Cylinder visual inspection.
The first part of the air-cooled cylinders inspection process is to perform a visual inspection. If the cylinder is showing obvious signs of scoring or in the case of a Nikasil cylinder if any of the coating is coming off, then it would be either discarded or set aside for machining/reconditioning.
Measuring air-cooled cylinders.
A cylinder that passes the visual inspection needs to be measured to see if it is still in tolerance. The first thing to do is to pull the size measurement specifications for that specific cylinder. Next check the tolerance stamp on the cylinder to confirm the size range that you need to measure.
Measure the cylinder in two locations at “D1” and “D2” locations. To find the ovality of the cylinder, subtract the two measurements made at “D1” from each other. The result will be the ovality of the cylinder. Repeat the process for “D2” as well and confirm that the cylinder is within the usable range.
Cylinders are considered worn out when they measure over by 0.100 mm from their original size. The cylinder should not exceed 0.04 mm in ovality.
To find the amount of wear of the cylinder, subtract the maximum number measured from the original tolerance number, that will be the amount of wear.
Piston and Cylinder Clearance
The next item to consider is the piston to cylinder clearance. Even if both the cylinder and the piston measure up to be within the usable ranges it is possible that the piston to bore clearance is still excessive.
To calculate the clearance, take the maximum bore measurement and subtract the piston measurement from that number. The result will be the piston to bore clearance number.
Another way to measure piston to bore clearance is to install the piston into the cylinder without rings. Then take a feeler gauge to measure the between the piston skirt and the cylinder wall. Use a feeler gauge that is as large as the allowable clearance. If that gauge goes between the piston and cylinder, then it is outside the tolerance. You will need to repair or replace.
Honing air-cooled cylinders
Remember to consider that if the cylinder is to be honed, (Cast iron or Biral Cylinders only), that the clearance will most likely increase. If the clearance is already close to the wear limit, then it would not make any sense to go ahead and hone the cylinder.
The other factor to keep in mind is the longevity of the engine you are about to build. Using a cylinder and piston that are close to the maximum limits will result in a much shorter life expectancy.
When to keep or replace a cylinder
You can replace just one piston and cylinder. On a newish engine, this is OK. This is normally only done if something went wrong with one of the cylinders. And, as long as you match the barrel height tolerances it is not an issue. Replacing pistons and cylinders in sets of six is more usual.
Cast iron or biral cylinders can be over bored and a larger piston installed. Usually the next size up will be 1 mm bigger. Then either a custom piston made to suit or an off the shelf piston that is already available can be used.
Nikasil air-cooled cylinders can also be replated back to their original size. Search for a Nikasil re-plater in your area. Just so long as the cylinder has not been overly worn or damaged. You will need to fit a new piston to the re-coated cylinder if you do not land on a standard size.