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Piston Weight

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On my M50 I run with no headgasket, but have developed a very slight oil leak from the drainage holes down the barrel after about 10,000 hard miles.It shows on the top couple of fins, even though I have "o" rings in a grove on the barrel. this is not a problem as when I lifted the head the "o" rings needed replacing anyway. My question is, I have a new hepolite piston (+30) ie same size as one in bike, but it weighs 381g as opposed to one I removed 405g. do any of you 'technical' people think 24g will effect the balance much?.  At the moment the bike vibrates a bit (Not as smooth as my Velo! ) at the top of the rev range, but is acceptable. I suppose I could just try the lighter piston and see if improves an otherwise great bike. Regards John O
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That's 6% change in piston mass.  Reciprocating mass includes piston plus half con rod (roughly...) it could be 4% change overall.  If balance factor was (say) 60%, its now changed to perhaps 60x1.04, or roughly 63%.  And since no two people seem to agree on precisely what's best....certainly to that precision...I don't believe there's anything to worry about.
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Many thanks David, I have noticed a very slight marks on the inlet valve, and corresponding mark on piston, so I will check clearance before running with no head gasket, and try both pistons to see if it makes any difference to the balance. Regards John O
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I have an ES2 engine with O rings fitted between the head and the barrel and have a dreadful oil leak. On investigation I found that the oil way in the head did not align properly with the oil way in the barrel, so that the O ring didn't seal properly.  Study the old O ring after removing the head to see if it is deformed and being squashed  into the oil way, thus allowing oil to escape around the side. Regards, Philip
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In my experience a replacement piston that is heavier than the original is likely to make its presence felt in terms of vibration, reducing the balance factor below the original 50%. As David said the lighter piston will increase the balance factor. This may result in a smoother running engine at higher revs. In your position I would fit the lighter piston and see how it performs. If you want to go gasket free, apart from the piston to valve clearance issue, you will need to measure the height of the barrel spigot and the depth of the corresponding head recess. The spigot should be 0.001 - 0.002" taller than the depth of the recess. This ensures that the spigot contacts the head first and provides the gas tight seal. The remaining small clearance to the larger mating surface will close up as the head is torqued down. The head may have to be skimmed to achieve the required relationship between spigot and recess heights. Edit - just realised that you are already gasket free so spigot to recess dimensions probably already sorted in your case.
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If there are marks - the clearance is not enough - it needs .040" (or thereabouts) cold (There are numbers somewhere).  Base gasket/compression plate needed perhaps?
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Thank you all for your comments, M P did my original gasket removal, and some more work, new valve seats and valves etc so I am sure he also machined the spigot clearance correctly, but I will check before I re fit the head. The 'O' rings were a little on the small size so I have obtained some 8mm x 2mm Viton seals from "Simply Bearings" (Lancashire)  as a Viton seal is supposed to stand a high temp. The original seal lasted about 10,000 hard miles before leaking so can't complain. David........I like the idea of a compression plate under the barrel......never thought of that. Phillip ........I recommend the Simply Bearings site for 'O' rings very informative( and inexpensive) many thanks Regards John O
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After cleaning the old piston a bit I have noticed that the exhaust valve cut out has also been catching the valve a slightly. I will definitely put the new (lighter) piston in, and carefully check the valve clearance with some 'blue tack' before final assembly.         The cut outs on the lighter piston are more in line with the valves than the piston I removed anyway. Regards John O

 


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