Yanmar 3GM30 Reboring One Cylinder Only

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robbieg

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I have acquired a Yanmar 3GM30 that has done less than 100 hours from new. Problem is it stopped because of a broken ring that has scored one cylinder. Having stripped it down the other bores are fine.

Can I just get the one affected cylinder rebored or will this affect crankshaft balance and shorten the life of the engine. At Yanmar prices 3 oversize pistons and rings is seriously expensive-although probably still worth it for what is basically a new engine.

TIA
 
robbieg said:
I have acquired a Yanmar 3GM30 that has done less than 100 hours from new. Problem is it stopped because of a broken ring that has scored one cylinder. Having stripped it down the other bores are fine.

Can I just get the one affected cylinder rebored or will this affect crankshaft balance and shorten the life of the engine. At Yanmar prices 3 oversize pistons and rings is seriously expensive-although probably still worth it for what is basically a new engine.

TIA
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i very much doubt that is a good idea / engineering practice
 
Think I would ask Yanmar. It could well be that the larger diameter piston is made to the same weight cos balance isnt just aboiut pistons - you balance crank rods and pistons together I believe. So Elton may well be right but I dont know.

If you do get an official answer, let us know please.
 
On a low reving diesel engine with an overbore of 10 to 30 thou? it won't be a problem but not what the purists or those with deep pockets would call best practice. Not sure if the engine has wet liners if it does then you could simply reline and bore to the same size as original.
 
robbieg said:
I have acquired a Yanmar 3GM30 that has done less than 100 hours from new. Problem is it stopped because of a broken ring that has scored one cylinder. Having stripped it down the other bores are fine.

Can I just get the one affected cylinder rebored or will this affect crankshaft balance and shorten the life of the engine. At Yanmar prices 3 oversize pistons and rings is seriously expensive-although probably still worth it for what is basically a new engine.

TIA
Click to expand...
Answer the question first, purists (of which there are many, until they have to put their hands in their pockets, on this forum!) would say do the three. Moi, I would do just the the one, the extra weight in the )/S piston would not IMHO, cause a prob.
I would also ask the question, how deep is the scratch? In my previous life as head of maintenance for an oil major in third world countries, I have polished out such scratches, and the engine has run happily on a re ringed piston in the same bore.
Stu
 
If the scratch is too bad to polish out (a la Stu) and it is a solid block ( not wet liners) sleaving the offending bore is an option. I had a diesel that I bought new oversize pistons for, before I checked with the machine shop. Two were worn beyond the oversize, so they sleaved them. The cost was a relief compared to returning the pistons for a bigger oversize and the time entailed. And you might be able to get away with reusing the piston, a considerable saving.
A
 
The extra weight (if any) of an oversize piston is inconseqential in the scheme of things. Take into consideration the weight of the con rod, crank, small end and big end bearing, which remains the same. And the fact that the new piston is only 10 thousandths of an inch (<1% greater) diameter. In any case, all the oversize pistons I've seen were the same weight as standard pistons.
 
I've done it on a two cylinder Volvo.No problems whatsoever.As long as the compression on both cylinders is within spec it's a perfectly sound engineering practice.
 
we are not talking about a highly tuned on the ragged edge racing engine here. I would hone the bore and fit a new rings, if the scores are really so deep that they affect performance (unlikely) then rebore and fit a new piston and rings. But only do the one bore, if you really want to be purist correct then have all the pistons graded (weighed and machined) so they weigh the same - I wouldnt bother.
 
boring ...

there are mobile engineers who will bring a borer powered by 240v supply and bore the one cylinder to suit the oversize required , on site , in situ .I have had it done , many years back , as long as there is clearance above the engine ? I will second the low revving deisel theory too these engines are not fragile highly tuned units , theyre plodders . if you really wish to restore micro- balance then remove another piston , weigh it ,then match the new one to it . its a simple job to match weights . also , you may get away with honing the score out or down to an insignificant depth then fit new rings ,
good luck , grafozz.
 
Although in theory the reciprocating masses will be different this will be well within tolerances of the crank. Therefore can I second the practical advice by others that a rebore on one will not matter.

Again as others suggest, I would have thought you could polish out the scratch in most cases then investigate oversize rings perhaps.

Brian
 
Correction

30boat said:
I've done it on a two cylinder Volvo.No problems whatsoever.As long as the compression on both cylinders is within spec it's a perfectly sound engineering practice.
Click to expand...

I must correct myself as I didn't read the OP properly.A rebore on one cylinder only will throw the balance out as the piston will be larger,heavier and produce more power due to a larger swept volume.What you have to do is to fit a new liner and piston of the same size as the other ones.This will keep things in balance.As said try and hone the scratches out but you can only go so far before loosing compression and start using oil.
 
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