When is a re bore neccessary

[Invicta]

Hi Having removed my firmly stuck piston rings and found that the wire wound pisto on my AJS model 18 is very oval. (Possibly I crushed it in the vice holding it while trying to get the stuck rings out, even though I barely held it in soft jaws enough to stop it turning. The engine had been left without its barrel on for many years. The barrel is smooth and unscored and is on standard bore. It has 0.007" wear across the diameter at the top below wear ridge and is concentric. I shall be replacing the piston, but my question is is it worth re boring the barrel when it has little wear. I have read that I should remove the wear ridge when fitting a new piston or rings to prevent the risk of the top ring breaking. How is it best to do this , Any advice would be greatly appreciated.

[g80csp11]

generally rule of thumb was that wear up to 0.0075 is right on the limit for a rebore
In the old days you used to be able to get pistons in 0.010 step , but now days only 0.020 steps
wire wound pistons originally ran a very tight clearance of circ 0.0015 when new , if you are running replacement non wire wound pistons clearance is 0.0015 per inch of bore to allow for expansion ie 82mm diameter would give 0.0048" clearance

[Joker_Bones]

The 53 Army maintenance manual agrees with Peter's rule of thumb...

bore.JPG

[Joker_Bones]

The Hepolite piston catalogue gives a range for piston ovality, piston to bore clearance is measured front to back at the bottom of the skirt...

oval.JPG