by Fast n Furious » Thu Oct 24, 2019 10:28 pm
I posted this explanation some time ago in another thread:-
If the piston is a high silicon alloy cast unit and measures 69.95mm then it is grade "A". These pistons in this size of bore need 0.05mm clearance. This is the correct piston for a 70.00mm bore.
If the bore measures 70.01mm then you need a grade "B" piston which should measure as 69.96
If the bore measures 70.02mm then you need a grade "C" piston which should measure as 69.97
If the bore measures 70.03mm then you need a grade "D" piston which should measure as 69.98
As you can see from above, the grading jumps in 0.01 increments.
Forged pistons don't necessarily follow this rule though because their thermal expansion rates are different. Consult the piston manufacturer for details on their grade system.
Usually, most Nikasil cylinders are stamped with the grade somewhere on the cylinder. This is only of any use with a new cylinder. A used cylinder must be remeasured to ascertain the correct piston grade.
This is because the nickel deposit on a new cylinder gets rubbed away within a few hours of use, leaving just the extremely hard Silicon Carbide surface to separate the aluminium of your piston from the aluminium of your cylinder. There is also some relaxation of the cylinder alloy when it has heated up and cooled down a few times. So, it it very important to measure a used bore even though you cannot visually see any signs of wear. Only an internal micrometer can measure this accurately enough. If you have access to the A-D range of pistons, you can use these and a pair of feeler gauges to measure which grade of piston is going to give you the required 0.05mm clearance. If you have too much clearance then expect premature failure of the piston with consequential damages.
I usually find that if it was a grade "B" when new, then the next replacement piston will be a Grade "C" then a "D" (If I'm lucky) then its ready for a re-plate or sleeve.
If you have a cylinder re-plated to match that Grade D piston then you wont be able to compensate for wear by fitting a higher grade of piston. Very rarely does anyone fit a grade D piston into a barrel marked as A or B because the barrel is usually knackered before it reaches this stage.
Hope this helps.