V-belt alternator Yanmar 1GM | 1GM10 | 1GML – HM29 (730 mm)

Description

Replacement alternator belt for the Yanmar 1GM series: the single-cylinder sailboat engine found in countless small sailboats and classic yachts. This belt runs from the crankshaft pulley to the alternator and is the only belt on this engine.

Yanmar designates belts by profile letter plus nominal length in inches. HM29 therefore stands for a 10 mm narrow section profile of 29 inches, which is approximately 730 mm.

✔️ The short version for the early 1GM and 1GM10 — prior to the alternator change at serial number E03691
✔️ Narrow section profile 10 x 6 mm, exactly as prescribed by Yanmar for the single-cylinder
✔️ Easy to replace yourself: the 1GM does not have a second belt that needs to be removed first

Dimensions

• Designation: HM29
• Profile: 10 x 6 mm
• Length: approx. 730 mm (29 inch nominal)

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

Suitable for

• Yanmar: 1GM, 1GM10, 1GML — the versions with the original alternator

Which of the two do you need?

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

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

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

Please note

• The 1GM and 1GM10 do not have a raw water pump belt. On this single-cylinder engine, the raw water pump is gear-driven from the camshaft. Sales lists that offer a second belt for this engine are incorrect. The two- and three-cylinder 2GM20, 3GM30, 3HM, and 3HM35 do have that second belt.

Maintenance

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

OEM for reference

• 128170-77350
• 12817077350