... has anyone tried boring a vintage kart cylinder (Mac or other) on a lathe? I was going to give this a shot, since I picked up a 1 1/4" diameter boring bar the other day for cheap. I figure that it doesn't have to be perfect, since I can straighten and true up the bore with my line hone. That is the hone I used on my MC5 clone block, to "bore" it 20 over and take out 10 thousands of taper ... the compression on that motor is huge, the hone did a great job.
FYI - a line hone like a Lisle 16000 will true up a bore, and you can work from both ends of a cylinder to remove taper. The familar cheap spring type hones are only for de-glazing cylinders, they do not change the shape of the cylinder like a line hone does.
Jeff
Re: boring cylinders on a lathe ??
Posted: Sat Jul 16, 2011 11:18 pm
by l brown
Jeff - You shouldn't have any problems boring the Mac cylinders. I have done the last two Mc -7's i run. If you want to align bore the cylinder, then it is a bit more involved to assure correct alignment to attain a bore that is exactly 90 degrees to the crank centerline. Plus, a bored cylinder when finished honed will be round and straight from top to bottom. Unfortunately your hone will take out more material in the port areas, so honing ideally needs to be limited to .0005 to .001 for best results. If you need any suggestions on align boring, let me know. The large diameter boring bar is perfect. You can email me if I can be of any help.
Larry Brown
Re: boring cylinders on a lathe ??
Posted: Sun Jul 17, 2011 11:57 am
by Larry Collins
Jeff,
I have bored several macs on my lathes, with mixed results. The cylinder liner in some I have worked on seemed to be very hard to cut, almost like they had been tempered (work hardened?). Assuming you have mounted the block in your lathe so that the cylinder axis is perpendicular to the crankshaft axis, and the cylinder is centered, the remaining challenges revolve around assuring that both your lathe headstock and your boring bar are both sufficiently rigid to prevent significant deflection. If one or the other are not VERY ridgid, you will find your newly cut cylinder has a great deal of taper, from end to end. My Dad's old 9" Southbend wasn't quite ridgid enough, but a 10" Atlas I have seems to be good enough in this regard.
Some suggestions: 1. Use only a carbide tipped cutter in your boring bar; 2. Weld a 1/2" thick flat steel plate to a piece of 1" or better 1-1/4" bar stock; then machine the plate in your lathe to provide a perfectly true, flat surface. Bore the plate for the Mac block, and then use your 4 jaw lathe chuck and a dial indicator to center up the engine you want to bore. 3. Take very light cuts with a very sharp tool. The ports in the cylinder can really cause your boring bar to vibrate, giving a rough finish that will take a lot of honing to smooth up.
I'm fortunate enough to have a Bridgeport mill in my garage, and I now do all my boring with it. However, even though a lathe might not be as convenient, a very good job can be done on a lathe, if it is big enough to be sufficiently ridgid.