While getting the junk off of my rear caliper the grinder with the wire wheel caught a corner and cut the rubber caliper boot.

Is there a good way to fix this or is replacement the only way to go?

The cut goes along with the folds around the caliper piston.

 

You can buy caliper rebuild kits. Don't know if the aftermarket has them for the 8 yet, but you should look at someplace like NAPA first. If they don't, the dealer should sell the boot individually.

You do want to get it replaced pretty soon. If you get grit in there you could score the caliper bore and then you gotta buy a whole new caliper.

 

Good to know thanks.

I was thinking of using some silicone adhesive to close it up, temporarily.
Then I will wait for the rebuild kit.

I have a set of rear calipers on the way.

 

When I had the same situation on my Mazda Protege, it was less work to buy a used caliper for $60-70 and replace it wholesale instead of doing the rebuild kit. The rebuild kit is cheaper, but not the simplest thing to install. There's a thread I can point you to about using the rebuild kit on the Protege, it's probably the same for the RX-8.

Of course, a used RX-8 caliper is probably going to be a bunch more money, so that may justify the extra effort for the rebuild kit.

 

You don't need to rebuild the whole thing. You just use the boot out of the kit. Admittedly, redoing all of the internal seals and such is a pain, but you don't need that here.

A quick glance at NAPA and CSK doesn't show any aftermarket calipers or rebuild kits yet, probably because that stuff isn't popular until lots of cars are off warranty. Looks like the dealer is your only choice.

Some silicone or RTV might work just fine actually. I did a ball joint boot in RTV years ago on a previous car and it held up for the 5 years I had it no problem.