Crank shaft part codes?
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The length of the rod doesn’t determine stroke, it’s the distance from the centre of the crank to the centre of where the big end is placed that determines the stroke. It was only early 1988 bikes that used 3bn barrels and cranks wasn’t it? Just get a crank from a later year model...
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Get a bit of clean cardboard, punch a hole so that one side of the crank can sit in it and spin. Then fully extend the con rod to tdc, mark a cross in the middle of the little end hole. Spin the crank to De-extend the conrod to bdc and mark a cross in the centre of the little end again. Measure the distance between the two...
That’s your stroke... -
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@spookdog yeah 3mb barrel but my bore is the 1988 bore size and from measuring my crank it seems the crank is the shorter type ofcourse there's the possibility my measuring was wrong but I'm doing my best it's a cheap crank if I get it wrong I'll just get a 3mb a bit of trial and error but I'll get there in the end
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@declan
The bore is irrelevant. The porting on the piston is the same, as is the ring pin positions. As long as it fits the bore a 3nb piston will fit a 3mb barrel and work, and a 3mb will fit a 3bn. You want a later 50.7 crank to make it right...
IMO
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@spookdog sure, but to find pistons that fit my bore you have to buy pre 88 pistons, standard pistons don't fit as the sizes don't match ofcourse you could get the cylinder bored to match the later pistons though If someone is using an early bore size they're likely using an early crank size that's my reasoning anyway
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You see what I’m saying? Your 3mb barrel started of life as a 56mm bore, not a 56.5 (3bn standard bore) the 3mb barrel needs the later 50.7mm crank. Not an early 3bn one...
The crank matches the barrel, not the piston. I just wanted you to get that right
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@spookdog yeah I understand what you're saying but to be truthful I have no idea what crank I've ordered it was literally the cheapest one on eBay that's in working condition so I'll throw it in the cases and if it fits the bearings okay I'll check the squish and see what stroke it is and act accordingly
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Crank came not long ago and test fitted without the engine innards and the play is exactly the same there must be some sort of defect with my main bearings despite them being genuine and only 4 minutes old I can't feel any play in the bearings with the crank out though it's proving to be a tad frustrating
