All,

I just found out I have a broken magneto on one of my D2 5U's. The stator shaft broke inside next to the bearing. It was working right up to this point. So, I need either a good used replacement or a rebuild service. Also, the starter Bendix and the gear they mesh with are shot. Bendix part number 9B8949. The gear that the Bendix meshes with is part number 7B2074. It might be simpler to get the whole cast piece with the gear, pony drive pulley, and shield.

Aaron

"AaronH" wrote:

All,

I just found out I have a broken magneto on one of my D2 5U's. The stator shaft broke inside next to the bearing. It was working right up to this point. So, I need either a good used replacement or a rebuild service. Also, the starter Bendix and the gear they mesh with are shot. Bendix part number 9B8949. The gear that the Bendix meshes with is part number 7B2074. It might be simpler to get the whole cast piece with the gear, pony drive pulley, and shield.

Aaron

Aaron, you don't say what brand mag you have, the Wyco or Prestsolight mags have a smaller shaft than the Amercan Bosch and probably more prone to breaking.

It is an American Bosch MJK. I don't think I broke it, perhaps I did. I am think it was broke before and just happen to get enough grab that it was turning.

Hi Aaron,
this scan may help. Check both for loose flywheel and sometimes very worn pony main bearings could contribute to this also.
Cheers,
Eddie B.

Eddie,

Thanks for the info. When I got home today I took a look at it. The flywheel is indeed loose. I will inspect the keys and reinstall per the instructions. The crank has quite a lot of end play as well. I don't know how much, but I'm sure it's more than spec. There doesn't seem to be much backlash in the gears, so that's a good thing. I still need to fix my mag, however. I can turn a new shaft on my lathe, but it will be a fair amount of work. I'd rather find a good working used unit to get it back running for now and send the broken one in for rebuild later.

Aaron

Some backlash is critical to shaft life. Fairly common problem. About the only way to correct is maybe swap mag gears or go to a thicker manifold gasket.

Hi Aaron,
the spec for end clearance is 0.010"-0.015" with max at 0.025"
As OM suggests as the flywheel gyrates around it takes up the clearance between the crank to cam gear and inturn the mag gear causing impact/shock/bending loads as the backlash clearance closes and opens. These loads both bend the mag shaft and also imparts torsional loads as well--all in all a hostile environment for the small mag shaft.
Cheers,
Eddie B.

The backlash problem I'm referring to doesn't really have anything to do with the flywheel and bearings. Has more to do with manufacturing tolerances.

I know it's hard to read but I didn't want to take the AB manual apart to copy, it's about five inches thick.

Thanks for the info. The backlash I was speaking about earlier is that between the crank and cam gears. My initial suspicion was that there was too much backlash causing things to chuck around. In reality there is some crank/cam backlash, but not what I would consider excessive. I don't know about cam to magneto backlash as I have already removed the mag but I will be sure to check that when I reassemble. The most likely culprit is the loose flywheel. I don't know why I didn't think of that first. When I was a kid working on junk lawnmowers I learned that a loose flywheel likes to shear keys and throw your timing all out of whack. I just hope the tapers and keyways are ok.

Looking at my books I can't quite tell what determines crank end play. Is it the main bearing or is there a separate thrust washer?

Anyone have one of these mags that they are willing to part with?

I have a magneto rotor shaft and a complet magneto with no gear
2a4144 on and have the bendix for the startor and more