Assessing a used Yanmar 8HP diesel engine

[Wotname]

OK, some words of caution here .

Try to assess the general condition of the liner before getting too heavy handed with the tools

I dunno if your kit includes a new piston but it might if it includes a new liner. I would expect the piston to wear more than the liner (but maybe I am wrong in that assessment). The point being is that you don't want to replace the liner in situ if you don't have to as it can be difficult to remove. Not impossible but difficult if you have only limited DIY experience.

If it is worn, scored or otherwise stuffed, then yes, it has to be replaced but being a raw water cooled engine, it won't like being removed. You may will have to fabricate some tooling to get it out. Refitting the new one is easy though!

Do you have the YSM service manual? It appears to be written for the DIY or shade tree mechanic and contains almost everything you will ever need to know about rebuilding the engine!

[grantmc]

I have a PDF copy of the original manual if you want one. It is for the YSE8 and 12, but almost the same as YSM. If so then message me with your email addy.

I persevered with my 8hp for several years but it became increasingly expensive. I recall the engine mounts and exhaust manifold were way expensive.

One regular maintenance item was replacing the head gasket every 3 or 4 years. It slowly corrodes (perhaps not the right descriptor) I sort of figured because it's raw sea water and being a horizontal engine. The gasket was always far more corroded at the bottom.

The model I had included a chain arrangement for manually turning it over. So a chain and a pully bolted on the front. Here's a video I just found that shows it quite well.

https://www.youtube.com/watch?v=5BMjk284osI

[Wotname]

Originally Posted by grantmc

I have a PDF copy of the original manual if you want one. It is for the YSE8 and 12, but almost the same as YSM. If so then message me with your email addy.

I persevered with my 8hp for several years but it became increasingly expensive. I recall the engine mounts and exhaust manifold were way expensive.

One regular maintenance item was replacing the head gasket every 3 or 4 years. It slowly corrodes (perhaps not the right descriptor) I sort of figured because it's raw sea water and being a horizontal engine. The gasket was always far more corroded at the bottom.

The model I had included a chain arrangement for manually turning it over. So a chain and a pully bolted on the front. Here's a video I just found that shows it quite well.

Yes the YSE manual is good but the YSM manual has way more information and detail. I have a YSE but for most (but not all) things, I use the YSM manual. Both is best . Likewise, I can email a YSM manual to the OP if needed.

It is my understanding the 'common' head gasket failure is due to corrosion occurring at the bottom of the gasket from condensation forming inside the cylinder and collecting on the bottom of the cylinder, especially at the head /gasket / liner joint.

Pro tip (not that I'm a pro ) - When leaving the engine shut down for more than a day or so, turn the engine over so the cylinder is at TDC on the compression stroke (use the decompression lever to assist). This ensures there is minimal airspace in the cylinder and both valves are closed thus limiting possible condensation.

[HorseshoeBay]

Thanks Wotname, with the BE rotated correctly and the end of the conrod lined up, the piston was pushed out of the head.

Diesel Works provided the YSM8/12 Manual.

While waiting for the piston kit, other parts can be serviced.

A service kit has also been ordered.

[becrux]

I have seen exorbitant prices from Yanmar for replacement mixing elbows, especially for older models. I saved about $300 by getting a replacement for my 3qm30 from HDI marine.
While I don’t see an exact solution for your engine, there appear to be some components that can be assembled that will fit your needs. They are all SS, which is better than any Yanmar mixing elbows. You can have a look at HDImarine.net
Hailey was extremely helpful.

[Pat_D]

I've had great fun with a YSM8. ya know there should be a decompression lever on the starboard side near the fuel pump. And get a crank-or make one. Very satisfying to hand crank her over.
The inside does look bad, the mixer elbow--no way to tell from the outside. Just pull it apart, scrape the inside and see if it looks too decayed. Mine never did. My problems were with the riser--it would eventually loosen up from shaking and wear the npt threads out.
East coast, neimiec marine has lots of oem parts and great customer service.

[HorseshoeBay]

Does anyone have a diagram of the riser/mixer?


Are there separated internal passages, or is it just a straight-through bent pipe?


If it's just a straight-through pipe and the threads can be filed and the walls aren't rusted through, could it be usable, without imminent danger to the engine?

[HorseshoeBay]

Hand cranking is a sensible backup, so I will look at making a crank handle

[HorseshoeBay]

Work on reseating the valves using grinding compound has started, with the heavily pitted exhaust valve first.

[becrux]

The mixer has an internal passage for cooling the pipe. The mixing takes place on the down-flowing part of the elbow.

[Pat_D]

The mixer's internal passage is more like a baffle; it's an inch or two baffle that directs the water the correct way (out). Not complicated and nothing to be afraid of. It makes it a little bit harder to scrape all the soot out.
The rusty look outside indicates sea water has been falling on it. ( The rusty inside the engine makes me wonder if the engine got submerged once.). IMHO it says nothing about the inside condition of the mixing elbow.

[becrux]

Originally Posted by Pat_D

The mixer's internal passage is more like a baffle; it's an inch or two baffle that directs the water the correct way (out). Not complicated and nothing to be afraid of. It makes it a little bit harder to scrape all the soot out.
The rusty look outside indicates sea water has been falling on it. ( The rusty inside the engine makes me wonder if the engine got submerged once.). IMHO it says nothing about the inside condition of the mixing elbow.

^^^ This is a better answer.

Don't be afraid to check out the elbow with a few hammer taps.

[skipperpete]

Here’s a screenshot of a mixer elbow ( found on another forum)

[Wotname]

^^ that is a very good picture of the problems associated with the standard Yanmar mixing elbow

I have added another common failure mode of these elbows that allow sea water to directly enter the combustion chamber via an open exhaust valve. If (or when!) the internal cast iron web fails on the "engine" side of the elbow, sea water drips toward the exhaust valve - see yellow lines. Such mixing elbows are a maintenance item and need replacing periodically - before they fail.

Replace on engine time, not on condition - unless you are a gambling man .

[HorseshoeBay]

The interna, walls have a red coating that doesn’t appear to be rust.

The exhaust elbow was severely rusted out, and the previous owner had taped over the rust, ouch!

What is the likely way the exhaust elbow rusts through?

Does the elbow rusting through indicate that the riser had rusted through and was sending saltwater onto the elbow?

A non-OEM stainless steel riser can be purchased.