+Edward Popham Great reply Edward, I was not aware Yanmar made the GM series with differnt bore sizes. I used an online parts list with explosion diagrams for my engine and similar models to get the correct parts numbers, easy when you are at home but not such a possibility in a 3rd world country. With regards to pressure testing this became an element of frustration for me, I did the primitive trick of turning the engine over by hand with compression on to see if there was resistance from each piston, and to feel if there was any difference between them after putting everything back together. Results were positive so I was happy. When the engine wouldn't fire I got the local Yanmar guys to do a compression test at great expense. They told me there was 0 compression on the 3rd cylinder which I didn't believe to be true. I bought a universal pressure testing kit from Amazon with the hope of testing the pressure myself because if I effected some repairs I didn't want to pay again for another test. The kit arrived but it wouldn't attach properly to my engine as all the different fittings were made for glow plug holes which the GM series does not have. I managed to get something to hold it in place over the injector ports and could get the pressure gauge to at least show something for each piston. With this information I went back to Yanmar and said 'what the hell!?' He told me he had done a leak down test not a compression test. Fine, but this is a 15 year old engine not a brand new one like the ones in all his brochures, I just want it to drive my 1970s boat in and out of the marina. I didn't really want to take his advice or buy his services anymore and I just use them as a source for parts now. I think the compression on my third cylinder is indeed less than the others but each piston turns over the engine independently at tick over so I am happy. The next job (and hopefully video) will be of a bottom end rebuild, I will take the engine out of the boat and replace all the piston rings. I already bought a new head gasket for that job as I saw one at a great discount online.

where did you get the explosion diagrams? Do you still have the sites listed anywhere? Thanks, I found this motivating.

j109.org/docs/Yanmar-3GM30-parts-catalog.pdf somebody is still hosting it online, its great cos you can download the pdf and use it onboard without internet connection

Merci

+Edward Popham The best thing about rebuilding this engine is that I now have a much greater understanding of the arteries and veins which keep the engine running and I am confident I could fix it in a pinch. Great to have some comments from you Edward, if you need some Yanmar parts sourced from the Netherlands I'm your man.

+Edward Popham Replaced mine on the 1GM10 with banjo connections and rubber hydraulic pipe so no more rusty pipes. They are stupidly over priced at yanmar.

Hi Arnest, better late than never with a reply I guess. The idea of lifting the decompression levers with the engine running is to see how well the engine is firing on each cylinder. I have no formal training so take this with a pinch of salt, but I noticed with my 3GM30 that cylinders 1 and 2 were providing the majority of the power and 3 was just kind of 'keeping pace'. This also gave the engine quite a heavy vibration. Cylinder 3 was not really able to build revs when running the engine by itself. 1 and 2 were building revs and working fine on their own. My suspicion was the piston rings needed replacing but me and my brother decided we wanted a newer, smaller and less maintenance boat as we both had kids on the way. Now I have a lovely little 1GM10 in our Sunway 25 which is fantastic.

+neil may I didnt use any gasket sealant on the head, not for any other reason than I wasn't aware it existed.

Bruce Wayne I would say the lube oil has leaked past your rings and into the combustion chamber. I would start by removing the injectors and shining a torch into the hole to see if this is indeed the case. Maybe turn the engine over by hand with a cotton bud down the hole and see if it picks up lube oil from the piston head.

I have 2 3GM30Fs on my catamaran.

Chris McLeod Its hard to say how many hours I put in, I did many evenings here and there and left it for a week or so more than once while I was waiting for parts or doing other things. If I had all tools and parts needed I suppose it could be done in a weekend. I watched the following video kzfaq.info/get/bejne/p9ZgaJl9vtzIgpc.html from another you-tuber for my original inspiration, I was just like you, knew a bit but never had the head off an engine. My honest advice is do it as now I know all the intricate workings of my engine and if it were to fail at sea I have the experience to diagnose and fix it. Be prepared to spend some money however, unforeseen costs will pop up and Yanmar parts are not only difficult to get hold of (depending on your location) but expensive. Off the top of my head I remember I had to replace a cross threaded bolt that holds on the rocker cover and it was €16 second hand. Any questions just fire away mate.

Email me.. chris@sarisdesign.ca