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Old 22-10-2023, 04:48   #1
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Assessing a used Yanmar 8HP diesel engine

The 1987 Mottle 820 I recently bought has what appears to be a Yanmar YSM8, which had a rusted out exhaust elbow. The engine still turned over driven by the starter motor, though wouldn’t start.

Interesting, there are lock tab washers on the big-end bolts, which being non-standard could indicate a previous rebuild.

I am unsure about the mixer/riser; it appears outwardly ok except for some surface rust. However, I don’t know if internal rust could be a problem or how to assess the working order of the mixer/riser.

Does anyone have experience with this?
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Old 22-10-2023, 06:01   #2
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Re: Assessing a used Yanmar 8HP diesel engine

Nope, but welcome to the forum, HorseshoeBay.
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Old 22-10-2023, 13:24   #3
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Assessing a used Yanmar 8HP diesel engine

Hi Horseshoebay, there’s nothing good about that mixer elbow, and the lagged section looks even worse. I’ve never had much to do with these tough little engines but a quick glance through the service manual clearly confirms that the lock tabs in your photo are original and not on the “big end bolts”. Those bolts and lock tabs are there to secure the crankshaft counterweights...... and the corrosion inside your crankcase is also a cause for concern.
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Old 22-10-2023, 15:51   #4
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Re: Assessing a used Yanmar 8HP diesel engine

The inside of that engine does not look good. Was there water in the oil? Since it turns over, does there sem to be compression? If so, next would going after the fuel system, changing filters, bleeding it of air, etc.
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Old 22-10-2023, 16:27   #5
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Re: Assessing a used Yanmar 8HP diesel engine

Yep, Skipperpete is on the money. Those bolts are for the crank mass counterweights and the lock tabs are normal. Usually there is no reason to remove them.

You can be sure the mixer and riser will be much worse inside than outside. They need to replaced - these days you need to hunt down aftermarket items or have them fabricated - which is not too difficult.

Note, there is no locking tabs on the conrod (big end) caps.

That engine does look like it needs some love but you could not get an easier engine for DIY work even if you only have basic hand skills. All that is necessary is a willingness to learn. If you have sufficient access either side, a lot of work can be done in situ, including replacing the liner. Hopefully you won't need crankshaft bearing as that requires a full pull down and they cost a king's ransom.

Aftermarket piston, rings, liner, conrod bearing, valves etc are available if you go looking.
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Old 22-10-2023, 17:21   #6
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Re: Assessing a used Yanmar 8HP diesel engine

A piston rebuild kit, including a liner has been ordered, along with a stainless steel exhaust elbow and service kit.

The head is off and the valves have been removed for cleaning.

Valve lapping compound has been ordered.

A crank handle hasn’t been found.

I am wondering about the best way to rotate the piston/big end to be able to remove the big-end bolts?
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Old 22-10-2023, 18:36   #7
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Re: Assessing a used Yanmar 8HP diesel engine

Various ways -

Turn the PTO pulley at the front of the engine
"Bump" the starter motor
Remove starter motor and use a lever on the ring gear on the flywheel
Put in gear and turn prop shaft by hand
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Old 22-10-2023, 23:44   #8
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Re: Assessing a used Yanmar 8HP diesel engine

Undid the big end bolts and removed the big end cap.

Next, should the piston be removed?

How best?
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Old 23-10-2023, 00:50   #9
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Re: Assessing a used Yanmar 8HP diesel engine

HorseShoeBay out of curiosity what is a rebuild kit worth?

Cheers
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Old 23-10-2023, 01:29   #10
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Re: Assessing a used Yanmar 8HP diesel engine

Quote:
Originally Posted by HorseshoeBay View Post
Undid the big end bolts and removed the big end cap.

Next, should the piston be removed?

How best?
With BE cap removed, rotate the crankshaft until the piston is at TDC then rotate the crankshaft another 90 (or 270) degrees. This will leave the piston at TDC. From the other side of the crankcase, insert a piece of wood and push against the end of the con rod; tap lightly with a hammer if needed.

Where did you source the parts?
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Old 23-10-2023, 04:13   #11
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Re: Assessing a used Yanmar 8HP diesel engine

Waiting for a rebuild kit from

https://dieselworks.com.au/

AUD$352

Been unable to get the piston out of the head.

It protrudes around 1cm.

Put a piece of wood against the end of the conrod and hammer strongly.

Won’t move further forward. Can move back easily enough.

Wondering what I am doing wrong or if it’s a mechanical issue.
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Old 23-10-2023, 04:18   #12
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Re: Assessing a used Yanmar 8HP diesel engine

Make sure the conrod is not getting fouled or catching on anything inside the crankcase (end of the liner etc).
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Old 23-10-2023, 14:38   #13
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Re: Assessing a used Yanmar 8HP diesel engine

Did you use a ridge reamer before trying to remove the piston. The piston rings may be stuck on the cylinder ridge .
Push it back in and check for a ridge. I may have some parts for your engine, where are you located?
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Old 23-10-2023, 16:01   #14
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Re: Assessing a used Yanmar 8HP diesel engine

Hi NorthCoastJoe,


Thanks for explaining about the rings.


I am in Melbourne, Australia.
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Old 23-10-2023, 19:09   #15
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Re: Assessing a used Yanmar 8HP diesel engine

Dieselworks is pretty good, I drive past their workshop on the drive to my boat and approached them about a rebuilt engine fo a friends yacht. Regarding the piston being stuck...... yes, most likely a ridge but it could just as easily be a ring of hard carbon in that liner dead zone, the usual procedure is to get at it with 180 wet or dry abrasive paper but only on the carbon band to actually get an idea of how bad the ridge is, often it’s only the carbon that blocks the piston. If you’re using a new liner just get into the ridge with a flap wheel in a drill, the main thing is to NOT damage the piston, crank journal or conrod.
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