Norton Commando Layshaft 3rd Gear 20T 040634 AMC Gearbox

Norton Commando Layshaft 3rd Gear 20T – Part 040634 (AMC Four-Speed Gearbox)

The Norton Commando layshaft 3rd gear 20T — OEM part number 040634 — is the correct replacement third gear for the AMC four-speed gearbox fitted to all Norton Commando 750 and 850 models from 1968 through 1974. If your Commando gearbox is jumping out of third, whining under load, or showing worn tooth profiles, this is the gear you need. Twenty teeth, correct ratio, correct material — this is what Norton specified and what these gearboxes were designed to run.

The AMC gearbox is a strong and well-designed unit, but like any gearbox that’s been working hard for fifty-plus years, the layshaft gears take a beating. Third gear in particular sees the most use on the road — it’s where you spend most of your time at legal speeds in the UK, and where a worn gear makes itself known with that characteristic clonk or jump. Replacing with the correct 20-tooth layshaft gear paired with an inspected mainshaft 3rd gear restores the tight, positive engagement these gearboxes are known for when set up properly.

As with any gear replacement, inspect the mating mainshaft gear, selector forks, and layshaft bearings while the box is apart. Replace worn components together to avoid a repeat stripdown. This gear pairs directly with the mainshaft 3rd gear in the standard AMC four-speed configuration.

Specifications

• Teeth: 20T
• Part Number: 040634
• Gearbox: AMC four-speed
• Position: Layshaft 3rd gear

Fitment

• Norton Commando 750 – 1968, 1969, 1970, 1971, 1972, 1973
• Norton Commando 850 – 1973, 1974

Stocked at Fair Spares New England. Questions about your specific gearbox build? Get in touch — we know these transmissions inside and out.