I noticed a coolant leak on my 2004 Mazda RX-8 and tracked it down to a cracked radiator housing. The good news is I think I caught this problem before it caused any serious problems, the bad news is that it's hard to get parts for a low volume car that's over 20 years old. I had no luck finding a (reasonably priced) replacement radiator locally, so I ordering one to be shipped to me.

What do I do in the meantime? I suppose I could continue driving the car, frequently topping off the coolant reservoir with water. I don't think that's a good way to go, though, because hot pressurized liquids have a history of making small cracks bigger. Possibly turning this radiator failure into an overheated engine failure. So I decided to try plugging the leak with some J-B Weld. I learned of this miraculous product from watching 24 Hours of LeMons video summaries, helping many teams get their car back out on the track.

This will be my first opportunity to use the product myself, and a great practice opportunity! Since I plan to replace the radiator soon, it won't really matter how well the product works. But I will gain the experience of using it, and a short period of seeing it in action. For some guidance I enlisted my friend Emily Velasco who has J-B Weld experience and here's what we did:

For surface prep, the surface around the crack was cleaned up and sanded down to give the epoxy a rougher surface to grip on to. The coolant level was drained below the height of the crack so it would quit weeping out which would interfere with epoxy adhesion. Once the area was clean, roughed, and dry, the first batch of J-B weld was mixed and applied.

For patch jobs like this, Emily recommends putting in something to help add structure to the blob of epoxy. A thin sheet of metal is common, and I remember seeing LeMons racers cutting up old license plates for this role. Unfortunately, we are dealing with a compound curve here so bending a sheet of metal to shape is difficult. As a workaround, our Home Depot run involved picking up a sheet of fiberglass cloth (Bondo 20128) along with J-B Weld. Emily recommended using the original formula and not the quick-cure type I saw in LeMons videos, because unlike them I am not in any pressure to get back out on the track.

A small square of fiberglass cloth was cut out and pushed into the still-soft first layer of J-B Weld. After the first layer had set (4-6 hours) but before it cured (15-24 hours) a second batch was mixed and applied on top. The result is not pretty, but it held well enough to stop further coolant leaks. The awkward position of the crack meant we got some J-B weld on adjacent components though I don't think they'll get in the way of future radiator replacement. I hope this old radiator's brittle plastic doesn't crack somewhere else while replacement radiator is still in shipping.

While I wait on arrival of that radiator, I decided to tackle other RX-8 projects on the to-do list starting with installing a backup camera.