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I've found that some starter cup base diameters are larger than the spotface diameter on the Mac flywheels. My Bonbright cup was sitting up on the outer edge of the spotface. I had to flycut the backside of mine about .03 deep so it sits down in the spotface. There's not any extra thread on the 101 crank, and this cut will give you an extra 1/2 thread. Terry Ives also counterbored the inside face of the cup for a bit more thread engagement. Of course, other cups may not have this issue, but I'd check the diameters! Ted

Ted,

Good piece of advice on the starter cup fit. I will add that I have found a few that did not have enough relief in the two half circle pockets that fit over the recoil arm bosses on the Mac flywheels so they would not sit down as intended even after assuring the base diameter and recess were sized properly. There are so many different Mac flywheels out there you just have to pay careful attention to the way everything fits when building up the motors.
Speaking of Mac flywheels.... I found one recently that is an 8 degree flywheel! I thought I had verified the magnet to key relationship on every flywheel in my collection of stuff and I just happened to pick one up from a motor I had apart to test a new tool I made and it was way off. At first I cussed myself figuring I had screwed up the tool but after checking about ten other flywheels I confirmed my tool was built right and I had a freak flywheel. Now I'll have to go out and test it to see how it runs compared to a "5"... might be a valuable piece.... or it might just be too much and I'll need a new piston after a couple laps
Check everything! You just never know with the old vintage kart stuff.
Steve O'Hara

Good advice on the starter pawl boss reliefs. If you have to flycut your pulley to fit into the spotface, you may bottom on those bosses. Ron C. and I were talking yesterday, and he measured the base diameter on some cast cups. They all fit down in the spotface O.K.. It may be only the Bonbright units that are of concern. They were originally sized for the 610/820 Benders, so the base O.D. is greater. Dave machines in the starter pawl reliefs with the wider spacing for the Macs when you order one from him.
I'm beginning to realize that you never take anything for granted in the vintage kart game, and not just with Macs! AH-58's are at least as diverse. 8 Degree flywheels, eh? That's a new cookie! Ted

Steve,
could you post a picture of that flywheel tool?
interested in how it's built.
dave