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Help 3LN1 Engine Rebuild

Discussion in 'Yamaha 250cc In-Line 4's' started by camarda, Jul 24, 2021.

  1. driftwood

    driftwood Well-Known Member

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    That'd be 7thou or .3mm
    my crosshatch was still evident. Chinese topring was still .2 after a lot of hard riding, so I'll put the tightest 2nd blackring in to get 0.2 there (my handwritten notes said >0.3mm here (can't remember where I obtained this fig) and renew oil control also. Oil usage was the reason for re-disassembly.
     
  2. jmw76

    jmw76 Well-Known Member

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    I don't think your ring gap is the issue. Are your bores true? Absolutely round and straight with very little if any taper.
    What about the ring grooves in the pistons? What is the ring fit like in the grooves.
    Maybe a new set of oil rings will solve the oil consumption issue.
     
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  3. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    I would have thought if there was excessive oil consumption caused by rings then you would be unlikely to still show any crosshatch in the bores and you would have had quite a bit of blowby on the pistons or piston depending on how worn everything is... have you considered that the oil consumption could be from Valve guides???
    If the valve are sealing this wouldnt show up on a leakdown test... but you would certinly have plenty of sludgy oil buildup in the exhaust runners... Just a thought... maybe some pictures of everything may trigger something in the combined graymatter on here :)
     
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  4. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    For what it's worth, the oil control rings on my FZR250 engine that I fixed up were totally worn out and had basically zero tension left in them. The valve stem seals were also dead based on all the burnt oil on the exhaust valve stems.

    I did not check the valve guides at the time, but after new rings, new stem seals, cylinder hone, valve lapping it ran very well.

    Guides are easy to check, fit the valve, push it out of the valve seat and try to wobble it. There is a spec in the manual but any more than 0.2-0.3mm (maybe less on these tiny swiss watches of an engine) means its time for new guides.

    Don't discount fitting new wristpins either, it makes a difference in free movement of the piston on the rod and friction is the enemy always.
     
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  5. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    FWIW on my engine that has ~55K kms I have measured the valve guides using carbide pin gauges they're less than 3.505mm, I suspect the bucket over shim has something to do with that, they valves don't experience any sideways thrust
     
  6. driftwood

    driftwood Well-Known Member

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    All your ideas have been taken on board. I'm just gonna ride awhile. It's running beautifully.
    The new stem seals solved smoke blow on startup, but usage wasn't good and with my mods I need to colour check plugs. Changed all rings. Old gaps 0.2, 0.3, & 2X0.6mm (worn to 1.56mm wide). Now <0.2, 0.2, & 2X0.25
    Dry compression check: 60psi!

    FWIW, fitting barrels really hard this time. Did in situ (easier than getting gudgeon circlips OR ENGNE out) but angle is unhelpful. Tips: to keep things squared up, used head nuts+spacer then breaker bar sitting horizontal on the maggy nut to give slight upwards piston pressure. And tie back the fixed camchain tensioner.
     
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