Timing marks on lightened flywheel

Looking for help from someone who has an airhead flywheel sitting around. I am changing the rear main seal and clutch on my R90/6 and when I got it apart I noticed the previous owner lightened the flywheel. In the process the O/T marks were removed. The F dot is still there and it looks like the OT mark was painted on at one point but I am not sure which paint mark is OT. Long story short I am looking for either a measurement from the F dot to OT or the number of teeth on the flywheel between the two so that I can add some white paint to the OT mark(s). New to the forum so hopefully I am posting correctly and the attachment with the image of my flywheel comes through.

P.S. I did mark and block the crank before removing the flywheel.

Welcome to the forum! The circumference of the flywheel should be 720mm for 360 degrees, so you can use 2mm along the circumference as 1 degree. Try to measure your circumference and see where is it that you get 720...it's probably at the outer edge of the teeth. For your bike, full advance should be 34 degrees BTDC. The static timing mark is 9 degrees BTDC. So you can start with your F-mark at full advance and work backwards from there to find what should be an OT position.

The flywheel will turn counter clockwise when looking from the back of the engine...I hope I have that right!!

20774 said:

Welcome to the forum! The circumference of the flywheel should be 720mm for 360 degrees, so you can use 2mm along the circumference as 1 degree. Try to measure your circumference and see where is it that you get 720...it's probably at the outer edge of the teeth. For your bike, full advance should be 34 degrees BTDC. The static timing mark is 9 degrees BTDC. So you can start with your F-mark at full advance and work backwards from there to find what should be an OT position.

The flywheel will turn counter clockwise when looking from the back of the engine...I hope I have that right!!

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Ah yes, the dreaded "swiss cheese" flywheel! I just happened to have my '76 R90/6 flywheel on my bench so I borrowed my wife's cloth sewing tape measure (sssh) and show a measurement of 64mm. When I looked at your picture again I realised that you can't use that number, as your ridge is now missing. The number of teeth is exactly 9, just where your very faint white mark is. I will send you a picture within the hour when my tech support gets home. I inherited my '78 R100RS from my childhood friend who was the original owner. The first time that I went to check the timing I made the same discovery as you have- marks gone. I was able to buy a brand new flywheel, surplus, from Motobins and plan to swap it in the next time that I pull the tranny for a spline lube. Luckily it was upgraded to electronic ignition so the timing will not have changed. If you wish to revert to stock too, PM me. While you are in there anyway, there are a few things that you might consider doing. These include flushing the trans and cleaning/lubing the throwout bearing. If you require some insight, let the collective know. This site is populated by some very experienced, generous (air)Hedze. It would be a good idea to state which year of /6 on which you are working.

Cheers, Russ '76 R90/6 (retired/ parts machine) '78 R80/7 '78 R100RS

Flywheel measurements

Thanks for the quick and thorough replies. I also borrowed my wife's fabric measuring tape but it's now got a big grease mark on it so not telling her will only temporarily keep me out of trouble. On the outside of the teeth it measures 738mm my guess is 720 is at the bottom of the teeth. I do have the Dyna 3 electronic ignition so I think beemeruss gets me close enough to help set it near TDC for adjusting valves. I am also putting in a used supposedly good transmission but if there is anything I should check or change on it I appreciate the advice.

lugnut said:

Thanks for the quick and thorough replies. I also borrowed my wife's fabric measuring tape but it's now got a big grease mark on it so not telling her will only temporarily keep me out of trouble. On the outside of the teeth it measures 738mm my guess is 720 is at the bottom of the teeth. I do have the Dyna 3 electronic ignition so I think beemeruss gets me close enough to help set it near TDC for adjusting valves. I am also putting in a used supposedly good transmission but if there is anything I should check or change on it I appreciate the advice.

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Tech support and I could not figure out the technique for sending my pictures yet so they will be delayed. I hope that your donor transmission is from a known source. My R90 that I had sold to my youngest brother 2,000 miles away was left sitting outside uncovered for 10+ years. When I got it back I found to my horror that the rotted speedometer cable boot had allowed rainwater to enter and flood the transmission. Put a new boot on your shopping list. It is also important to ensure that it is compatible for your bike. I understand that the '74 (first year) 5 speed boxes were suspect, but any up to '80 should fit. After that the whole clutch assembly changed to a superior style but the input shaft length changed. Whenever I pull a transmission I usually slosh a litre of diesel around inside and pretend that it is a cocktail shaker. Screen the dregs and look for any swarf that may be present. Those R90's are great bikes and I hope that yours is a keeper.

Russ

Top Dead Center

Hey guys, lots of good stuff here.
On and off over the past year of getting my soiled dove of a 78RS back on the road, I thought of lightening the flywheel, Now I have one reason in the do not do it column, a missing timing mark.

I don't want to be a jerk but top dead center can be determined by looking in the spark plug holes while rotating the engine slowly. Of course, I can see the problem with a missing marker, what can you substitute in it's place once you do find TDC that will hold up over time so you can set the timing with a timing light?

Personally, I don't think a scribe line engraved in the teeth would cause a lot of problems and would last longer than paint.

Alright, I have blabbed my nonsense and I will go back in my cage to be quiet. Cheers, St.

STEVENRANKIN said:

I don't want to be a jerk but top dead center can be determined by looking in the spark plug holes while rotating the engine slowly.

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That works for finding the TDC ballpark, but you have to have a lot better eye than I do to get it exact for ignition timing purposes. It takes a very sensitive and accurate depth gauge to be sure. The lateral piston motion with 3 or so degrees of crank rotation is too minuscule for my eye to be sure about.

Thanks Paul

I have added a bit more knowledge to the addled brain, Thanks Paul. St.

Timing marks on ANY airhead flywheel or clutch carrier

This article of mine is a full treatment (I think....>>so, any confusion, or need for clarification, ask me) of the flywheel (aka Clutch carrier on late models) markings. How to PRECISELY determine TDC (OT mark), or, any other mark, precisely. How to properly install it, ETC (rather a lot of ETC):
https://bmwmotorcycletech.info/F,OT,S,Z.htm

This is one of the professional ways of doing it. It explains how to use a piston stop device (available inexpensively, or, make your own).

Snowbum