Yanmar 1gm10 low compression.

[Tammany]

Rebuilt the top end of my 1gm last year as it was suffering from poor starting and would stall when selecting gear. New valve seats, new exhaust valve and grind and it cured the problem and engine starts and runs well. I've got the engine in the workshop at the moment as part of the boat refurb and the compression is low, I can turn the crank pulley fairly easily. Adding oil to the intake and the compression goes high so I can't turn it over manually unless I use the decompressor lever. The engine starts and runs well but seems to me their is some wear in the rings.

Leaving it a few days the compression from the oil being added does go down but it takes a long time so the oil is seeping past the rings very slowly. When the head was off a visual examination of the bore showed no signs of wear or scoring at all. I'm also pretty sure the conrod isn't bent as the piston was sticking out the top of the cylinder wall.

Here is a video of the result of last years top end build. As you can see she runs lovely and is very responsive on the throttle. Is it worth the hassle of replacing the rings though??

 

[Champagne Murphy]

Is the decompression adjusted properly? Had a similar problem on a Bukh.

 

[RichardS]

If you can turn over the engine by hand just holding the crank pulley rather than any kind of lever then that doesn't sound too good to me.

I think that if I intended to keep the boat a while I would check the bore and rings properly with a feeler gauge whilst the engine is out.

Richard

 

[lw395]

It may just be a case of cleaning the piston to free the rings, if they are sticking.
This probably means new big end bolts though. Not stupidly expensive.
Rings are pricey at £78 from Marine power, I wonder if anything cheaper would fit?
I've seen Carillo rings 1/3 that price....

I think people sometimes over-react to being able to turn an engine by hand. When running, there will, one hopes, be oil on the bore wall. And the air in the cylinder only has a short time to leak out, even when cranking at a few hundred rpm.

 

[Tranona]

Sounds like the con rod may be bent. Common problem with GM engines. To check remove the injector and put a piece of 1.2mm fuse wire in, turn the engine over and the piston will squash the wire. The critical dimension is 0.5mm. If it is more than that you do not have enough compression - that is the volume above the piston is too big. The procedure is covered in detail in the Yanmar workshop manual.

If the test is positive, alternate causes are leaky valves or head gasket.

 

[NormanB]

If you did a good job on the head (and it sounds as though you did) and if I am reading you correctly the engine was running fine before you moved it ashore but now because you can turn it over by hand you 'think' there is a lack of compression witnessed by the fact that oil in the bores improves compression.

Well that situation would exist on a perfectly good engine too so I would exercise caution in assuming there is a ring/bore issue. The fact is that you will not achieve a perfect ring seal on a cold engine and that is a feature of design and practicality. As the engine achieves designed operating temperature the piston and rings expand to design size.

Best bet would be a compression test, your manual may have cold blow spec but if not you can just record the readings (do not add oil to the bores) and record what you get - if all is well they will all be the same +/- but if there is a problem it will be dogs bollocks obvious. If they are all the same it will avoid you having to strip.

 

[RichardS]

Best bet would be a compression test, your manual may have cold blow spec but if not you can just record the readings (do not add oil to the bores) and record what you get - if all is well they will all be the same +/- but if there is a problem it will be dogs bollocks obvious. If they are all the same it will avoid you having to strip.

Surely it's a single cylinder?

Richard

 

[NormanB]

Oops my bad! My sincere apologies- I missed that.
I would still do a compression test.

 

[anoccasionalyachtsman]

Sounds like the con rod may be bent.

The OP reports that a squirt of oil corrects the low pressure.

 

[Tammany]

A higher compression with oil in the cylinder always seems to me either sticking, broken or worn rings but I have little experience in diesel engines but plenty in 2 stroke petrol engines. I don't think it's the rod as the piston was clear sticking out the top of the cylinder wall by a couple of mm's when the head was off. I haven't done the crush test however. It might be that it's just really cold and they may be a bit jammed up as the engine hasn't been run since June.

 

[RichardS]

A higher compression with oil in the cylinder always seems to me either sticking, broken or worn rings but I have little experience in diesel engines but plenty in 2 stroke petrol engines. I don't think it's the rod as the piston was clear sticking out the top of the cylinder wall by a couple of mm's when the head was off. I haven't done the crush test however. It might be that it's just really cold and they may be a bit jammed up as the engine hasn't been run since June.

2 or 3 mm above the level of the head with the head gasket removed sounds about right so a bent conrod is indeed very unlikely.

Richard

 

[Tammany]

Pulled it apart to satisfy my own curiosity in case it was a bent rod. Doesn't seem bent to me and piston is in perfect order too. The rings also seem to be very good shape, they still have a nice sharp edge to them. I'm a bit confused now as too what is causing the poor compression. The head seems to be in good order.

 

[Tammany]

Will fit new rings as a matter anyways and see how it goes.

 

[Tranona]

The OP reports that a squirt of oil corrects the low pressure.

As oil will fill the combustion chamber and is incompressible tells nothing. The crush test is the recommended test. Look at the clearance - only 0.5mm - not detectable by eye.

 

[<152587>]

0.5 mm is 0.020 inches, just less than a spark plug gap, suggest you best consult specsavers

 

[<152587>]

Good to examine exhaust valve seat, they can corrode if motor stopped on the exhaust stroke. I always turn single cylinder engines to compression stroke when storing, elimanates corrosion caused from residual liquid in the exhaust system

 

[Tammany]

Had a new seat and valve fitted when redone head. Still might be the head thats not 100% though. At least I know for sure the bottom end is good.

 

[JumbleDuck]

0.5 mm is 0.020 inches, just less than a spark plug gap, suggest you best consult specsavers

 

[RichardS]

Will fit new rings as a matter anyways and see how it goes.

If you carefully remove the top ring and push it squarely down the barrel a cm or so with the top of the piston you should be able to find a ring gap maximum on the service manual which you can measure with a feeler gauge as I suggested in post #3.

If the gap is out of spec there is a bore / ring wear problem.

Richard

 

[lw395]

I suspect the squish clearance will need to be a long way out to be making the engine easy to turn over by hand.
Now the engine is apart, I'd measure the ring gaps.
Feel for slack in the small end too.