The water passage O-rings are often quite mangled now that the bikes are 30+ years old and have perhaps seem several engine openings; they’re one way that coolant can enter the crankcase and contaminate your oil so it’s cheap insurance to replace them unless you know they have been done recently. There are two O-rings around the water passage dowels, located here:
A closeup of these:
These ones are in dire need of replacement – they don’t even slightly look like they have a round section any more. Replacements are available new (on eBay, often for US$0.99ea) and are part number 94608-50000:
Given the price it doesn’t make much sense to me not to replace them if they haven’t been done recently; given that you have to remove the engine from the bike and then the rear engine cover to access them, it’s a lot of effort for ~$2 of O-rings. They’re 23×2.8:
Dig the old O-rings out, remove the dowels, clean them up if necessary and place the new O-ring around them:
Clean out the block where the dowels and O-rings sit, then put the dowel and new O-ring in place:
Now for the locating dowels. There are two of these, circled in red (upper left, lower right):
If yours are corroded beyond use or missing, a replacement part is available new and is part number 94301-10160 (also available on eBay):
Here’s what a new one looks like:
Clean the holes out of gunk and insert the new dowels:
If you are doing this as part of a triple or quadruple bypass, click here to go back to that page.