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chasbmw 01-18-2013 04:38 AM

lightened flywheel
The flywheel on my R90/6 has been lightened, it weighs 2.380 KG, here are a couple of photos

This works very well and the engine has been balenced to suit.

However, I do have a problem wear of the starter ring gear. This wear was caused by fitting an 9 tooth starter on a flywheel meant to have an 8 tooth one, coupled with the starter relay sticking on twice, and each time it took me too long to get the tools out to disconnect the battery. as a result I have a very noisy starter, which does not always properly engage and disengage.

What is the solution?

AntonLargiader 01-18-2013 05:58 AM

I'd say that flywheel is not a problem, and you need to get the starter overhauled.

Wirespokes 01-18-2013 06:59 AM

I'll second that.

disston 01-18-2013 07:54 AM

I agree, it's not the ring gear. Since the part you are likely to need for the starter is the unavailable Bendix (not sure this is proper use of the term but think you know what I mean) you will either get one of the modern replacements that cost around $300. Or get a used OEM Bosch. I like the idea of a used starter. They can be had on Ebay sometimes for less than $50.

Very pretty flywheel.

Airhead Wrangler 01-18-2013 09:21 AM

Not a lot of meat left on that bone. Nice work.

supershaft 01-18-2013 10:34 AM

I think a couple of people here have said they have replaced the gear ring. Reportedly it's an interference fit.

chasbmw 01-18-2013 10:53 AM

I dont think that I explained my issues properly, The starter motor was replaced with a ND after the damage was caused to the Ring gear. By both the 9 tooth starter and the starter sticking on.

Currently it works OK most of the time, but on occasions baulks and does not engage properly. The engine has 9.5 CR pistons and requires quite a bit of effort to turn over even with powerfull starter and battery.

Options might be:

1 Clean up Ring gear with dremel, take off the nicks and burrs.

2 Find new ring gear and fit, rebalance flywheel.

3 Replace flywheel, get it lightened and rebalance

The problem with all the options except 1 is that the engine was balanced together with flywheel and clutch and I don't want to go down the rebalancing route due to time and cost!

Kai Ju 01-18-2013 12:24 PM

wear pattern

Was that wear pattern caused by the incorrect starter motor. I would have to assume so since none of the flywheels I've looked at lately look like that. Makes me wonder if the starter motor gear hangs up on those v-shaped notches instead of engaging the ring gear teeth.

Maybe you can have the ring gear turned/machined to eliminate the notches and restore normal function.
If you can find somebody that can remove the ring gear, just take this one off and flip it over so the good side faces the starter.

Ducatiparts.110mb 01-18-2013 01:30 PM

I'm not an expert on air cooled BMWs, so please forgive me if I'm talking bo11ocks.

However taking the essence of the problem as I understand it -

The starter doesn't willingly engage or disengage sometimes
The starter struggles to turn over the engine

Aren't these both symptoms of having raised the starter gearing by having changed from an 8 tooth pinion to a 9 tooth one? This has effectively raised the gearing by 12.5% so that the starter tries to turn the engine 12.5% faster than it used to, and the engine resists the motor 12.5% more than it used to.

If the teeth on the starter are going past the teeth on the ring gear 12.5% faster then it will be harder for them to find the point of mesh.

Also if the starter gear is effectively 12.5% bigger diameter (on account of being 12.5% bigger circumference) then the backlash is reduced again making it harder for the gears to mesh. This would also account for the teeth on the ring gear looking like they've been hobbed out to a different toothform in the photos.

Finally if the starter is higher geared then it will struggle more to turn the engine over.

I would expect the V shaped notches to be intentional and provide a "lead-in" for the starter pinion teeth as they move into mesh with the ring gear.

Why did you go for a 9 tooth starter pinion?

If I were you I'd go back to a 9 tooth pinion and clean up the burrs with the dremel as you suggest. In the car world it used to be standard practice to shrink the tooth ring onto a flywheel. I'm a bit out of touch with modern cars so not sure if that's still a current practice. However looking at the pictures of both sides of your flywheel there doesn't seem to be a clear line showing where the flywheel stops and the ring gear starts - it looks more like the teeth are machined directly into the flywheel.

Nice machining by the way. If my explanation is wrong I'd be happy to be put right.

Chas - Just seen that you're in Bath. Me too. Oldfield Park.


disston 01-18-2013 01:59 PM

Unless I have miss understood this there were two problems. incorrect 9 tooth starter was used for a while. The flywheel is an 8 tooth starter flywheel it doesn't like a 9 tooth starter. When this is done, this is not the first time somebody has used the wrong starter, it damages the gear teeth on the ring gear. The wrong starter gear will not match the gear spaces of the ring gear.

2...the starter relay froze on several times and the wrong starter ran on and on causing a terrible racket and more damage.

3...a new Nippon Denso starter has now been fitted. It has the correct 8 tooth gear but it sometimes doesn't engage all the way or fails to crank the high compression motor.

What Chas suspects is that the damage to the ring gear are preventing the starter gear from engaging.

I still think the flywheel is not damaged enough to cause his complaint. I think the ND starter needs brushes or cleaning. Maybe the battery is weak or the charging system is marginal. I could be wrong, that's just what I think at this point.

Those V marks in the gear teeth do not look like they are caused by any starter bendix. The damage to the gear teeth seems to be, to me, a little bit of banging on the top corner edge. This sort of damage is often caused by the correct starter.

Maybe somebody with a bunch more experience than I have will see more here.

I have a rather elaborate suggestion. Put a stock flywheel with good ring gear on the bike now. Reassemble and see if the problem is now solved. You'll have the use of a much slower bike for a while but then if the problem is solved by the stock flywheel you will know it is the problem

chasbmw 01-19-2013 01:04 AM


Dissiton is correct about the sequence of events.

The bike currently has a year old new ND starter, 450w alternator and 2 year old Pc680. I had hoped that the new ND might have sorted the problem, but it remains to a degree.

I don't really want to go to a new flywheel, the flywheels or 9 tooth are still available from Motobins at a bargain price, but then I lose the advantages of both the lightening, the engine has been balanced to suit the flywheel and I would have to find a 9 tooth gear for the ND starter.

There is an old school bike workshop machinist in Bristol called Pistonbroke, I might see what he can suggest about cleaning up the ring gear

Can anyone post a close up picture of lightly worn ring gear with an emphasis on the the starter side of the ring gear?

Thanks to you all


boxertricks 01-19-2013 03:01 AM

boxertricks 01-19-2013 03:07 AM This flywheel is from a 1976 /6 with about 40000 miles on it, Hope it helps, if you want any more let me know, Ray

Kai Ju 01-19-2013 09:33 AM

44,000 mile flywheel
Here is a pic of mine that I just removed with about 44,000 miles on the clock.

and a gratuitous picture of the flywheel that replaced it.

chasbmw 01-19-2013 10:43 AM


Yours shows benefits of CNC stuff. Mine is slightly lighter at 5.24lbs.

Is the picture of your old flywheel showing the starter side of the ring gear?

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