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ET: Engine (Twin) Piston Choice


Little Honda

Well Known and Active Website User
VOC Member
the 7.3 cr pistons are 8:1's with the piston crowns turned off i believe they fail after a large mileage with hairline cracks across the crowns i have seen these after the event in Bury. There is also an issue with the gudgeon pin circlips which move with the usual catastrophic consequences on the pin milling a groove in the bore . Mahle via the manufacturers in england issued a warning with pictures of such failures. and recommend that the circlips be fitted with the gap horizontally or vertically positioned. to prevent this happening . they accuse the user of over revving the motor for this failure . Sounds like a design fault to me. if the pistons are not scuffed and the ring lands are not worn you could treat it to some new rings
Ken, read again, if possible: Circlips should be fitted with gap showing down, never horizontal, as the accelaration of piston operates against the spring pressure of the circlip, if mounted horizontally, which
is prone to open due to loss of spring pressure. Same reason is their recommendation, to use circlips without
ears. What do you think, are the horizontal recesses for in the piston bore ? If you would fit your circlip horizontally, you wouldn´t be able to grab the circlip, as then there is a gap.
 

vibrac

Well Known and Active Website User
VOC Member
Well I have done a lot of searching there are quite a few motorcycle pistons of 90 bore honda aprilla and others but nowhere do they give relevent dimentions pin size distance to crown skirt length etc etc so unless someone has found and used one from another make I am on a hiding to nowhere to get a low compression 90 bore
 

Little Honda

Well Known and Active Website User
VOC Member
Well I have done a lot of searching there are quite a few motorcycle pistons of 90 bore honda aprilla and others but nowhere do they give relevent dimentions pin size distance to crown skirt length etc etc so unless someone has found and used one from another make I am on a hiding to nowhere to get a low compression 90 bore
Why not reducing compression by decompression plates?
 

vibrac

Well Known and Active Website User
VOC Member
The packing is too much leaves a gap at the top and most importantly I have works grey flash pushrod tubes that wont take large variations in angle its a 90mm flat top piston im afraid
 

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