If you mill inlets and outlets of the barrel, absolutely used eye and respiratory protection. Take care, that right protective measures are taken.
The extension of the outlet is often hailed as the most important measure. I'm not so sure. Much more important are the measures below the piston.
According to Kieltsch (1) the outlet port width may amount to 44 mm width (chord) in maximum. To be on the safe side I stopped 42 mm width. The wider the outlet is the more are the piston rings stressed by deflecting.
The chord is measured best with a pair of dividers. The milling on the barrel is not without risk. You can easily ruin the barrel. Best you proceed as follows:
Set the dividers to the desired width. Mask the desired width with adhesive tape strips. Check the space between the strips width with the dividers and correct them if needed. For the milling you now have the desired width marked. The exhaust port should have the shape as in figure 1.
Remember that the piston rings will be stressed more than before. To prevent breakage channel edges has to be rounded seriously. According to Kieltsch (1) the rounding radii should be 3-5 mm left and right and 2 mm above. The upper edge should slope with 2-5 degree into the barrel.
In the figure the transfer ports are shifted upwards 1.5 mm. The effect arises from the thicker foot gasket (2mm instead of 0.5 mm). The barrel head must be sanded off, to keep the compression constant.
Drawing: Porting series RD 350 air-cooled and the same barrel with 2 mm base gasket and enlarged outlet.
- Horizontal numbers are channel widths, tapped as a chord in the barrel.
- Vertical heights of figures are also tapped in the barrel