bsa A10 piston rings

[eugenius]

Hi and sorry for the simple question..  I have new std rings for my A10. However, when dismounted and cleaned up above the pistons they are marked as 70.5mm. So I suppose standard rings are not good. Should I change those with +10 rings?
Thanks

[berger]

+ 20

[eugenius]

thanks!

[Swarfcut]

Like bergs says, not 10's, you need plus 20 thou. Half a mm is  the same as what we call plus 20 thou oversize.

 Standard Bore ........  plus 1.5mm is  Plus 60 Thou.
                                 plus 1mm    is  Plus 40 Thou
                                 plus 0.5mm is  Plus 20 Thou


  The conversion is not exact, but is within accepted convention.  The bore needs to be accurately measured, pistons will be slightly smaller diameter to give the running clearance, typically 3-4 Thou, but see Service Sheet 704 for specific model recommendations. Are they the right pistons in the right sized bore?

 More confusing is that the original bore is quoted in mm, but replacement pistons and rings were always expressed as being so many "Thou's" oversize.

 Swarfy.

[eugenius]

thanks Swarfy for the precise information. In fact, above my pistons it is clearly incised the size 70.5mm. I have also measured them by the caliper and that's the size. I have ordered Hepolite rings +20 as you adviced. Regards!
Eugenio

[BSA_54A10]

ans when you get the rings ask for their recommended clearances and end gaps

[Black Sheep]

Of course if you can only get oversize rings, filing the ends down until you get a correct fit is a possibility.

[trevinoz]

You can never get a correct fit by filing over size rings down.