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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@spookdog yeah but this engine uses the early bore size so I'm not sure what crank is in it
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@declan
You said it’s a 3mb barrel?Later cranks have a 50.7mm stroke
Early (3Db1) ones were 50mm
Just measure your crank. The difference between it fully extended at tdc and bdc lowest point...
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... -
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... -
If it’s a 3mb barrel I’d get a later 50.7 crank to match it...
I couldn’t find markings on the crank, only the con rod. I’ll check them out in a bit... -
@spookdog I've ordered a 2rh crank it's listed as a 1988 by another seller so I'll see how it matches up theres no other useable cranks on ebay at the time all the rest are either pricey or have stripped threads
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@declan
Fair enough. You are running a 3mb barrel though aren’t you? I seem to remember it was. Cause if it is you really want a 50.7 crank to bring the piston to tdc. Otherwise your squish will be redundant. As well ayllwer compression...@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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@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
@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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@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@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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@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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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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Just out of curiosity: What carb do you have on your bike? Is it the early 88 round slide 26mm or later flatslide 28?...
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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
@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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@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
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
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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
Removed the crank seal so I can see exactly where the play is coming from its the the crank surface where it presses into the race (as suspected) but the crank fits very tightly into the race has to be pressed in its almost like the race is tapered and I've got it reversed, main bearings aren't tapered at the race are they?
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Removed the crank seal so I can see exactly where the play is coming from its the the crank surface where it presses into the race (as suspected) but the crank fits very tightly into the race has to be pressed in its almost like the race is tapered and I've got it reversed, main bearings aren't tapered at the race are they?
