Cutting a piston down
Posted: Sun Mar 29, 2020 1:43 am
I needed a Cushman 2.875" +.075 piston and nobody sells them anymore! So i decided to turn a 3" piston down to 2.950. Thought youall might want to know how or just watch n poke fun.
first I measured carefully that the wrist pin diameter and height is the same. I measured the ring land OD and noted how much smaller than the major skirt OD. Then I measured the cam grind IE diameter inline with the wrist pins and the diameter 90 degrees to the wrist pins. this measured .010 a large cam grind but hopefully they know the expansion characteristics of their pistons, so I'll go with it So I cut two pieces of .0025 feeler gauge as long as the jaws on the chuck. Then I put the shims under the jaws on my 4 jaw chuck that were over the center of the wrist pin bores. then I centered it carefully
Then I turned it to size and checked it in the cylinder it was going in to.
Once it was the right size round I noted the chuck key location of the jaws inline with the wrist pin bores and marked them with a dry erase marker. Then I loosened one of the jaws inline with a wrist pin bore and removed the shim on the other side and tightened the jaw I loosened until the piston shifted against the dog that didn't have a shim. and with the tool post still set for the last round cut, I took the offset cut.
Then I loosened the jaw that had shims under it and removed one shim and taped the piston over enough to get the shim between the jaw that had never been moved and tightened the one that wes loose back to the mark. Checked that it was back to center with an indicator on the inside witness. then loosened the jaw that had never been moved and removed the opposite shim and tightened it back up. Then still not moving the tool post from the final round cut took the last offset cut.
Then i turned the piston over in the chuck and centered the ring land and turned it to size
Tried the rings in the land groves and got lucky as hell! I expected to need to cut the ring groves deeper but they were ok.
Sure was easier 50 years ago when there were a few shops in town with a piston cam grinding machine.
Dusty
first I measured carefully that the wrist pin diameter and height is the same. I measured the ring land OD and noted how much smaller than the major skirt OD. Then I measured the cam grind IE diameter inline with the wrist pins and the diameter 90 degrees to the wrist pins. this measured .010 a large cam grind but hopefully they know the expansion characteristics of their pistons, so I'll go with it So I cut two pieces of .0025 feeler gauge as long as the jaws on the chuck. Then I put the shims under the jaws on my 4 jaw chuck that were over the center of the wrist pin bores. then I centered it carefully
Then I turned it to size and checked it in the cylinder it was going in to.
Once it was the right size round I noted the chuck key location of the jaws inline with the wrist pin bores and marked them with a dry erase marker. Then I loosened one of the jaws inline with a wrist pin bore and removed the shim on the other side and tightened the jaw I loosened until the piston shifted against the dog that didn't have a shim. and with the tool post still set for the last round cut, I took the offset cut.
Then I loosened the jaw that had shims under it and removed one shim and taped the piston over enough to get the shim between the jaw that had never been moved and tightened the one that wes loose back to the mark. Checked that it was back to center with an indicator on the inside witness. then loosened the jaw that had never been moved and removed the opposite shim and tightened it back up. Then still not moving the tool post from the final round cut took the last offset cut.
Then i turned the piston over in the chuck and centered the ring land and turned it to size
Tried the rings in the land groves and got lucky as hell! I expected to need to cut the ring groves deeper but they were ok.
Sure was easier 50 years ago when there were a few shops in town with a piston cam grinding machine.
Dusty
