can use book figures on its location, and modify the procedure.    These procedures allow you to make your own marks, or, find out about your camshaft, or, almost anything you think of!

It is beyond the purpose of this article to tell you how to work with every possible situation.  You need only to THINK a bit!

1.  Rotate the crankshaft until either piston is slightly inwards from being full-out.   That distance is not critical, but (by eyeball) ~3/4" is usually nice.  You need not measure this unless you want to, and there is nothing critical about that distance at this point. The piston must NOT be fully outwards.  Install the piston stop into either one of the spark plug holes.  It is usually more convenient to use the LEFT cylinder, as it is on the left side where you can easily see the timing hole.  If your engine is dual-plugged, USE THE TOP SPARK PLUG HOLE.

2A.  Adjust the piston stop so that it GENTLY contacts the piston. The piston stop will be contacting the piston before the piston is fully outwards.  If the piston stop cannot be adjusted deep enough for contact, remove it; then, rotate the crankshaft to move the piston more outwards.  Do NOT try to have the piston fully outwards is important for the piston to be lower than that.  Reinstall the piston stop & adjust it so it GENTLY contacts the piston.  You want to make SURE the piston & piston stop are in solid contact.  If your piston stop has a locking nut, lock it now.

2B. NOW:  Maintain some pressure via the rear wheel or other method, so the piston stop & piston are pressured when determining the readings.   Do NOT take readings unless you have some pressure.  READ the degree wheel opposite wherever you installed a pointer.

3.  Rotate the engine VERY GENTLY & VERY SLOWLY in the other direction.  When the piston again reaches the piston stop, maintain a bit of pressure via the rear wheel or other method, so the piston stop & piston are slightly pressured when determining the readings.  READ the degree wheel again. 

4A.  Rotate the engine until the degree wheel indicates exactly half-way between the two readings.  At this position the pistons are, or should be, 100.00% fully outward, the OT mark should be precisely centered in the timing hole if the flywheel is mounted correctly.   If the flywheel is not showing OT;  read this NOW!!!:

4B.  BLOCK THE CRANKSHAFT, ...and then unbolt the flywheel and re-position it so the OT mark IS showing.

4C.  While you do not have to reset the degree wheel, I think you should and recheck your work.  Reset the degree wheel so that it indicates 0 after finding the half-way point again.  Then, rotate the engine forward and backwards to the piston stop point again. You should get identical readings away from 0.   Be sure you understand what it is you are doing.  It should now be obvious to you that you can position the crankshaft for any number of advanced or retarded degrees with reference to the OT Top Dead Center position.  You can MARK the OT position if it does not exist on your modified flywheel, etc.  You can add any markings anyplace you want, at known degrees.  EASILY! 

By moving the crankshaft forward and reverse to the piston stop position, half-way is obviously the true Top Dead Center (TDC, also known as OT).  Doing it my way ENSURES no errors, as the piston will stop moving for a slight amount at TDC/OT, as the crankshaft is moved a teeny little bit.

I purchased this piston stop from OMC (Outboard Marine Corp.) many long years ago; part number 384887.  J-23648 is also stamped on its body.  Note the 14 mm standard spark plug threads at one end & the SMOOTH full radius tip, to avoid injuring the piston.  Note the dark-colored locking nut.  I have also made simple piston-stops from sparkplug bodies.  You don't have to make one from a spark plug so it is adjustable, just long enough, but making it adjustable makes for easier use. I suggest you PURCHASE your piston stop.  My degree wheel is the 180° type, & came from Chrysler's Racing Division, called "Direct Connection".  I have used this degree wheel on race cars & motorcycles.  I made an adapter, due to its large center hole, to fit the Airhead alternator bolt.


It is relatively easy to get an accuracy of one degree (& even better accuracy is not difficult to obtain).

The method of using a degree wheel with piston stop is a good & reliable method, compared to trying to measure the piston movement with a dial-gauge; unless the head has been removed, or a very carefully used dial indicator with offset feeler is used.   Using a degree wheel and piston stop is economical...and you can make one or both tools if you wanted to.  Use of a pencil or rod, etc., in the spark plug hole to 'feel' the piston, is NOT SAFE, you do NOT want to break something off inside, nor do you want to jam whatever the 'tool' is, against the spark plug hole threads.  The method needs to be accurate enough to mark the flywheel.  

6. There are engines (NOT Airheads) that have their ignitions timed by piston movement amount. This is usually done by means of a dial indicator method on the piston, whether the head is off, or with head in place. The spark plugs are usually centrally located and angular measurements are not, therefore, a problem.   It is possible to convert piston movement to crankshaft degrees, but a fair amount of mathematics is involved.  It is NOT as simple as you might think.   An example of the piston movement method for timing would be many two-stroke engines; and,  BMW Classic K bikes (K1, K75, K100, K1100, K1200).  I have put the mathematics involved & a LINK to a website article that has software to do it all for you automatically, here:

7.  The degree wheel & piston stop method removes nearly all bearing play & other problems from indications; OTHERWISE, trying to find OT precisely, from such as pencil movement or eyeball, you would find that the piston does NOT MOVE as the crankshaft moves, even as much as several degrees in some circumstances, when the piston is at TDC (OT).  It is not only various bearing clearances that cause this; but think about how the rod small end moves in the piston pin, a slight amount before the piston reaches the top, to the same position after, while the flywheel moves more.   Obviously, if you are just trying to position the Airhead flywheel (or clutch carrier) in the correct crankshaft threaded holes, you don't need piston stops, degree wheels, or anything else, because there is such a large movement of the Airhead flywheel between each of the flywheel mounting holes.  Since there are FIVE holes, you can calculate how many degrees between holes, eh?  (math challenged? divide 360 by 5) (and, I already told you, earlier).

8.  When you are done & are SURE of your work, you can mark the flywheel or clutch carrier through the inspection hole, if you need to.  Any mark you make should be dead-centered in the hole. For sure, any existing flywheel or clutch carrier marks for OT should be exactly the same that you got when doing your piston stop work.

9.  You can mark the flywheel or clutch carrier for anything you want, since the degree wheel can be read for any number of crankshaft degrees in either direction from TDC/OT. Have the engine at the already determined absolute OT Mark, centered in the window, which SHOULD BE EXACTLY AS THE DEGREE WHEEL METHOD DETERMINED (and should be IDENTICAL as BMW's mark) TDC point.  Rotate the engine forward or backwards & mark the flywheel for what you want in degrees.   You can put markings BTDC & ATDC, or anything else.  Obviously, you can determine F or Z or S markings, if you know what the degrees are supposed to be for those.

10. You "degree-out" (determine specifications) camshafts this same way (BUT at the proper specified valve lift and initial clearance).  The process, for BMW Airheads, involves the degree wheel, dial indicator, setting minimum possible valve clearance, & then at BMW's specified 2 mm valve lift, or, the amount specified by the camshaft maker.

11.  When working on Airheads, you may occasionally need to know the spacing between markings on the flywheel (or clutch carrier).  You may need to know the distance per degree on the circumference.  The diameter of the larger engine'd Airheads flywheels are all the same & there are 2 mm between each degree.  For the R45/R65, it is 1.5 mm between each degree.  These are rounded figures, but accurate enough.

12.  While there are some variances with exact engine dates and numbers, & with the camshaft sprocket & the ATU, the following should suffice for STOCK engines:

The STATIC timing (S mark) should happen at 9 degrees before TDC (before OT) on the /5 & /6 models.   The tolerance is +- 3 degrees.

Later models are at 6 degrees before TDC (OT), tolerance of +- 3 degrees.  The Crossover was in 1977.

The maximum advance is typically at ~2000 RPM on the earliest automatic timing units & ~3000 rpm on later ones.  You could have any ATU or combination of ATU parts, as they all fit mechanically interchangeably, from the /5 models through 1978.  The number stamped onto these ATUs, a Bosch number, is for the entire assembly.  You could have a different pair of springs on the ATU, BMW/Bosch had more than one strength, one of the reasons for 2000 RPM and 3000 RPM maximum timings.   BMW also had different DWELL timing on the ATU's.

The marking for point of maximum advance on the flywheel (F dot or Z line) should be at 25 degrees before OT, tolerance of +- 2 degrees, through the 1977 machines, later engines were 26 degrees.

The mandatory transition of the 1977-1978 machines for the USA occurred NOT at the start of production after the company annual vacation, but on January 1, 1978, to comply with government mandated emissions laws. However, all 1977 machines seem to comply with the later timing...but this is not necessarily absolute, but only what has been reported to me.  See the bulletin in the next section.

For much more on Automatic Timing Units, see

The points dwell angle has varied, and is listed in the above .....
but there is additional information here:

Dual-plugged bikes have their own article:

Below are two ways to access an old advisory bulletin from Butler and Smith (BMW importers-distributors, from long ago).  Here is the flywheel marking bulletin, and some notes on it from me: CLICK!

If you prefer to access it from the Internet:

Information on Butler and Smith is located here, in a large red-outlined box:

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, add emphasis where appropriate.
08/13/2011:  Add the information on identifying degree positions of various sorts.
09/27/2012:  Add QR code; add language button; update Google Ad-sense code; minor editing for clarity.
2013:   Remove language button as the javascript code interferes with some browsers.
05/21/2014:  Update article strictly for clarity.
01/27/2015:  Add sections to the methods and add the data on timing degrees at the end of the article; minor code changes; mark up the photo.
08/09/2015:  Add section on "Simple Method...."; and make minor changes to the over-all text to emphasize being careful and referencing the last part, etc.
12/11/2015:  Revise meta-codes; increase basic font to 14pt, clean up article, clarify details for the newbie, etc.
02/20/2016:  Minor changes to horizontal lines & metacode....and change a few font sizes to larger, miscl (Minor).
03/03/2016:  Final changes to metacodes, layout, clarifications, etc.
06/18/2016:  Re-do entire article to clarify ideas, methods, procedures, layout.
11/26/2016:  Add link to Butler and Smith flywheel markings article & B & S info.
11/28/2016:  Final go-over of excessive HTML changes, and improve clarity of timing marks information.  Fix homepage link.  Possibly to be revisited in near future.

© Copyright, 2014, R. Fleischer

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Last check/edit: Wednesday, September 13, 2017