V-belt alternator Yanmar 1GM10 | 2QM15 | 1GMY – HM31 (790 mm)

Description

Replacement alternator belt for the later Yanmar 1GM10 single-cylinder, the twin-cylinder 2QM15, and the 1GMY. This belt runs from the crankshaft pulley to the alternator. On the 1GM10 and 1GMY this is the only belt; the 2QM15 also has a separate, shorter belt for the seawater pump.

Yanmar specifies belts using the profile letter plus the nominal length in inches. Therefore, HM31 stands for a 10 mm narrow profile of 31 inches, which is approximately 790 mm.

✔️ The long version for the later 1GM10 — after the alternator change at serial number E03691
✔️ Also the prescribed alternator belt for the classic 2QM15 and for the 1GMY-C and 1GMY-EC
✔️ Narrow profile 10 x 6 mm, exactly as specified by Yanmar

Dimensions

• Designation: HM31
• Profile: 10 x 6 mm
• Length: approx. 790 mm (31 inch nominal)

Belt lengths are specified in various ways: Li is the inner circumference, Lw/Ld is the pitch length, and La is the outer circumference. When in doubt, measure your old belt.

Suitable for

• Yanmar: 1GM10, 1GML (later version), 2QM15, 1GMY-C, 1GMY-EC

Which of the two do you need?

During the production of the 1GM10, Yanmar installed a different alternator with a different pulley diameter, which also changed the belt length. The parts catalog places the change at serial number E03691 (1GM10C at E00336, 1GM10D at E00341, 1GM10L at E00218):

• HM31, approx. 790 mm — this item, for the later version, and always for the 2QM15 and 1GMY
• HM29, approx. 730 mm — for the version with the original alternator

Sources contradict each other regarding which exact version falls before or after that boundary. Therefore, the safest approach is simple: measure the old belt before ordering, or compare its length with the two sizes above.

Please note

• The 1GM10 and 1GMY do NOT have a seawater pump belt. On these single-cylinder engines, the seawater pump is driven by gears from the camshaft. Sales lists that offer a second belt for these are incorrect.
• The 2QM15 DOES have two belts: this alternator belt plus a short belt (HM23, approx. 584 mm) for the seawater pump. It is recommended to replace them together.

Maintenance

Yanmar does not specify a fixed replacement interval for the GM and QM series: check the tension every 250 operating hours and replace the belt if you notice cracks, fraying, or a glazed flank. Having a spare belt on board is not a luxury — without an alternator belt, the battery will no longer charge.

OEM for reference

• 105582-77790
• 10558277790