Christopher - here are the basics.
When the piston goes in the down direction - it compresses the fuel oil mixture in the case. The transfer ports are like tubes connected motor case to the cylinder above the piston - so as the piston comes down it acts as a pump - gas/ fuel is pumped through these channels and to the area above the piston in to the cylinder. This is how the cylinder is "charged" with gas fuel.
If you want to see it in action when you have your motor split - its kinda cool to put the cylinder on half the case and rotate the crank - watch as the piston comes down. You will see that those holes above the piston pin and relief on the side of the piston become pathways for the gas fuel to enter the transfer Chanels.
Remember - the area below the piston is compressed by the piston lowering - so the gas is under pressure. It uses the holes in the piston above the pin and those received channels to escape the crank case and go in to the transfer ports.
The cut out at the bottom of the piston below the piston pin is there to do the same. But as the piston skjirt comes down, it starts to close off the transfer channels - so having the relief on the side of the piston allows gas fuel to continue to flow in to the channels even after the piston skirt is closing things off.
Modern pistons are designed with this "relief" built in.
Would it be fair to say relieving the sides basically enlarges the transfer ports? Looking at my DR177 it never closes off the transfer ports do to the size of the hole in the skirts. Wondering if it is worthwhile to do this to a DR177? Makes me curious when I see new pistons, such as the new DR177 which arenít relieved and the VMC that is. Also see the BGM piston is relieved.
Yes - it would.
In my view - the BGM is one of the better designed pistons.
Really optimized for flow to transfer ports with deep indentation.
There are limits to what you can do with a stock piston when carving in to the side due to thickness of material.
Others can comment on what has worked for them on the DR177 - I don't know it very well.
I like the holes above and to the side of the pin - keeping a horizontal full thickness connection between the piston pin and the piston skirt. If you watch more of FMP - you can see a lot of different carvings he's done.
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