An expanding mandrel used in a lathe is the best way to square cylinder bases, however there is something to be aware of:

Expanding mandrels are, at best, a compromise at any given diameter. The way to make sure the mandrel is correct is to regrind it for your given application.

Here's how I fixed mine:

On the mandrel I use for Yamaha cylinders, I reground the "sleeve" to be perfectly round and straight at standard bore size (52.0 piston + ~.003). First, I got the sleeve to "stick" on the mandrel a bit using dykem -- It gives just enough stick without adding any thickness that can deform the sleeve. I also "backed up" the sleeve on the small end of the mandrel with a tapered bore collar that I made. I then made sure the centers in the mandrel were perfect (just a bit of a touch-up was required to get the mandrel running dead true on each end). I then picked it up in a cylindrical grinder and very carefully started grinding on the OD of the sleeve (very carefully... since there is essentially nothing but friction and a touch of dykem holding the sleeve in place), making sure the machine was set up to grind dead straight. I actually had to bump the expanding sleeve "up the taper" once or twice to get enough material that I could grind it clean all over at the desired diameter (you'll be very surprised how far off things are when the sleeve is shoved up the taper a ways). After a bit of work, I had a sleeve that was dead round and straight at the diameter I wanted.

Not only does the mandrel "grab" the cylinder much more solidly than a "stock" mandrel -- I'm also confident that it is dead true to the bore centerline, (something I don't think is possibly with an "off the shelf" expanding mandrel). A side benefit of doing this is that the mandrel doesn't make a mark on the bore whatsoever... you can finish hone your cylinder, lock it onto the mandrel and true the base (and top if desired), and when it's removed, there is not a single mark in the bore.

The other thing to be aware of when using an expanding mandrel is that it must run dead true on both ends in order to be certain the base is absolutely square. I always put a "tenth" indicator on each end of the mandrel after I put it in the lathe... and make sure it's perfect on both ends before I skim the base. It's also a good idea to make sure the tailstock on your lathe is lined up with the spindle centerline (stick an indicator on your chuck and "sweep" the tailstock spindle... with the tailstock fully retracted, as well as with it almost fully extended).

As with most things in karting, simply buying an "off the shelf" piece of tooling rarely does the job as accurately as is possible.

Pete Muller
Muller Machine