Steel/stainless bolt for radiator drain?

[D60]

I'm assuming there's a good reason plastic radiator tanks also get a plastic drain bolt or petcock? Presumably thermal expansion rates along with less likelihood of damaging tank threads?

I need a Mazda plug like so

It's just an M10x1.25. I've got some Dorman offerings coming in plastic, but wanting to clarify why a basic flange head bolt is a bad idea? Assume any o-rings or sealing washers are easily transferred.

 

[Rand]

Probably multiple reasons, to prevent damage from overtightening.
to prevent dissimilar metal reaction etc.

They all use plastic so no need to reinvent the wheel get a plastic one.
2.2 ft-lb is pretty loose.

 

[spasm3]

Plastic was probably cheaper for the oem.

 

[zzyzzx]

I'm assuming there's a good reason plastic radiator tanks also get a plastic drain bolt or petcock?

I always thought that these things were plastic because it's the drain plug gasket o-ring that does the actual sealing.

 

[The Critic]

Threads are probably stripped in the radiator, just drain/refill using the lower hose and take the antifreeze shower.

 

[D60]

So the interweb led me to believe it's M10x1.25 but I got the old piece out (score one for LH drill bits) and it's M12x1.25

I finally realized I actually had M12x1.25 drain plugs in my oil pan assortment. I installed one and I'm inclined to just leave it.

The old one failed because the head twisted off. It was leaking and I went to snug it. It spun by hand but with even a TINY 1/4" ratchet it was clear the head was already mostly severed.

In thinking about it, if you place any preload/stretch on the bolt, that plastic bolt has to stretch with heat cycling and almost surely becomes brittle with time. You have to preload it a little to maintain a seal at the flange head.

The petcocks with an o-ring PAST the threads are arguably better because they just rely on an o-ring to plug a hole. But here, it's literally a plastic bolt and seals like your oil pan drain. I think metal might actually be a better choice here.

 

[D60]

Old unit. The apparent hole in the body is from LH drilling

 

[K-Man]

Check the threads before you get a replacement. I got an aftermarket all-aluminum radiator for my Ranger, and its drain plug has left-hand threads. Probably not common with OEM radiators, but you never know.

 

[JHZR2]

I suspect the reason for not using metal includes CTE mismatch, ability to torque a metal bolt much more (plastic will self protect), and probably the fact that a nylon bolt is self lubricating into the PP/PE/GF tank, so there’s little chance of damaged threads. It could well be of a soft enough material to self-seal.

 

[D60]

Check the threads before you get a replacement. I got an aftermarket all-aluminum radiator for my Ranger, and its drain plug has left-hand threads. Probably not common with OEM radiators, but you never know.

Nah, the standard RH plug shown in Post #6 went in beautifully and snugged very well.

Plus, a LH drill bit backed the old one out with ease. Thus, it's RH threads.

 

[Donald]

I have blue plastic wing drain plugs on my Volvo Penta boat engine. They come out for the winter to winterize. The plastic gets hard after a few years of heat cycles and snaps. I replaced them.

 

[johnmyster]

Stainless threaded fasteners are notorious for galling. Avoid whenever possible. Especially stainless on stainless without lubrication.

Corrosion incompatibility is largely overhyped. Coolant touches copper, brass, aluminum (heater cores and radiators), iron (block), steel (coolant pipes), stainless (water pump shaft), etc. Coolant systems are a giant mashup of materials protected by coolant corrosion inhibitors.

 

[D60]

Stainless threaded fasteners are notorious for galling. Avoid whenever possible. Especially stainless on stainless without lubrication.

Corrosion incompatibility is largely overhyped. Coolant touches copper, brass, aluminum (heater cores and radiators), iron (block), steel (coolant pipes), stainless (water pump shaft), etc. Coolant systems are a giant mashup of materials protected by coolant corrosion inhibitors.

Stainless isn't gonna gall in plastic female threads.

But yeah, the worst ever was a cable rail I once installed that used stainless threaded eyes to tension. They were 5/16‐24 and under the type of tension necessary for a cable rail, the threads would just gall and lock up. A competing company used coarse threads which is a WAY better choice for this application.

 

[D60]

Well O'Reilly delivered the M10 pieces that I didn't actually need. At an average cost of ~$6 each I'll just keep them for a just-in-case.

Turns out M12x1.25 is less common for radiator drains, but Dorman does offer some of those, too.