LCGB Forums

I've just purchased a cylinder kit which is graded "D,"
What would be next if it's worn out by high mileage, needing new rings etc?
Nicasil lined kit. Always had cast barrels and can't find any specific info.
Thanks.

steel liner would be a good option.....

I can't remember what I was told years ago about the A to D grading but you can have the barrel relined with nikasil or chrome if it is worn or as Mick has said steel lined.If you send the chosen piston to the engineering company they match the cylinder to the piston and it doesn't seem to matter what piston grade you send. Have a look at this thread about lining
viewtopic.php?f=7&t=152&start=10#p729

Thanks for that. What I was thinking unless I've got it totally wrong is, should I go back to the dealer and ask
for a grade A or at least a B so I can just move down the list of pistons rather than having it replated or lined?
Cheers Martin.

Martin s wrote:Thanks for that. What I was thinking unless I've got it totally wrong is, should I go back to the dealer and ask
for a grade A or at least a B so I can just move down the list of pistons rather than having it replated or lined?
Cheers Martin.

I think grade A or B is always grade A or B, wear is irrelevant,unless someone tells me otherwise.

The piston is graded to the cylinder a b c or d to maintain the tighter bore to piston clearance required on plated ally cylinder kits , a worn cylinder will require a replate regardless of the grade it started at ,you dont just stick the next grade piston in it and rings are all one size and require gapping on fitting . Also a gade d kit can be replated & honed to a grade a piston . Hope this helps to explain

That's great thanks, knew I was asking a daft question.

Exactly - my RT was a Grade B out of the box. When the bore wore MB recommended buy and A grade piston and get it replated to suit.

The difference A - D is about 0.04mm

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.

Thanks for the info.