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Old 23-02-2021, 03:00   #1
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Melted pre-combustion chamber

Greetings to the hive,
My question is what is the likely " collateral damage " frm getting combustion temps up high enough to melt pre-combustion chamber.
Back story: Engine is a yse 12 Yanmar that had a block access plate that incorporates the block breather installed inverted. This caused engine oil to be fed into air intake resulting in a runaway. AFAIK the engine was stopped between 30 secs & 60 secs.
Also reported that exhaust v'v is a bit burnt.
My reaction is that top end is toast but as I haven't experienced this I thought I'd ask those that have what damage did they find?
i.e. are the piston & rings junk?
If anyone has a Yanmar YS12 series parts book that would be helpful as well.
NZ agent can't supply the part & wont give the part no.

Be interested to hear from anyone who has melted a precom chamber.
cheers P
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Old 23-02-2021, 13:34   #2
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Re: Melted pre-combustion chamber

Following with interest 'cause this is an unusual failure (I think).

Meanwhile, "NZ agent can't supply the part & wont give the part no." Doncha just wonder why we read here on CF how wonderful the world wide support for Yanmar engines is?

Good luck with the repair!

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Old 23-02-2021, 13:43   #3
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Re: Melted pre-combustion chamber

Never had a runaway, but rebuilt a few that did. No Yanmars. Runaway caught early, no apparent damage. Longer run, high rpm usually rod, valve damage. Sometimes valve marks in piston tops. High rpm usually broken rod and other damage that made rebuilding not worth it.

Runaways usually happen in worn engines. Discover the cause. Probably a combination of worn bores, and worn rings. Could also be from the injector pump. My guess, the engine needed an overhaul anyway. Head is probably ok, send to a head rebuild shop. Probably new valves and springs if the rpm was high. Once an engine is apart, I always replace pistons, rings and bearings.

NZ isn't the only place in the world to buy parts. You should find the parts book online.
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Old 23-02-2021, 13:51   #4
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Re: Melted pre-combustion chamber

Quote:
Originally Posted by Lepke View Post
Never had a runaway, but rebuilt a few that did. No Yanmars. Runaway caught early, no apparent damage. Longer run, high rpm usually rod, valve damage. Sometimes valve marks in piston tops. High rpm usually broken rod and other damage that made rebuilding not worth it.

Runaways usually happen in worn engines. Discover the cause. Probably a combination of worn bores, and worn rings. Could also be from the injector pump. My guess, the engine needed an overhaul anyway. Head is probably ok, send to a head rebuild shop. Probably new valves and springs if the rpm was high. Once an engine is apart, I always replace pistons, rings and bearings.

NZ isn't the only place in the world to buy parts. You should find the parts book online.

Yes I know NZ isnt the only place to buy them. In fact I NEVER buy a part from the local yanmar mafia ( Dont enjoy being reamed ) but a client wanted to.
Have located a pre-com chamber in Singapore

If you look again at first post you'll read the cause of the runaway.
Thanks for likely damage comments, thats what I was after.
He's just installed new piston, rings & liner so doesnt want to replace them after 5hrs use.
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Old 23-02-2021, 14:26   #5
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Re: Melted pre-combustion chamber

Back in the late 70s Volvo had issues with pressed in pre chambers in their 40 series engines that were the result of extreme engine overheats. Supposedly the prechamber was made of stellite, and if the overheat was severe enough the prechamber would loosen in the head, either because the iron head relaxed enough that the interference fit was lost, or the prechamber grew, stretched the head and the interference was lost, or something.....

In any event, the fit between the head and prechamber was lost and if running continued the prechamber could hammer around and eventually fall into the cylinder, with predictable results.

If it was me building this engine I would pay close attention to the fit of the new prechamber in the cylinder head....
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Old 23-02-2021, 14:57   #6
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Re: Melted pre-combustion chamber

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Originally Posted by DougR View Post
Back in the late 70s Volvo had issues with pressed in pre chambers in their 40 series engines that were the result of extreme engine overheats. Supposedly the prechamber was made of stellite, and if the overheat was severe enough the prechamber would loosen in the head, either because the iron head relaxed enough that the interference fit was lost, or the prechamber grew, stretched the head and the interference was lost, or something.....

In any event, the fit between the head and prechamber was lost and if running continued the prechamber could hammer around and eventually fall into the cylinder, with predictable results.

If it was me building this engine I would pay close attention to the fit of the new prechamber in the cylinder head....

thanks for the tip DougR but I think it should be ok as they have a pin in them but I havent pulled one out yet.
I couldnt work out how the pre-com chamber could melt before the piston did but maybe it hasnt melted but the bloke who pulled it out destroyed it & just said that. No evidence for that, I'm guessing there.

Wow, the thought of a pre-com chamber falling out makes me cringe.
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Old 23-02-2021, 15:07   #7
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Re: Melted pre-combustion chamber

To be honest, I can't see how the pre-com chamber could melt. I could (perhaps) see the tip burn / erode a bit from extreme heat.

As you say, more likely broken trying to remove it incorrectly.
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