Not 316
... remember that one or two of you had them made from 316 stainless.
This is for Ed but if it seems like 'duhhh' to you, Ed, it can be for people who don't know as well.
Chainplates should always be of type-304, not type-316. Type 316 is more corrosion-resistant, but also more brittle. It will not withstand the often sudden loads of rigging that may be slightly out of tune or in conditions that may lead to shocking the rig (such as what might flex the hull) as well as will 304. That said, consider:
You are changing out type-304 chainplates on a 30-odd-year-old boat. So don't worry about needing 316 for corrosion resistance if 304 lasted this long!
Type-316 is more expensive and typically less common in most welding/SS-fab/sheet-metal shops.
The clevis pins are Type 316. Avoid galling.
Provided the rest of the fittings, from the mounting bolts to the top clevis pin, are all sized, tensioned and maintained properly, it will all work as one cohesive system as designed; so let that be your primary consideration. I have seen only one fail in all my days, on the mizzen of Cherubini 44 #6 (1977; it was 28 years old at the time) and the rest of the rig held up the spar till we got it apart and replaced them all.
Do NOT use threaded bolts that are too short. Threads should never be employed as a bearing surface. For definition: 'bolt' = pin with no threads. 'Screw' = has threads. For convenience and cost issues, chainplates are typically put in with machine screws, not 'bolts' -- but for best results use a hex-head cap screw that has enough threadless shank (the 'bolt' part) that ALL of the chainplate and all of the wood ride on the part with no threads. You can use washers on the other side if you have to space out the locknut. (The same goes for rigging tangs hung from the mast.)
Treat all mounting holes in the wooden structure with epoxy and do NOT encapsulate bolts, pins or the chainplates themselves in fiberglass, epoxy or anything like. Chainplates need air. That said, a fillet of 5200 around the deck opening is better than silicone for it keeps out water (an enemy of stainless-steel when there is insufficient oxygen) and allows both sufficient flex and sufficient limits on the flex to enhance strength.
If you've already made them of 316, it's no biggie. You just probably paid a little too much and at the very very worst case (and extremely unlikely scenario) you would, theoretically, need to maintain the rigging tension a little more keenly. But any good yachtsman already does this; so he will most likely never know the difference.
