Machining down skirt on a chinese barrel

I thought the build for my 911 engine was going a little too smoothly with all the chinese parts in it, so it was a relief to finally run into a typical chinese part related problem. Hopefully this is the token problem done with :P

I got a Polini 50cc  5 port replica top end from danny at pocket racing bikes as well as a stock banshee cylinder to put on. I decided on the polini replica top end because the port timing is MUCH better (very close to the real deal polini) even though the banshee barrel is a little better in casting and the ports are cleaner but that can be fixed on the polini rep.

So the problem i ran into was the skirt at the bottom of the barrel was cast a little too thick (or not machined down one or the other) and wouldnt fit into the crankcases. Machining the cut out in the crankcases out would have been faster but i wanted to use a variety of barrels in the future, so the only option was to turn down the skirt on the barrel. 

First i used my temporary jig (made from some nylon - going to remake a much more elaborate version out of 6061 soon) and 4 jaw chuck to hold it in the lathe. Dial indicator was also used and the runout was about .05mm or so which is as good as i could get it due to the jig being (soft) plastic. But that is still fine for this, as concentricity isnt totally essential.

Doing an interupted cut like this usually isnt a problem at all but the casting being quite thin and made from cast aluminium meant it was a concern that the skirts could snap off and then the barrel would be ruined. So i had to run the lathe at 200rpm (any faster and the whole thing would vibrate too much) and use a woefully slow feedrate, the enitre operation took about 2 hours. to remove the 1.5mm of metal.

Taking the first few cuts - depth of cut was about .175mm at maximum to lessen the risk of breaking the skirt off. i could have gone to about .25mm im pretty sure but considering the risk i was happy to be patient :)

 Done! i did leave a little ~.3mm step at the bottom to save plowing the cutter into the base and gouging it, but the .5mm thick gasket means thats not a problem. Another bonus of doing this was it cleaned up the pretty daggy castings on the piston skirt. Looks like it could have been a polini in some other life haha! maybe..

Next, more cag engine work and assembling this 911 motor some more.