Part 8 - Machining a large piston

by Jason Ballamy

The next part was the piston. This is supplied as an iron casting and the first job was to get it running as true as possible in the 4-jaw and clean up the holding spigot.

Then holding it by the spigot in a 5C collet I could machine the end, OD, skirt and put in the two groves for the piston rings.
It was then transferred to the mill and held by the spigot in a 3-jaw mounted on the rotary table to clean up the rectangular hole for the little end, also drill and tap for the screws which retain the wrist pin.
Without rotating the table it was reset in vertical mode and the hole for the 3/8" wrist pin drilled and reamed, I added a simple jack to reduce any risk of the piston sagging under pressure from the drill.
The spigot was then sawn off and the piston returned to the lathe with protection between the jaws and taken back to finished length.
The rings were just turned from CI bar.
While the cast iron dust was spread around the workshop I decided to do the fuel tank. First job was to clean up the two mounting lugs, as you can see by the shiny edges they were a bit chilled.
Luckily the middle was soft enough to allow them to be drilled and tapped, the fuel pipe connection was also done at the same setting.
The same could not be said for the filler hole, this was chilled all over and although the indexable milling cutter cleaned up the face to show the extent of the chilling there was no way it could be tapped like that.
The solution was heat, I used the propane torch to get the filler boss upto red heat and kept it there for about 15mins then let it cool very slowly.
That did the trick and a brass cap finished off the tank.

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