I hate to ask a dumb question, but what is the purpose of the plug on top of the radiator (D1)? When adding or changing out the coolant, wouldn't I just add through the expansion tank?

 

You can remove that plastic plug and replace it with a $2 brass 1/2 inch NPT plumbing pipe plug from the hardware store and a piece of teflon tape. It will crumble eventually.

It makes a good place to add coolant, etc. I fill my rad all the way up, then fill up the coolant bottle to the 1/2 way line, then squeeze the upper hose to get out as many bubbles as possible.

 

For adding just go through expansion tank. The plug is necessary for bleeding the system. And you are gonna wanna replace that plastic plug with a brass plug from the hardware store.

Edit: savannah beat me to it lol.

 

It's not 1/2" NPT, not on my '98 Discovery 1.

The plastic is under-rated. It's a brass radiator and if you use a brass plug, you run the risk of stripping the threads -- especially if you use the wrong threads. At least with plastic, you can make a mistake and you just lose a $2 cap.

You can get brass plugs made for Land Rovers but they're costly and they can still seize or strip the threads. If you do use brass, at least use some anti-seize on the threads.

The plastic plugs are inexpensive. You can afford to keep a couple spares for the same price as brass. They'll last a long time and they'll never damage your radiator. Also, the plastic plugs use an o-ring. Although the threads seal the radiator without the o-ring, the o-ring under the cap's flange seals it again even if the threads are damaged. The o-rings are sometimes sold separately, so be sure to get both. There is no need for teflon tape.

And, I agree, the purpose of the cap is to bleed the radiator of air. I believe it's the highest point on the cooling system so air will accumulate there and it can be bled out. DII's are totally different.

 

The problem with the plastic plugs is that as they age, and with the heat, they become brittle. Then at some point they can shatter when you're removing them and you can end up with bits of plastic down in your radiator which probably isn't a good thing. At least I wouldn't want it in mine.

I wouldn't use a plug from the hardware store. A hardware store will only have an NPT thread plug. These are BSPP thread.
The reason they have an o-ring is because BSPP stands for British Standard Pipe Parallel which means the threads do not serve any sealing function. It's all handled by the o-ring. That's why if you use one from the hardware store you have to load it up with pipe sealant of some type, which, at best, is a bodge.

The other issue with hardware store plugs is that some people use either a galvanized or black iron which have a much higher anodic index than the brass of the radiator and you'll eventually get galvanic corrosion where they touch.

The threads in the bung are 1/2" BSPP. Rovahfarm has the brass plugs for 3 or 4 dollars (at least I assume they are BSPP, I haven't asked Trevor).
I like the better design of the ones I use, but they cost more too, about $13.

Aside from just lasting forever, the brass plug allows you to remove it as often as you want without damage. This lets you drain your radiator when needed without having coolant go everywhere when you pull the lower hose. Instead, remove the plug, feed a large clear vinyl tube in to the hole to the bottom of the tank and siphon out the coolant. Then when you remove the lower hose, you spill a minimal amount.
This also works well when doing any maintenance that requires opening anything in the coolant system when you don't want to completely drain it. Just siphon out some to lower the level.

The downside to the brass plugs is that the plastic will act, in theory, as a pressure relief. But if you don't let your truck overheat, that won't be an issue.

 

Oooo...great idea. Why didn't I ever think of that. I do it all the time with other fluids, just never thought about it with the radiator.

 

X2!!! Just dont drink the green koolade

 

I would point out that the one featured in the tech sticky is a 1/2 inch NPT. Now that does not make it right, but for someone that has a daily driver knocked down due to a plastic plug, it certainly is a quick option. Also, a nylon plug would also be an option, to preserve the brass threads. As it is not a tapered plumbing thread, perhaps there is a stock bolt that would fit better?

 

I used the brass NPT plug. I put a rubber o-ring around the top from my garden hose parts bag (the o-ring is a round black rubber one not the squared edged ones si it just rolled right on there) then I put some Teflon tape on the threads and screwed it in by hand then just snugged it up. Fit like a glove and never leaked. Not saying I'm right just thought I would throw that out there about the rubber o-ring.