1949 D2 Pony Magneto Breaking shaft and/or gear

What sort of gear lash do you have in the crankshaft/camshaft/magneto drivetrain? Any chance you are using the same gear on each mag? or did each one have it's own gear whan you installed it?

Another line of questions...how high does your pony rev before the pinion kicks out when the diesel starts? Have you noticed that the magneto shafts broke when the main engine started up? Or do you go to start it and it won't start?

If it's the latter case, maybe you should not let the carburetor run out of gas completely before shutting the mag off. I have always been one that doesn't use the mag switch, but I do usually idle the pony down before I let it run out of gas. Might be it should be left at a higher rpm so there would be less torsional stress from the pony popping as it runs out of gas?

Unfortunately I did not measure the gear back lash when it was apart. I do not remember it being super tight though and the teeth on both gears look good. I have had to change the gear once due to it breaking which I got new from some place in the northwest as I recall. I have not messed with the pony clutch as it does seem to kick out fairly quick after the diesel starts although that is one of the things I am wanting to double check. When the mag breaks it is always when I have started fueling the diesel and squirting some ether. I don't even have the mag ground wire hooked to the switch so I always just run it out of fuel...but I do throttle it down a bit. I should have the kill switch hooked up though in case the pony starts to run away. The diesel itself is a strong runner once it startsbut as I said it does seem to like ether but maybe I am not letting it warm up enough since I want to get the pony shut down before it breaks another mag. The pony also runs real good after the rebuild. I made the front crank bearing and bought the rear crank bearing as wells as rod bearing and rings. I know I should probably take the pony off and tear it apart but that is just not anything I look forward to.

Check this service letter. Sorry it's a bit hard to read.
Suggested fix is to swap mag gears to hopefully pick up some backlash.
Other option is a thicker manifold/top gasket.

Thanks a bunch to both of you for your suggestions. From the service letter it looks like I can check the backlash using a screwdriver through the pipe plug you look through for the timing marks. I thought the first mag broke because of the front main bearing being wore out on the pony motor but in retrospect that was probably not the case as all the other gears in the gear train were solid and had no play in their shafts. I am thinking I can pull the intake manifold and make a thicker gasket without having to pull the pony motor off. Once I get into the little beast I think I will also look at the magneto mounting flange as I may be able to use a die grinder on the mounting holes to raise it up just enough so it don't get slammed.

Hi Team,
also be aware that on the small horizontal ponies a loose flywheel can cause magneto shaft breakage also.
Scanned is a Serv. Mag, Aug. 14 1953, article on this.
They also talk of an upgrade to the manifold machining to provide greater gear clearance--alas they do not cite Serial Numbers, so you may have one of the early covers that is close fitting with regard to gear clearance.
Note also the torque of 150-165 ponds feet for the flywheel nut.
Cheers,
Eddie B.

I got the magneto off the pony today...the gear was broke and the shaft was bent. The flywheel was most definitely loose, I was able to turn the nut off by hand. The keyway in the crankshaft is a little worn but the keyway in the flywheel is twice as wide now as it should be. I am going to use some cast weld rod, heat the inner bore of the flywheel up with a torch and then arc weld most of the keyway shut. Then I will use the lathe and turn the weld down to match the taper. I don't have any taper guides for my keyway broach set so I will have to make one to fit the flywheel so I can cut the keyway back into where I weld. I am tempted to pull the intake off and add some gasket there as well because this is the last good gear and armature shaft I have. I have some ideas on fixing the armature shafts but I have never cut gears although my milling machine could definitely cut them with the right hand at the controls. I had the little beast split in half a few years back to change the main clucth and pressure plate and sure wish I would have machined the hole for an electric starter in the bell housing...although I really do like using the pony motor.

Thanks Old Magnet and Eddie for providing those scans, it is amazing to think a loose flywheel on a pilot motor with good bearings could cause the magneto shaft to break.
Regards
Mike

Thanks Old Magnet and Eddie for providing those scans, it is amazing to think a loose flywheel on a pilot motor with good bearings could cause the magneto shaft to break.
Regards
Mike

When I bought my HT4 the pony flywheel was loose, it was knocking real loud. The nut was tight but the flywheel would still rock 5 degrees or so. The gear on the mag came loose and fell off when I went to pick it up. When I got it home I slapped on a different flywheel off a parts pony. It tightened up real good and has never given any more problems.