While you're at it, if you have an older boat you might want to replace all four. I had one break on my 1975 Cat-22 while sailing in a strong gusty wind 3 years ago. The bolt on the port forward shroud had corroded in the area that you couldn't see from the deck or the cabin and I suppose it had been over tightened at some point in the past. Quite a surprise. Luckily the mast held and I dropped the mainsail and hooked up a jury-rigged line between a stanchion and the shroud, and sailed home on my storm jib. Catalina Direct sells a beefed up set of chain plates. they have larger diameter bolts and larger backing plates.