Well, these are not really my pics but this is how it works.
This is what you want to take off of the crankshaft... the blue stuff that is...
Make sure you take off the right amount on the right side.
So when the crankshaft is in the BCD, or top dead center (lowest point, so that means that the piston is in the lowest possible position), you take off 5mm off on top and 15mm off on the bottom.
This can be done with a CNC Machine (Computer Numerical Code Machine) or with one of those other pieces of briliant tools, which I actually do not know the name of anymore in English...
This is what you end up with.
For example, you can improve the flow by taking away this part:
And making it so:
You basically have to round it off.
About the mouth of the engine, I can't seem to find any pics.
The inlet can be done so:
- Take a 13mm drill.
- Do not drill deeper than 18mm
It is not really important how you drill out the inlet, as long as you end up making it bigger; especially higher. The reason a Speed Engine (by Polini) has its intake higher up against the crankshaft is so that the flow is better.
Can be done like so:
And finished so:
What you end up with then is something like this:
I'll try to find the other pics of the mouth part of the engine. What you basically want to do, however, is to extend the ports of the stock engine. The ports from the carter which work in combination with a stock cylinder are not going to be the best for a, for example, 70cc Polini or 70cc DR kit.
Therefore you take a look at the top and side ports of your cylinder and by seeing those you adjust these to your engine by extending the engine ports; make them deeper into the carter, wider and more to the back (in the direction of the inlet).
I will find pics soon!