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I've had a Supermiata crossflow radiator in my car for a few years now. I went to drain the coolant and noticed the seals on the two drain plugs have degraded and are leaking. I can't find a replacement online. Has anyone dealt with this before?
Right now i am just debating on sticking an o-ring in there.
A 3/8” bonded sealing washer (metal-backed rubber washer) works really well and is sold at any hardware store. Unlike a regular rubber washer or crush washer, they grip the bolt/plug tightly to provide a radial seal, and they are meant to seal well with minimal torque.
An O-ring won’t seal as well under the head of the bolt since it’s not designed to be squished like that. A normal copper crush washer will need a lot more torque on a plug that’s comically easy to strip.
Those will work, as will this style that is sold at most hardware stores. I used 3/8" washers instead of M10 since it made them extra tight radially around the threads, but any bonded sealing washer for a bolt should be tight enough to seal to the threads in addition to a face seal.