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A method for boring Lawn Boy D cylinders

840 Views 5 Replies 5 Participants Last post by  Mike72
Lawn Boy cylinders have liners that show very little wear even after hundreds of hours of running time and unless they become deeply scored from using straight gasoline or from built up carbon deposits, can usually be brush honed and fitted with new rings and sometimes a new piston.
As far as I know, the .030 oversize piston was the only one ever made for the D engines. I would have preferred a .010 and a .020 oversize along with the .030, but many cylinders out there became scored and the idea probably was that a single .030 over bore would clean up almost all of them.
Pictures 1 and 2 show a cylinder mounted to the true flat surface of a precision fixture. The fixture with cylinder are then attached to the lathe face plate, centered on its diameter and the 4 nuts are tightened securing the fixture to the face plate. Next, the cylinder is centered by shifting (tapping) it around and checking both ends of the bore with a dial indicator to average out the readings. The cylinder bolts are now tightened and a light cut is taken to verify the bore is centered. This particular cylinder cleaned up at .010 over, oh well.
The .030 oversize piston measures 3.405 diameter. Allowing .0035 clearance, the cylinder was bored to 3.4085. Picture 4 shows the actual boring operation which went smoothly using a HS tool bit in the bar. After the boring operation, a 180 grit silicon carbide brush hone was used to get a fairly good finish for the rings, and any remaining sharp port edges were filed. The cylinder and piston are going to be used in a future engine build.
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Was there a part number on piston ? Also never run across oversize Piston or rings. Rings part number ?
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