Initial Compression Test of my Yanmar 1GM Diesel Engine - Voyage Dreams 84 

That was very interesting. Thank uou for taking the trouble. Best wishes from Plymouth, England

sorry about your buddy. a tip of the cap to you for having the brass to tear into this. I will keep watching

Looking forward to watching this little project as i have the YSM12 that may soon need attention like this.

Very interesting

I did the learning while doing just like you are. It was a good experience. Great channel !!

Another fun video.

I have a 3GM30F and am planning to do a compression test on my engine soon with the same adapter from Tractor Supply. According to my service manual for all the GM engines, the bore x stroke are 75mm x 72mm. As the Yanmar specified compression ratio is 23:1 for all GM engines, at sea level (1 bar) the engine compression should therefore be 23 bar, approx. 334 psi. Based on the compression ratio and cylinder bore x stroke, the combustion chamber volume can be calculated to be 14,459 mm3. I measured the adapter with a calliper and found the volume of the adapter is 4,496 mm3 (8.6 mm diameter x 77.4mm depth). This additional volume will reduce the compression ratio to 17.8, so at sea level (1 bar) the compression tester when using this adapter should read 17.8 bar, approx. 258 psi when the compression is on spec. EDIT: using the official average of 1013 millibar at sea level, the compression of the engine should be 23.3 bar, approx. 338 psi. Factoring in the adapter volume, it should be 18.03 bar, approx 262 psi.

I'm all invested in this particular sub-plot! Show no fear Owen!

So overthinking this

I'm surprised there aren't some cheaper Chinese knockoffs of these engines. I'm looking at older boats I can afford and most have questionable engine hours and are worn out. To be expected for their age.

You're not looking for an exact measurement, it's not going to be like a gas engine where you can do leak down test and such. That's why diesel mechanics make so much money doing repairs, you have to tear down and take measurements. You may not be able to avoid a teardown.