Yanmar 2GM20 Rebuild, common faults and everything you need to know
The little 2-cylinder that powers a generation of 25-35 foot cruising yachts. Few components, mechanical simplicity, a full rebuild is realistic.
The 2GM20 is one of the most rebuildable marine diesels in its size class - a purely mechanical design with no electronics, a robust cast-iron block, IDI with a separate swirl pre-chamber, and spare parts still available for 40-year-old examples. The engine shares its block, cylinder head, crankshaft and all mountings with the original 2GM (the 2GM20 is a 75 mm rebore of the 72 mm 2GM), so Yanmar's 2GM column in the factory torque table applies in full. But it has one documented villain: the mixing elbow. Just as on the siblings the 3GM30F Yanmar's own service instructions recommend replacement every 500 running hours or every other year - hydraulic lock from an old mixing elbow is the most documented cause of death on the GM series. Remove it at every major service. We carry out cylinder head reconditioning, cylinder boring, crankshaft reconditioning and complete engine reconditioning in our own machine shop with 12-month warranty. See also our boat engine rebuild hub page.
The 2GM20 at a glance
Sourced from the Yanmar 2GM20 workshop manual (FSM, spec p.1-5 and the 2GM20 spec page), supplemented with Hallberg-Rassy Club and Wikipedia datasheets. Power is stated as one-hour rating and continuous DIN6270.
Variants of the GM family
| Variant | Cyl / cooling system | Power |
|---|---|---|
| 1GM10 | 1-cyl, raw water cooled | 9 hk |
| 2GM (bas) | 2-cyl, 72 mm bore, 586 cc | 15 hk |
| 2GM20 Current | 2-cyl, 75 mm bore, raw water cooled | 18 hk |
| 2GM20F | 2-cyl, heat exchanger (fresh water), ~114 kg | 18 hk |
| 3GM30 (sibling) | 3-cyl, 954 cc | 27 hk |
Note - torque: Yanmar publishes the 2GM20 only as a hp rating (16 hp cont. / 18 hp max) because marine propeller load is governed by power, not a tabulated Nm torque figure. There is therefore no factory-tabulated peak torque in Nm - the Nm figures further down the page are exclusively tightening torques for bolted joints, not the engine's power output.
2GM20 tightening torque
All figures from the Yanmar 2GM20 workshop manual, torque table p.12-44 to 12-46 (the 2GM column), cross-verified against Service Standard p.13-13 and the torque diagrams. Yanmar states values in kg-m; the Nm conversion is shown alongside. Oil the threads where the manual requires it.
Cylinder head procedure: oil the threads, torque the six M12 nuts in sequence 1-2-3-4-5-6 in three stages (1/3 -> 2/3 -> full 98 Nm), re-check the torque and then re-adjust the valve clearance. Note that a factory re-torque in service (chapter 13) is a lower re-tightening figure, not the installation torque. No glow plug and no separate oil drain plug is tabulated - the GM series has no glow plugs (manual decompression start) and the 0,9 kg-m figure refers to the bottom cover sheet-metal bolt, not a drain plug.
Common faults on the 2GM20
Cross-verified against Practical Boat Owner, Hallberg-Rassy Club, SailNet, Cruisers Forum, Yanmar's own service instructions and Practical Sailor.
Mixing elbow corrodes - hydraulic lock
500 hours / 2 years • Critical- Symptoms
- Salt water in the exhaust, possibly water straight into the cylinders via the exhaust path, the engine refuses to start after a period of standing idle.
- Cause
- The mixing elbow mixes salt water with exhaust gas. Yanmar's own service instructions specify replacement every 500 running hours or every other year. Hydraulic lock from a corroded elbow is the most documented cause of death on the GM-series diesels.
Raw water pump shaft seal leaks
Standard maintenance • High- Symptoms
- Salt water drips from the shaft side of the pump, the shaft becomes scored, and cooling water flow drops.
- Cause
- Natural wear on the shaft seal in the rubber-impeller pump. On replacement the shaft is checked for scoring - it must be replaced if scoring is present. The housing bolt is torqued to 24,5 Nm on assembly.
Camshaft seal leaks oil
High running hours • Medium- Symptoms
- An oil trail along the side of the block, oil drips under the engine when standing.
- Cause
- The seal ages. The 2GM20 has a gear-driven side camshaft; the end nut (M20) is torqued to 68,6 - 78,5 Nm on assembly.
Cylinder and piston wear
5 000+ hours • Medium- Symptoms
- Falling compression, rising oil consumption, blue smoke on acceleration.
- Cause
- Natural wear. The block is bored and honed during the rebuild; the connecting rod bolts (M7) are torqued to 24,5 Nm.
Injector nozzles coke up
3 000+ hours • Medium- Symptoms
- Rough idle, black smoke under load, falling power.
- Cause
- The pintle needles coke up and atomisation deteriorates. Serviced and tested on a flow rig; the nozzle nut (M20) is torqued to 98 Nm and the retaining nut to the head to 19,6 Nm.
Heat exchanger blocked (2GM20F)
10+ years • Medium- Symptoms
- Overheating under load, falling operating temperature, reduced cooling flow.
- Cause
- Scale and salt deposits on the raw water side of the heat exchanger. Applies to the F variant with a closed fresh water system; check the sacrificial anode at the same time (49,0 - 58,8 Nm).
2GM20 rebuild prices
Small engine, less work. Based on 2 cyl 75 mm. Excl. VAT and parts. Main operations - cylinder boring, crankshaft grinding, cylinder head resurfacing - carried out in our own machine shop.
Cylinder head reconditioning
Bottom end
Full rebuild
Mixing elbow
Cylinder machining
| Service | Price |
|---|---|
| Cylinder boring 2 cyl (75 mm) | 1 659 kr |
| Plateau honing 2 cyl | 583 kr |
| Block O-ringing 2 cyl | 1 467 kr |
Crankshaft work (2-cyl)
| Service | Price |
|---|---|
| Crankshaft grinding 2-cyl (3 main + 2 rod) | 1 807 kr |
| Polishing standard | 1 022 kr |
| Cleaning 2-cyl crankshaft | 588 kr |
Cylinder head reconditioning 2-cyl
| Service | Price |
|---|---|
| Resurfacing cast-iron cylinder head 2 cyl | 1 075 kr |
| Pressure testing cylinder head 2 cyl | 1 580 kr |
| Valve seat cutting 4 valves | 495 kr |
| Valve guide replacement 4 pcs | 790 kr |
See also
Other Yanmar engines and related services.
Questions and answers about the 2GM20
What is the mixing elbow fault?
The exhaust mixing elbow corrodes in salt water. Just as on the siblings the 1GM10 and 3GM30, Yanmar's service instructions recommend replacement every 500 running hours or every other year. Hydraulic lock from an old mixing elbow is the most documented cause of death on the GM-series diesels. Remove and inspect it at every major service.
What is the cylinder head torque on the 2GM20?
The cylinder head's six M12 main nuts are torqued to 98 Nm (10 kg-m) per Yanmar's workshop manual. Oil the threads and torque in sequence 1 to 6 in three stages (1/3, 2/3, full), re-check and then re-adjust the valve clearance. The two extra M8 bolts are torqued to 24,5 Nm. The figures come from the Yanmar FSM torque table p.12-44 and the cylinder head sheet p.2-18.
What does the F in 2GM20F mean?
Freshwater-cooled - a fresh water cooling system via a heat exchanger. Fresh water circulates in the engine, salt water runs through the heat exchanger as a secondary circuit with the same rubber impeller pump. The base 2GM20 (without F) is directly raw water cooled and weighs ~106 kg; the 2GM20F with heat exchanger weighs ~114 kg.
Does the 2GM20 have glow plugs?
No. The whole GM series (1GM10, 2GM20, 3GM30) cold-starts with a manual decompression lever and has no glow plugs. Yanmar's torque table therefore lists no glow plug entry. Nor is there any separate oil drain plug tabulated - the 0,9 kg-m figure refers to the bottom cover sheet-metal bolt.
What does it cost to rebuild a 2GM20?
Cylinder head reconditioning 4 600-6 400 kr labour. Complete engine rebuild 18 000-27 000 kr labour, plus parts 4 000-10 000 kr. The mixing elbow always adds 2 500-5 500 kr.
What is the difference from the 3GM30?
The 2GM20 is a 2-cylinder (636 cc, 18 hp) while the 3GM30 is a 3-cylinder (954 cc, 27 hp). They share the same basic design, torque table and weak points (mixing elbow, raw water pump). The 2GM20 suits 25-31 foot yachts, the 3GM30 suits 30-35 ft. Spare parts overlap within the GM series.
Have a 2GM20 that needs looking at?
Call us or send a quote request. Reply within 24 hours. We service all types of boat engines - see boat engine page. All rebuilds with 12-month warranty.