Airsal had core shift problems in the past which had the exhause and transfers higher or lower than desired which created a shyte show on the consumers who were trying to just slap on a kit and go. You can set datum and index for these deviations from norm. I use the extra fine sharpies for the piston marking, I can get those at a dollar store in a 5 pack (blue red green 2xblack ) but any permanent marker that is extra fine is good.
To check if you might have gotten a core shift old cylinder somehow, you can set datum but putting the piston on the conrod, add the gudgeon pin (no need for fighting with clips or rings to measure), introduce cylinder without a base gasket and put piston at TDC. Do not use any lube on the cylinder, as there are no rings it should slide easy enough dry and any lube will erase or blur any sharpie marks you draw. Set the datum by shimming the cylinder up using feeler gauges until the periphery of the piston crown is level to the deck of the cylinder. Note how much shim thickness you need for datum and also use a sharpie and draw on the piston where the floor of the exhause port is on the side of the piston.
Rotate the crank to put the piston at BDC and check that the periphery of the piston crown alignment with the floor of the exhause port. Use feeler gauges to make these two points in alignment. That is the thickness of base gasket you would need to have free flow of exhause and transfers gasses into the cylinder. Make note of the thickness you need for this index mark. Leave that thickness at the base and rotate to TDC and mark to the piston where the floor of the exhause port is on the side of the piston.
Any extra that you need to shim up the piston to go from datum to index will be a slight loss on compression. You can correct that by using a thinner head gasket. This is where copper gaskets of different thickness is handy. Otherwise you will need to decide of you want to have a machine shop shave a little off the deck for you, or a little off the head, to restore compression ratio.
Any less shim from datum to get index is a different issue as you would want to maintain datum, so the piston does not pop up out of the cylinder at TDC. You need to gauge this within reason though, if it is minor then you can just use a thicker head gasket (again copper ones are handy) to avoid contact between head and piston crown. Or you can leave it to the thicker datum thickness and shave the floor of the exhause port and the transfers to align the floors of the ports to the periphery of the piston crown. HOWEVER before you shave, remove the cylinder and check on the skirt of the piston for your sharpie mark(s). If shaving the floor will give less than a 2mm margin between the future mark and the bottom of the skirt, to not shave. That 2mm margin is your guarantee that the hot gasses of combustion will not be migrating into the carter and cause more than just a bit of a issue.
> Justin Caswell Wrote:
> Did you have to do a lot of shimming to get the ports lined up with the
> stroke, or did you just install it as shipped and that gave you enough
> of a hp gain?
> I understand they all require a bit of a case match, but seems to be a
> mixed bag when it comes to exhaust port alignment.