Interesting page that:
GRUBEE ENGINE MOTORIZED BICYCLE INFO
About beveling the edges of pistons, I have done it on the exhaust port side to test for the effects of raising the exhaust port without destroying a cylinder. Jennings recommended it in his book and I tried it before porting the exhaust. I progressively removed a slice of material off the top of the piston to test the effect on rpm band on a 74 CR250 and a 76 DT250 I had owned. Cylinders were expensive, pistons replaceable, so modify the piston first.
I had tried this on several snowmobiles but found exhaust timing changes messed up the clutch engagement, so I saved myself some expensive grief at the cost of replacing a well worn piston.
This was not so much a bevel, as a cut across the top of the piston crown. Down perhaps 1mm max down towards the ring and 10mm in toward the center of the piston on the crown to test porting mods. Like this picture from the GRUBEE ENGINE MOTORIZED BICYCLE webpage, but more in toward center of the piston.
I don't remember any effect on crown burn patterns, but I wasn't looking for any either. It was a temporary mod to test the effect on rpm range before I ground the roof of the exhaust port. When I had the effect I was looking for, or actually when I had gone too far on the piston, I ground the previously successful measurement into the roof of the exhaust.
Neil and I did not use this method when we ground his Blaster exhaust port. We followed the port height of his DT200 which we knew how well it worked. Actually we had planned to play it safe by cutting less than this but he found the metal came off faster than he was aiming for and we ended up on the full measurement.
I do recall that the cut down pistons had a bad effect on rings. Piston showed much more blow by. Overall, I would not recommend this as a permanent modification, but just as a testing tool.
As for the beveled transfer port, worth testing but considering the dynamics involved (dropping piston edge, need for a sharp initial break, effect on timing) I do not think it is a good direction to go. Hard to tell if the poster's results are from improved flow or changes in transfer timing. I'd suggest transfer timing.
Wide powerband modification webpage
About the "pocket" in the transfer ports: Sorry to say, the DT200 has the same pocket when I looked harder. Neil and I have decided to not mess with it. Other than the higher intake port (which I am not convinced has any effect) and higher exhaust port (and powervalve) the porting on the DT200 seems identical to the Blaster. This is good news. An easy cylinder head mod and exhaust port grind and you are into the realm of doubling your horsepower. Triplecrown's Blaster has proven this to be true.
So, Neil and I are leaving this area alone at the moment.
Next cylinder mod is stuffing the reed box to reduce volume and keep flow velocity up.
Plan is to fill the sides of the box up with epoxied aluminum plates to have the reed block fit tighter, and to build up the intake port divider and sides with epoxy for the same reason. I'll try to get pictures and we will see how it works. Neil's Blaster is working pretty darned good as it is, getting too darned close to catching up with my KTM!
I have a couple heads to be modified in the machine shop. Going to machine one over 50% squish area , one under 50% squish area, both aimed for about 20cc volume and a simple toroidal sort of shape. We will see how they turn out.
