10 years ago I set up a 6421 for a local wood carver. He bought it along with a big bore top end (84cc) from Baileys. At the time, we ran it side by side against my 371, 20" chisel on both. The 371 had the slightest edge at the time. For these ten years I've been bugging him to let me to to at least gut the muffler, and preferably, install the NWP top. "No thanks," he repeated time and again, always with some version of "warranty; it might develop a problem; yada yada, blah, blah, blah."
Well, this summer he offered to trade me the whole works for one of my many 372s. The switcheroo happened before he could even blink his eyes. Lots of smiles on both of our faces indicated our mutual satisfaction.
Immediately I pulled the cat from the muffler and opened up the exhaust outlet. Big difference! A month or so later I swapped the top ends. Old piston looked fine, no questionable marks on it or cylinder. NWP cylinder came with 2 spacer plates of different thicknesses. With no base gasket, I was able to achieve about .020" squish by using the thicker plate. I'd have to review my notes. It may be that squish is .021. When I first tried the thinner plate, I found that at TDC the piston was contacting the top of the combustion chamber.
Saw is running perfectly, is much stronger than when 64cc, pulls a 28" skip tooth .375 just fine. In fact I compare all saws to my hopped up 288s that I used for years when felling timber up in Idaho and Oregon.
Recently I packed the Makita (it's lighter) for felling 48 aspens around a million dollar home up the hill towards Purg from Durango. These trees were 16" to 28" DBH, mostly 100' tall, on a slippery, steep slope, often leaning towards the house, and with some rot in many. A springboard to climb up above the rot at the bases of some of them would have certainly been helpful. We put the winch line in some of them to get them to initiate in the desired direction.
The homeowners (from Texas) were so enamored of the project that they were baking 2! batches of fresh cookies for us each day. I avoided the bathroom scale throughout the project.
My only criticism of the saw is that is does not have a full wrap handle bar on it yet. Each day when I made my first cut, I'd have a big grin on my face, second only to the burst of laughter that comes from the first time in a day that I dawg my 288 into a cut.