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2024-06-11, 20:02:05
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2024-06-08, 18:30:04
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Fitting Japanese Pistons to a MK3

Started by Clive, 24 May 2007 at 04:33

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Hi Everyone ,

Has anyone fitted O/S Japanese pistons to a MK3, there seems to be a lot to choose from ie 62-63mm range
Honda Suzuki, Yamaha,which are the best ?  I have read write ups on T35s fitted with these pistons and the amounts to be machined off the cylinder and head to maintain the compression ratio . Has anyone done this conversion to a MK 3  which does not have such domed pistons .
What are the pit falls, i know the weakness of the cylinder near the base , but there dos,nt seem to be a lot of spare cylinders out there so this convertion seems a good way to get another life out of the old bits .

Alan Cun

Hello Clive, dont know if this is of any help but about 4 months ago I picked up a heap of mark Doug parts that were going to used for a historic racer. In among the parts were two barrels fitted with honda pistons and never used.     They are a 4 valve piston  one side gudgeon says Honda.    The other A25 (hole) /5 .  On top of piston by the reconditioner is 2.4398.    I bought the package from Kevin  Hodges Traverston via Gympie Qld , very experienced in old bikes, so reckon he knew what he was doing. I am sure others have had more results to pass on. regards Al



Thank you for your replies , i was hoping for more detailed responce but never mind will have a go myself and then i can perhaps pass on a more detailed account of the convertion .


          I have fitted Honda CB250N pistons in Dragonflies. All you have to do is change the small end bushes for specials with a 15mm bore and have the cylinders bored to the required size. Just to be on the safe side, I also filed a couple of flats on each piston to clear the valves, but I have heard of owners fitting the pistons without filing the flats. If you check the Honda pistons, you wil find they have the gudgeon pin slightly off centre. I ignored this and fitted both pistons with the exhaust cutaways to the bottom on both cylinders, and the engine ran fine. The resulting lower compression doesn't seem to be a bad thing with our modern unleaded fuel - it appears to give more torque without sacrificing too many revs.


[Thanks Eddie,
Will acquire a couple of Honda pistons and have a go and thank you to everyone that replied to this Post



I also have CB250 pistons in my Mk. They have been in there for some 12 plus years and had no problems, at one point I had the barrels off and there was no sign of wear.