piston clearance 75mm bore

[mauricejota7604]

Hi Technical specialists can someone tell me if the piston to cylinder clearance should be checked at the gudgeon centreline or the bottom of the skirt.
And what should that minimum clearance be with cast Mondial pistons, there seems to be varying opinions.
Thanks,
M.

 

[Rob]

Useful info from factory attached (RGS workshop manual extract)
HTH

 

[Paul Marx]

Clearance will of course vary with the use you intend subjecting the bike to and with the patience or lack of for running in.
Paul

 

[mauricejota7604]

Thanks Paul and Rob.

 

[breganzane]

It's usually just a little up from the bottom of the skirt, but well down from the height of the pin.  See how the sizes change as you go to the bottom.  Clearance as per the manual Rob posted for stock pistons.

 

[sfcpiet]

Maurice,

If it says 75 on the piston crown, the bore should be 75mm, standard Mondial, Mahle, Asso, KS and probably just about any other european piston manufacturer, practice.  For general use, the ensuing clearance will be quite ok.  The bore is stated because of the difficulty of measuring pistons precisely.  If fitting used pistons, keep to the bore that is stated on the crown, even if you do measure slightly less at the skirts due to wear.  Pistons do not expand evenly.  Other un-worn areas need the stock clearance just as much, otherwise you risk having the piston nip up.

FWIW, clearance for cast pistons in the 60-90mm diameter range is usually around 0.06-0.08mm, forged can do with slightly less.

piet

 

[phil37]

  The old "1 thou per inch of bore clearance" is not too far away for a cast piston in a sleeved bore. - Regards - phil

 

[Dellortoman]

Or if you prefer to think metric, divide the bore by 1000.

75mm bore -> 0.075mm clearance should be in the right neighbourhood.