Heat exchanger cap partial failure
I mentioned a coolant leak issue a couple of weeks ago on my Westerbeke W-52. While I haven't ruled out other problems like gasket failure, it is fairly evident that I am losing coolant directly from the heat exchanger pressure cap. These fail in time, and if that's all it is, it's not a big deal to replace. The cap reads "14 lbs. PSI max. pressure" or something along those lines, and it is a silver metal cap that I suspect is the typical 2 1/8" size. It still dogs down, but seeps whether it's running or cold, due to what I am guessing is the "head pressure" of the expansion/fill tank, which is a couple of feet higher than the heat exchanger.
My question: Should I be able to find something cheaper than a "marine part" for $40 shipping and handling, like say at an auto parts store? It looks like a freaking radiator cap to me, and if I found one with "15 psi" on it, would I be "taking a big risk"?
Yes, I am new to diesels and heat exchangers.