David,
Have the pistons oversize to begin with.
Drill and tap the piston and screw in the rod until it is really tight, you could put a bit of loctite on it to help grip.
Remount the piston rod in the chuck, and using razor sharp HSS tooling, preferably with a nose radius, take the piston down to size in one cut. It might take a bit of playing about to get to the right size, use a bit of ali or brass first to get the cutter into the right position and then lock up your cross slide gib. If you try to take fine cuts on PTFE it will 'string' badly. Plus a lot of airbourne dust will be produced, which isn't good for you in large quantities.
So to step thru it.
Cut pistons to length, but oversized on diameter (you can wack off 1/2" in one cut). The larger the cut the better, less dust.
Drill and tap pistons and fit rods.
Using a bit of ali or brass, bring the cutter to give you size, and lock cutter in position when size is reached. Retry for size just in case cutter has moved during lock up. It shouldn't be a tight fit in the bore, just a nice sliding fit. You can't lap in a PTFE piston.
Mount up pistons using rod in the chuck.
Take one smooth cut to bring piston down to size.
Check for fit in bore.
Hope it works for you, and if you have any questions or complaints, I am on holiday for the next few months.
Bogs