| Also known as | E3971 |
| Diagram | Fig. 1 — CRANKSHAFT, PISTON AND TIMING GEARS · ref 39 |
| Subsystem | engine bottom |
| Listed in parts books | 1963–1964 |
ROTOR NUT T20 SMALL DIAMETER ROTOR (5/8)
Rotor retaining nut for the T20 Cub's small-diameter (5/8) alternator rotor.
This is the rotor nut for the T20 Cub, the small-diameter (5/8) alternator rotor version. It sits at the end of the crankshaft, on the timing side, and holds the alternator rotor in place once it's fitted over the shaft taper. It shows up in the crankshaft, piston, and timing gears section of the parts book, and the older factory number for it was E3971 before the modern numbering system took over.
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If your Cub has the small-diameter rotor, this is the nut that keeps it seated and torqued down against the crankshaft. Rotor size matters here - there's a separate nut for the large-bore rotor setup, so it's worth checking which rotor is actually on your engine before ordering.
It's a small, inexpensive item to keep on hand, and since it only fits the small-diameter rotor application, double-check your rotor before you buy so you don't end up with the wrong nut for the job.
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