Friday, April 8, 2022

Changing the starter motor on a Yanmar 2GM20

Whilst weather bound in Newlyn there were 2 days with no sun and insufficient wind to operate the wind generator (the harbour is well sheltered from the West) and I had been using power for the heater, cool box etc., so I decided to run the engine for a while. All I got was a click from the starter solenoid, having checked those connections I could and finding nothing the engine finally turned over after several dozen attempts. I stopped and started the engine several times without further problems but at the beginning of a long trip I was not inclined to take the risk of the fault re-occurring.

It was not practical to check every connection and switch but they did not appear to be the problem and the battery voltage was fine so the starter solenoid was the likely culprit. These cost £40 - £50 but there are two types and before I got the motor off I did not know if it was a Bosch or Hitachi. Checking further I found that a new Bosch starter motor and solenoid could be got for £125 (plus vat), as the motor was likely original and 25 years old I went for a complete unit and had it shipped to the harbour masters office at Newlyn.

Getting the old motor off was a challenge, the bolt heads were perfect but with an 8 inch lever on a socket I could not shift either, and I could not get a longer leaver (even if I had one) onto the lower bolt and a longer lever directly onto the top bolt would have risked the lever hitting the aft fuel injector and / or the fuel return union - not to be risked. Also I could not bring a lump hammer down on the leaver to free the bolt.

The original Hitachi starter motor, the alternator and air filter
removed, note how clean the retaining bolt is.
In the end I shifted the bolts by using a short extension, a UJ and the long extension to get the ratchet handle just clear of the front of the engine and with the extension rod resting on something solid. I then had to stand on the leaver and use my whole weight to jerk the bolts free, after moving the bolts about one flat the bolts came out easily just using finger pressure and are in perfect condition.

Without the starter motor.
The new Bosch motor and solenoid fitted.
Getting the alternator back on is a challenge, the standard loom
is short so you need to support it from underneath and hold it
in position with a knee whilst connecting the wires.
An added complication this time was a wire that had been
cut very short by whoever fitted the external regulator no longer
being securely fitted into the plug so I replaced the two wires
and the plug with a couple of connecters.


It is tempting to take the solenoid off of the motor with the motor in place, as I did, that is not recommended, refitting is tricky with the motor off the engine and even now knowing how it is done I am not sure I could do it with the motor in place.

Make sure you have a decent socket set preferably hex rather than 12 point, long extensions, a good UJ and a long T Bar., my 3/8" set was barely long enough. I will be adding a long 1/2" extension and TBar to my tool kit (which already has a 1/2" to 3/8" converter).


  1. Hi John,
    good work. I am wondering if there was a risk of tearing the bolt and if so - what in such case?

    1. It is unlikely that you would shear a bolt of that size that was in good condition, if it did happen you would have problems, usually you would need a pro with an extraction kit, drilling a hole in the stub then putting in a tool, essentially a screw with a left hand thread so when fully in it should undo the remains of the bolt.