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Old 11 June 2009, 07:10   #1
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Merc Rebore & upsized pistions

As you may or may not know, I have decided to do a complete rebuild on the Merc. That way I know everything will be solid and I get an essentially new engine of known history!

I haven't got as far as extracting pistons yet, but as a pre - emptive Q are any of you Barrus / Merc type bods allowed to tell me how much of a skim of the cyl liner triggers the use of the +0.015" pistons & rings? I ask because from my current limited view through the exhaust port I may not need to remove too much metal (only feels like about 0.002" using my nail - not very high tech, I know....).

I am wondering if it needs "just" a couple of thou off to remove the scratches can I use the original pistons & new "standard" rings OR it it a case of "Skim 0.015 & get the upsized pistons or leave well alone"?

It would appear I have a "650" series, based on the part numbers of various lumps I have removed.

Thanks in anticipation.
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Old 11 June 2009, 07:38   #2
mdt
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If it is just light scratches you need to deal with can you not just hone it? And fit new rings?

If you where to remove any real depth of metal from the bore and to use standard rings you would have compression issues as you would have blow past the rings.

As an aside it is standard practise in the car world to bore the block out a proper amount and stick in new pistons and then adjust the fuelling etc to suit. I have never played with a outboard or a 2 smoke so I may be talking rubbish mind you.
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Old 11 June 2009, 07:43   #3
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Exactly!

Hence my asking the question - how light is a light scratch? (and essentially how big can the cyl go before the blowby starts) problem is at the moment I don't know just how bad it is - there's a limit as to how much of me I can get through an exhaust port!
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Old 11 June 2009, 07:47   #4
mdt
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you really cant go over the top with a hone. I would do this first and see how it looks.

Can you not hone it out then stick a ring in and measure the gap and see if it is still with in the tolerance?
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Old 11 June 2009, 08:49   #5
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Agreed, The plan at the moment is to decide on the action after I find out just how bad it is in there, and then hone, skim or scrap (tho' I doubt it'll come to that!) to suit. Problem is that until I get the flywheel & the underlying gubbins off I can only "assess" the small bit of wall I can see / reach with my fingernail thro' the exhaust port!

I think a better question might have been "what is the max cyl dia before you need to bore up 15 thou' to the bigger set of pistons & rings?"
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Old 11 June 2009, 08:58   #6
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take the obm serial number to a merc/mar dealer and buy the factory service manual ? Price not known
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Old 11 June 2009, 09:05   #7
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Quote:
Originally Posted by uncle al View Post
take the obm serial number to a merc/mar dealer
And there lies the problem.... the remains of the plate tell me it's a "...ury". the rest is long obliterated!

(And the insert in the block is missing as well - hence my other post!)
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