The part number of the Yamaha seal is
11H-12438-10
Courtesy of Randall in Minneapolis
Contrary to Don in Oz's observations, my experience with the US-market '78 CX500s is that the original mechanical seal is smaller than those currently available, either from Honda or Yamaha, and that there is NO STEP between the mech seal seat and the adjacent oil seal seat.
Today, we verified using a caliper that the original mech seal is smaller than the Yamaha replacement seal. This is consistent with the Honda replacement I used last summer. (To my knowledge only the later, larger seal is available from Honda.) Again, contrary to Don's observations of the Oz-market bikes, this photo from Rob Davis' website clearly shows a step between the mech seal and oil seal seats, with the larger diameter at the waterpump side. I assume this to be the case with the later CXs in the US, as well.
(EDIT: An afterthought - given the age of these engines, it's possible that this example was previously machined to receive the larger seal.)
In both cases, I created a step using the method described by Rick Hoad near the end of this page from Rob's site. I also fashioned a press based on Ian Shearer's comments on the same page.
You'll need to obtain a Dremel flap wheel (Dremel #504).
It's important to enlarge the waterpump side of the hole, only as deep as the weep hole. Enlarging the engine side will prevent the oil seal seating properly.
Using the flapwheel in an electric drill, hone the hole a little at a time, checking the size frequently. Today, we used the caliper as a gauge, set to the original hole diameter plus the difference in the mech seal cup sizes (23/1000ths in, about 1/2mm), Last summer, I checked using the cup. (I was more confident with the caliper.) Don't hone the hole to the full measured size of the cup. It needs to be a snug fit.
The final result doesn't have the clean step of the factory-machined example above, but both the mech seal and the oil seal fit properly.
EDIT: Be sure to clean out the weep hole after honing. It hasn't yet been done in this photo.
The press is made from a short length of 3/8" threaded rod, two large washers, two nuts, and about three inches of 1" ID schedule 40 PVC pipe, sanded smooth on the end. The pipe fits perfectly over the rubber parts of the seal, contacting only the flange of the metal cup.
One end of my press is a little out of square, so I had to rotate the pipe and press it again. In the end, it's a perfect fit.
A reminder from R80Dave, our mentor this weekend: Don't touch the rubber or ceramic parts of the mechanical seal with your hands. The oils in your skin can degrade the surfaces.
