I just started porting a year ago, so take anything I say as friendly encouragement, not sage advice.
There is a lot of info on this site about porting and research is your best tool in modifying chainsaws.
I don't know either of your saws very well. I believe an 020av has the same cylinder as a 020t. If that is so, setting a tight squish and opening the muffler is about all you need to do. Gains after that will become difficult.
DCS6421 is probably going to benefit from opening the muffler (and bypassing the catalytic converter, which I think it has?). Tighten up the squish. Widen exhaust and intake. Run it. Most people who mod 60cc Dolmars seem to go for a larger Piston and Cylinder. I believe a 79cc top end can be bolted directly to the saw you already have. That would be a change bigger than the very best port work.
My limited experience is that open port saws only need changes to intake if the intake port is too small (or short in duration) to adequately supply the case and transfers. More often the carburetor is a bigger limiting factor. If you want more intake duration, trimming the piston skirt is a less expensive way to experiment.
Generally speaking, any port job follows a basic progression.
1. Start with a saw in good running condition.
2. Open your muffler outlet. 80-120% of cylinder exhaust port area. (Mufflers can be replaced or patched.)
3. De-limit the carb (not all carbs have limiters)
4. Measure squish and stock timing numbers.
5. Make a plan. I usually aim for .022-.018 squish, 16-22 degrees of blowdown. Less
blowdown on wider exhaust ports. Everything else is relative to the specific saw.
The "Farmer Jones" approach is to modify squish by deleting or changing the base gasket thickness. Exhaust and Intake ports can be altered with a dremel and or files. Mostly they are simply "blended" so that casting flaws and areas of bad flow are fixed. Ignition timing might get advanced.
The intermediate approach is to use a Lathe to cut some off the base and sometimes make a "pop up" piston. Larger carburetors are sometimes used. Unlimited coils start to be helpful. Ports often have more serious alterations in width or timing.
The advanced approach is a no-holds-barred mission that may include all of the above, as well as boring the combustion chamber on a lathe, changing transfer widths, heights and angles, adding extra transfers, adding or removing stuffers, boring carb venturis, partially filling ports with epoxy, and anything else that seems effective.
If you don't have a lot of specific tools, you start with the "Farmer Jones". That can be good because it's harder to make catastrophic mistakes. Set a good squish and do conservative work in the exhaust. Lowering the floor of the intake port or raising the piston skirt will increase intake duration, but too much will cause the saw to run poorly. Not all saws will benefit from increasing intake and few need major changes there.
I recommend collecting a few simple tools:
Flex Shaft grinder (Foredom, Dremel, or
Chinese knockoff with carbide burrs and grinding stones)
Degree Wheel
Measuring calipers (accurate to 0.001")
0.030" Solder
Hopefully I'm not spouting a bunch of stuff you already know. I really enjoy messing with saws and you probably will too. If you're not afraid to mess up, then you have nothing to lose.
And show us what you end up doing!
