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A method for boring Lawn Boy D cylinders

951 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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Interesting. I had never heard of an OS piston for those engines and I rebuilt more than I could count. I loved them: simple, reliable and that sweet sound.馃グ
Outstanding work Mike! I have a .030 over piston that I think will now be mated to the old original jug that came off my 3054. I don't have the machinery, or skills, to do the boring myself though, so off to the machine shop for that.
Interesting jig for that job, I didn't know they could be bored at all.
Was there a part number on piston ? Also never run across oversize Piston or rings. Rings part number ?
Was there a part number on piston ? Also never run across oversize Piston or rings. Rings part number ?
The only part number on the piston is under the crown: 606607, same place as on a LB piston. From what I know, these .030 oversize pistons were aftermarket sold by Rotary, part number 23 2724 as a oversize replacement for the LB 606607. They were complete with rings, pin and retainers. This exact same oversize piston was also sold by other aftermarket suppliers under their names. I never saw the rings sold separately. They were discontinued not that long ago, and still show up on ebay from time to time. I bought some when they were still available and also on ebay. Thanks for asking.
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