Confronted with some old metal sharknoses that ran too fast and after a lot of research and head scratching, the alternate regear method was developed; but never applied to these.
The power trucks on older diesels and trolley cars had very similar motor shaft mounted worms with a 14/1 gear ratio and ran too fast. After measuring, it appeared that a 1/2" cube neodymium magnet would fit into a standard MU-2 power truck.
First check magnet polarity with a compass. Then remove cover plate screws or drill out rivets. Remove wheels and bearings, Some versions have long rivets under cover plates securing bearings. Remove old magnet and pole pieces by removing screw or rivet. Rivets are usually steel which partially shorts out magnetic field. Remove brushes and springs, leaving mount in place.
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Magnet and pole pieces mounted on cut down truck.
Remove all extensions on top of housing, to lower new magnet, by using a mill or rotary tool and file. The cut should be almost to the spring mounting pin on that side.
Install brushes and springs. Check new magnet polarity and mount with pole pieces pushing against brush mount. The magnet will extend slightly past the opposite edge of the pole pieces, but this causes no problem and reduces flux minimally. Test motor by applying low voltage. There may be some drag on the armature, which can be eliminated by adding shims in the housing slots to angle bottoms outward slightly until clear. Magnet will hold pole pieces, but a little CA may be added to be sure.
In spite of the the fact that the steel cover plate provides a short magnetic path across pole piece ends, the speed reduced from 140 to a more believable 82 SMPH at 12 V and current was cut from .43 amp to .30 amp.
