Why does the Mag rotor keep breaking

Not sure if this applies to the rotor, but mag shafts breaking can be attributed to a loose pony flywheel and/or worn pony crank bearings I've heard.
Is your flywheel tight on the crankshaft, and what's the play like in the crank? Don't want any more than about 015 radial and 025 axial to stay in spec. Lots of ponies run looser than than but it does risk the mag shaft.

300 for a rotor sounds very high. There are a couple of shops referenced on here that I'm sure can do much better than that price. By the way, update your profile (up there top of page on the right "My Profile" to update your city and state. There may be a member close by that can advise)

Thanks Neil.
It was a complete mag that I bought for 300. I talked to CAT they still offer a rotor that they have a few around for 110$ or so. I found another place in Chicago that wanted 140$ I think.

I've heard the same thing from the salvage guy I got the mag from. I thought it was hard to believe that you could get that much play in the cam (enough to break mag shaft or climb a tooth) without it being completely wore out. If I'm going to pull it apart to redo the bearing might as well rebuild the whole motor I suppose. Any idea what I'd be looking at for that?

Thanks for the Feed back.

Good timing as I've just done two lately. Rod shells are 30 from Cat per rod. Mains I can't remember but think are in the 50-100 range - need two of those, although for my two ponies, my machine shop picked up some generic oilite bushings and turned them to suit. Some folks make their own out of aluminum. It's basically a plain bushing with some oil channels cut into it, and a dowel hole that is drilled once the position of the bushing is established for crank axial float (010-015, 025 max). Head gaskets are cheap but you can also just reuse your existing ones. There are quite a few gaskets but gaskets, nuts and bolts from Cat are cheap. If you don't have a parts book for you serial # range, pick one up from ebay, in order to provide your parts man with the list of gasket parts. A carb kit runs about 50 from a few suppliers listed in the stickies up top. I had a mag reconditioned by Mainely Magnetos a couple of years ago for around 350 - it's like new.
Main thing in addition to all this is to thoroughly clean the coolant passages as they tend to clog up with sediment over time. First rod them out being sure to a) get the little holes at the bottom of the head and the corresponding hole in the block, and then the passage that crosses over the base of the block. That one is easiest to clean out with an old speedo cable. Alternatively you can pop out the frost plug on the underside and get in there that way.
Once you've done all that, it'll start on first or second pull, and idle sweetly. I install the carb idle jet/port/plug a little different to as-factory which for me greatly helped with the ability to idle. As-factory has it installed with the notch on the inner end more or less facing down, and with the parting line on the outer end vertical. Instead, I install it with the larger of the two notches on the inner end at bottom dead center so that the fuel can flow more easily. I found on mine that the throttle plate when closed obstructed the notch preventing mixture flow and thus no idle. As soon as I rotated that notch to BTDC, it idled like a couple of old Southern gentlemen sitting on the bench outside the store.

Is the magneto distributor shaft and /or bushings worn enough for the rotor to strike the cap?

Before you rebuild the entire tractor check that you have back lash (clearance) at the magneto drive gear. American Bosch parts diagrams refer to both the distributor rotor and magneto armature as rotors, the armature one being "rotor, magnet"
There is a AB Service Bulletin published about 1948 that states "due to accumulation of manufacturing tolerances" the total of some units may eliminate the backlash clearance at the gears. They suggest trying another magneto. If that doesn't cure it a thicker manifold gasket may be required to gain some clearance. In any event if there is pressure on the magneto drive gear or severe pulses due to loose flywheel or worn clearances you can expect rotor, magnet breakage.

Cat no longer has any pistons for the pony engines [D2/D4]

Found the service letter.....tough to read I know.

Old magnet.

Is this issue only with the Bosch mags or the wico also?

You would think cat could've made a new mag gear a shade smaller to allow for tolerance that could be easily replaced.

Old magnet.

Is this issue only with the Bosch mags or the wico also?

You would think cat could've made a new mag gear a shade smaller to allow for tolerance that could be easily replaced.