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Old 26-01-2012, 16:14   #16
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Re: Oversized Yanmar 3GM30 Cylinder bore

Like I mentionned before, I am now at the top of the normal spec for cylinder diameter(2.9540"). Due to the light rust on the cylinder wall, some of it has to go. I will then be overspec and do not wish to push it. I would rather do the job now in the winter with the engine in my garage then in July in the middle of my vacation. Thus the oversized boreing.
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Old 26-01-2012, 16:19   #17
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Re: Oversized Yanmar 3GM30 Cylinder bore

I know, I agree with your reasoning.

Just asking about a ridge to help in thinking about what the original wall clearance may have been.

No ridge, may mean yours happened to be on the looser side when first built.

Some ridge, may indicate the older engine has a different (tighter) tolerance than the new engines listed. But i doubt this, and suspect you have a ridge that you can barely catch a fingernail on.

Hope that makes sense. I'll not hinder you further.
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Old 26-01-2012, 17:35   #18
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Re: Oversized Yanmar 3GM30 Cylinder bore

Not much of a ridge if any. I was a bit surprised since my boat is 32 years old and I believe this is the original engine.

Don't worry, you don't hinder me. I appreciate your help.
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Old 26-01-2012, 18:23   #19
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Re: Oversized Yanmar 3GM30 Cylinder bore

Good engine design, the oval piston is a factor.

In rereading the thread I realize another of the many mistakes I've made.

I misread, thinking you had measured the piston and bore to come up with .0036, thus including the wear.

In fact you used the piston and bore numbers from the manual (as I did) and used math to get that figure. (thought I was offering another viewpoint) But you did the math correctly the first time and didn't humble yourself like I did.

So the factory numbers "imply" the clearance is .0036 without the benefit of a proper range of clearance. Never seen this before.

The newer three cylinder requires .0018 - .0030, but has a piston that is a half inch larger diameter. Not sure how that might figure in, but I don't think it does.

Not sure where I'm going, just thinking out loud, trying to decide what I would do. Maybe measure the pistons and bores for more data. And try to decide how much actual wear is present, compare to recent observations of the exhaust stream, and come up with my own decision about the bore clearance. The only time I've seen .0035 or more to setup is on an air cooled engine, or one built exclusively for drag racing.

Here's to hoping a pro who has done several builds on this engine will chime in and set us straight.

Best of luck.

Greg
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Old 26-01-2012, 18:39   #20
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Re: Oversized Yanmar 3GM30 Cylinder bore

Are your pistons out yet? If so I would give them a good look... especially the lower ring "land". Seen two 3GM30's with cracks there.... maybe another reason to just go with the rebore and new pistons.... I had no idea they've made that engine for 30 years. My 85 Hans Christian had a much differnt 30 hp than my '92 Cat which had 3gm30's...?
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Old 28-01-2012, 05:18   #21
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Re: Oversized Yanmar 3GM30 Cylinder bore

Hi All on this thread,
I have a pair of Yanmar 3GM30's on/in my Priv 43 Cat in Thailand
Both are down on compression (200psi instead of 300)
The Port enging has so much crankcase pressure it blew the dipstick out!!
Just saved the engine fortunately.
I'm thinking new rings/pistons or rebore and rering and prob a head job too, definately a valve grind.
Big question is, how much are pistons / and rings in your part of the world.
I need a comparo as things can be a bit expensive in Thailand
thanks to all
regards
Phil
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Old 28-01-2012, 10:52   #22
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Re: Oversized Yanmar 3GM30 Cylinder bore

Quote:
Originally Posted by MysticVoyager43 View Post
Hi All on this thread,
I have a pair of Yanmar 3GM30's on/in my Priv 43 Cat in Thailand
Both are down on compression (200psi instead of 300)
The Port enging has so much crankcase pressure it blew the dipstick out!!
Just saved the engine fortunately.
I'm thinking new rings/pistons or rebore and rering and prob a head job too, definately a valve grind.
Big question is, how much are pistons / and rings in your part of the world.
I need a comparo as things can be a bit expensive in Thailand
thanks to all
regards
Phil
Welcome aboard!

OK Pierrre, What did you pay for your set?
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Old 28-01-2012, 11:27   #23
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Re: Oversized Yanmar 3GM30 Cylinder bore

Quote:
Originally Posted by MysticVoyager43 View Post
Hi All on this thread,
I have a pair of Yanmar 3GM30's on/in my Priv 43 Cat in Thailand
Both are down on compression (200psi instead of 300)
The Port enging has so much crankcase pressure it blew the dipstick out!!
Just saved the engine fortunately.
I'm thinking new rings/pistons or rebore and rering and prob a head job too, definately a valve grind.
Big question is, how much are pistons / and rings in your part of the world.
I need a comparo as things can be a bit expensive in Thailand
thanks to all
regards
Phil
One of mine had a broken piston and low compression/hard starting. I had it overhauled in USVI for ~$2300 US. This did not include head work as that had been done/misdiagnosed in error in Antigua. The mechanic insisted on replacing all 3 pistons though. This was a simple overhaul and not complete rebuild as the engine only had 2300 hrs on it. (Pistons, rings bearings) unfortunately, i dont know how much of that was the parts as it was about 10 years ago... not much help I guess...
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Old 28-01-2012, 16:39   #24
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Re: Oversized Yanmar 3GM30 Cylinder bore

There is two Yanmar dealers here in Dartmouth, NS (Dartmouth is just across the harbour from Halifax). One dealer is selling these pistons with rings for $128.50 (Canadian) each. Thus, $385.5 for all three. The other dealer sell the exact some piston with ring for $257 each (exactly double the price). Thus, $771 for all three. Guess were I am getting mine!
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Old 28-01-2012, 17:37   #25
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Re: Oversized Yanmar 3GM30 Cylinder bore

Hi Pierrre
Thanks for this info
I will check with a couple of Yanmar dealers in Brisbane tomorrow and my supplier in Phuket where the boat is.
I will then post that info for all.
It is heartening to know that the pistons are reasonably priced somewhere in the world.
What about hoses and gaskets?

Waterpumps.....!
(Dont start on that...)
I put new impellers in mine on Jan 5 this year to be sure to be sure.
Put the boat back in the water (it lives on the hard in Chalong Bay Phuket) and the Port one didnt work even with a new imp.
Drat...
It wouldnt raise a sweat on a hot day, let alone water.
New pump cost $750 + or -.

If you could get a new little cam it would be just great, but guess what....unprocurable.... a bloke could make a fortune supplying them to the world.

Putting the boat to sleep on the hard for a while

One last thing for beginners...when flushing the sea water out of the engine by removing the inlet hose from the pump and connecting a fresh water hose to it.
Run the motor for a minute or two, check the water is coming out the exhaust all over the next boat. Then ...TURN THE WATER OFF and 5 sec later STOP THE ENGINE.
Guess what happens if you dont turn the water off...
Why is water coming out of the air filter???
DOH.
Thank goodness for deco levers!!
This is the confessional booth.

More later

Phil
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Old 05-02-2015, 09:16   #26
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Re: Oversized Yanmar 3GM30 Cylinder bore

I am in the process of freshening up my 3GM30F Yanmar. I would like to take the glaze on of the cylinder bores. I've have not been able to identify what grit I should use for the hone? I did read that if the rings were a moly coat that I should use 320. But I also read that 240 will do also. Anyone have any thoughts on the matter?
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Old 25-02-2015, 23:01   #27
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Re: Oversized Yanmar 3GM30 Cylinder bore

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Originally Posted by larossa View Post
I am in the process of freshening up my 3GM30F Yanmar. I would like to take the glaze on of the cylinder bores. I've have not been able to identify what grit I should use for the hone? I did read that if the rings were a moly coat that I should use 320. But I also read that 240 will do also. Anyone have any thoughts on the matter?

Either is fine, it more important you get the angle of the hone pattern correct, approx 60 degrees, this requires some speed in the up and down motion, but if you slow the drill down its easier. That is to ensure the new rings break in quickly.

Wash the cylinder out well before reinstalling the piston assembly.


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