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Old 24-01-2016, 17:43   #1
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yanmar 2gm20 rebuild piston ring gaps

hey, just wondering if anyone can help me, I have pulled the head off my 2gm20 and the pistons look good to the untrained eye, however one of the chambers where the diesel is injected seams busted, all the holes are joined and will need replacing, what might cause that? years of running I guess.
Anyhow I looked on youtube and people are inserting the rings into to cylinders without the piston and messuring the gap in the ring, I did this and both cylinders messured .017-0.018 but I cant seam to find any info anywhere on what the gap should be. Any idea?? Also is it possible to change the rings without oversizing the pistons and boring etc?? Hope to just change rings and bearings as everything looks good ( to the untrained eye).
one cylinder was leaking a little oil after filling them both to the top and leaving them for 2 days, but not much perhaps less than 10 mil. my pistons have a no 2 mark on the inside assuming that this means it is oversize already?
thanks for taking out the time to read this
cheers
brian
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Old 24-01-2016, 20:41   #2
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Re: yanmar 2gm20 rebuild piston ring gaps

People often change just the rings. The ring supplier may have the ring gap info. However, you should have a repair manual for your engine. It should have all the info you need. You are going to need the torque numbers for the rod bolts and the head, maybe others. Also, the cylinder is usually slightly honed. Enough to restore cross hatching. When new, your cylinders were honed to final size. At the end of honing there is a technique to put scratches of about 45 in the cylinder wall. The purposes of the scratches are to allow the piston carry oil to the upper part of the cylinder. Once the cross hatching is worn away, the rings and cylinder wear very quickly. Restoring them adds life. Your cylinder may still have the x-hatching.
I don't know your engine, but it's probably bored w/o a sleeve. The #2 could be anything. 2mm is about .078" more than a usual oversize. I don't know the point of putting oil on top of the piston. Depending on the temp and viscosity all pistons will pass the oil. Also the bores taper towards the bottom, so the lower piston would have more clearance.
If you need a hone, better auto shops rent them. Also some rental stores. I know you can get rings & gaskets on the web, but if a local store sells them they may rent you tools. Get some type of assembly lube so the new bearings and rings aren't dry.
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Old 24-01-2016, 21:02   #3
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Re: yanmar 2gm20 rebuild piston ring gaps

Thanks for your advice, very helpful, will go to Minards diesel for the rings and bearings, bought a honing tool on eBay for $23! Wish me luck

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Old 24-01-2016, 21:13   #4
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Re: yanmar 2gm20 rebuild piston ring gaps

You may be able to find the service manual online. The torque specs for the rod caps, main caps, and head bolts are important.

I have a manual and can look stuff up for you if you need. But not tonight, must sleep as I was up most all night last night with high winds.
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Old 24-01-2016, 21:44   #5
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Re: yanmar 2gm20 rebuild piston ring gaps

Are u out sailing? Where are u at?

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Old 24-01-2016, 22:31   #6
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Re: yanmar 2gm20 rebuild piston ring gaps

On the hook in central Florida, west coast.

Finally heading further south in a week or so. It got cold here and had to wear long trousers, and socks, too! That ain't right.
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Old 24-01-2016, 22:43   #7
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Re: yanmar 2gm20 rebuild piston ring gaps

Ring gap for a new ring is I think, .010 to .016, but could be .017. My new rings all measured 0.013 which is what the old rings with 1100 hours on them were too. I think service limit was somewhere around 0.025 ish, but could have been even more.. Don't have the manual handy and would have to go dig it up.

0.017 should like it's still good. THough the rings are pretty cheap at $25 ish a few years ago. More for oversized.
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Old 25-01-2016, 05:40   #8
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Re: yanmar 2gm20 rebuild piston ring gaps

Yanmar 2GM20 Service Manual
Boatinfo - Yanmar Service Manual for 1GM10, 2GM20, 3GM30, 3HM35
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Old 25-01-2016, 05:55   #9
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Re: yanmar 2gm20 rebuild piston ring gaps

I assume the block is out? If so have a machine shop hone it, or if you do it yourself do not use one of those dingleberry hones, they are essentially junk. You want your cylinders walls perfectly straight, even and parallel. The dingleberry hones don't maintain that, if you use one anyway, only break the glaze, do no more than that.
Once re-assembled, you do not want to be easy and baby the engine, if you do, you will likely have an engine that burns oil forever, you want to "seat" the rings, and that is done under significant load.
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Old 25-01-2016, 06:04   #10
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pirate Re: yanmar 2gm20 rebuild piston ring gaps

Please keep us in the loop. Same engine here. SC, you only had 1100 hours? That doesn't sound like much for a diesel.
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Old 25-01-2016, 06:17   #11
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Re: yanmar 2gm20 rebuild piston ring gaps

Yes keep updating and take photos. Thanks

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Old 25-01-2016, 14:27   #12
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Re: yanmar 2gm20 rebuild piston ring gaps

Thanks for all the good advice everyone, yea I haven't looked into seating the rings and need to read up on it, but I have a lot of work ahead of me before the engine will be starting again. And I'm doing this rebuild really slowly :/

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Old 25-01-2016, 15:18   #13
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Re: yanmar 2gm20 rebuild piston ring gaps

I rebuilt my YSM8 yanmar & found that parts are about 1/2 the price ( or less ) in the USA as compared to Aus/NZ. You will need a USA address for most distributors but if you buy a few parts it pays for itself easy. Are you sure it's not carbon blocking the pre-combustion chamber holes? Ditto for advice re workshop manual. If you have lots of time you can research crossover parts as well as yanmars are used in tractors etc.
Good luck with rebuild
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Old 25-01-2016, 16:27   #14
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Re: yanmar 2gm20 rebuild piston ring gaps

If anyone has info on crossover parts for for the 2gm20 or the 3gm30 please post the info. I've been calling different supply places, china etall and cant find matching parts. You would think something would be avaliable. Secret stuff I'm sure. Thanks

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Old 25-01-2016, 16:31   #15
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Re: yanmar 2gm20 rebuild piston ring gaps

Check all the grooves etc on your pistons very closely for cracks.Especially the lowest groove.
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