the flat rad cap for a s4 is 17.00 on mazdatrix, can i just use a radiator cap that is rated for the same as the other one instead?

 

Yes.

 

No you can't. Pressure relief caps are only for when the fluid has somewhere to go, i.e. an overflow bottle. Use the correct cap.

 

Both caps have the same upper sealing washer. So, when the relief cap opens, it does the same thing as the flat cap, the fluid has nowehre to go. As long as there is a second relief cap (at the water neck), and a hose leading from it to the overflow canister, this will work fine. I've done it.

 

Or put a completely flat cap and drop a thermo sensor in there like I did, lol. I don't need to fill from that spot since I can do that on the top of the Fluidyne radiator.

 

But that's not the highest point in the system, the filler cap is. Must be a bitch to bleed...

 

Well I guess I should clarify a bit. If I have drained the entire system, then I do add from that filler neck. If I just need to top off a bit, then it's the top of the radiator. But you are right though, it's a bit more of a pain when it comes down to bleeding the line, but the opportunity to make that plate for my thermo sensor was too good to pass up at the time.

 

Ive been meaning to ask what people with aftermarket rads do. I just got my koyo in the mail and all of a sudden it dawned on me that I know have a spot for two radiator caps. So does everyone with an aftermarket rad use two caps or what?