Yanmar Engine Ring Gear Replacement

Описание: The end of our 2015 season the starter on our Yanmar engine was becoming unreliable. We obtained an “aftermarket” starter during the winter which was fitted and worked perfectly until halfway through the season we ran out of diesel on the way back from a regatta. We were able to hoist sail and make it back to our mooring ok and the Thursday afternoon of the following week I added fuel, bled the system and tried to start the engine. There was a clunk and the starter stopped dead. I decompressed the engine using the decompression levers and turned the crankshaft pulley by hand to make sure the engine was free. I then cranked the engine successfully with the starter but as soon as I dropped a decompression lever the engine stopped dead again. I removed the starter and found the nosepiece had broken off.

I then knew we had a major problem, probably with the ring gear on the flywheel that meshes with the starter pinion.

After the boat was on the trailer, in position for the winter and cleared out I removed the engine compartment, lifted out the batteries and assembled the engine crane. The saildrive was braced and strapped to the keel after the propeller was removed. The engine was unbolted from the bellhousing and the engine mount to hull bolts removed. The engine was then moved aft and turned to allow access to the flywheel for removal. As the engine was being moved the starboard engine mount fell apart!

The first problem was to remove the coupling plate to allow access to the flywheel to crankshaft bolts.

In her previous life GTR, then called Jabberwocky, had sat with a lot of water in the cabin for some time. The water level was almost half way up the engine so the coupling plate was rusted in place. The mounting bolts came out ok but the plate would not move.

It was persuaded to come out by using a bearing drawer leg put through the centre hole and tightened on to a piece of channel iron The “tide” mark can be seen on the flywheel!

The flywheel was then unbolted, removed and taken back to my workshop for cleaning and examination. The ring gear teeth were counted (97) and examined. It can be seen that the teeth were in a pretty poor state.

A new ring gear and a pair of engine mounts were ordered along with a couple of hoses and filters for the annual service.

When the parts arrived it was easy to see just how worn the teeth were. The outside of the new ring gear was determined at this stage.

After checking that the new ring gear was correct preparations were made to remove the old one. With access to oxy acetylene it is possible to expand the old ring gear of the flywheel but I no longer have that access. I instead reverted to a method I used when building racing car engines.

Two spaced centre punch marks are made on the ring gear. A drill depth is then measured against the new ring gear to prevent damage to the flywheel. Two holes are drilled in the old ring gear and it is split off using a cold chisel.

The flywheel was then cleaned and the new ring gear suspended on a rod from a vice in preparation for heating. It is necessary to heat the ring gear sufficiently to expand it to a grater diameter than the flywheel. This takes around 200 deg C. Any excess heat can ruin the ring gear but trying to fit it if it is not sufficiently expanded will also destroy it. I measured the diameter of the flywheel and set my callipers 1 mm above that figure. I also used a grease pencil that melts at 200 deg c to allow me to monitor the temperature.

The ring gear was heated evenly until the vernier fitted in side it. It was then held in welding gauntlets and dropped over the flywheel. It immediately gripped the flywheel and became cool.

Re-assembling the flywheel to the engine requires to use of a torque wrench to be sure the bolts are tight enough but not too tight. The torque figures are given in the engine service hand book. A tool had to be made to hold the flywheel to allow it and the coupling plate to be torqued. A length of aluminium bar and 2 M8 socket head cap screws mated up with balancing holes in the flywheel. This tool allowed the flywheel and coupling plate to be properly secured. The engine was then re-installed with new engine mounts, serviced and the engine compartment re-assembled in preparation for testing.

We hope you enjoyed our latest video documenting the rebuild of our Mumm 36. Thanks for supporting the team at Bottom Up Boats.