| Also known as | T3991 |
| Diagram | Fig. 8 — GEARBOX SHAFTS, BEARINGS AND INNER COVER · ref 9 |
| Subsystem | gearbox |
| Listed in parts books | 1970, 1974 |
Mainshaft High Gear (22T)
The 22-tooth mainshaft high gear for the gearbox shaft cluster, listed as 57-3991 (formerly 57-1947 and 57-2318).
This is the mainshaft high gear, a 22-tooth gear that lives on the mainshaft inside the gearbox, part of the shaft and bearing assembly shown in the gearbox shafts, bearings and inner cover section of the parts book. It's the top gear in the cluster, the one that gives you high gear drive off the mainshaft.
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The part number 57-3991 replaced two earlier factory numbers, 57-1947 and 57-2318, so if your old parts book or a gear pulled from an earlier box shows either of those numbers, it's the same physical piece under a different catalog listing. The pre-1973 alias T3991 is just the legacy letter-prefix form of the same part number.
Our catalog record shows this one appearing across gearbox listings from 1970 to 1974. It doesn't spell out a specific model, so check tooth count and spline fit against what's currently in your gearbox before ordering, and give us a call if you want help confirming it lines up.
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