This was inspired by my broken plastic radiator nipple that connects to the hose that goes to the overflow tank, and also by the following threads:
It seems that several people had good luck with the brass fitting repair of the overflow hose fitting in the stock Berh radiator.
Well, guess what, it appears that before I got the car ('82 300D), someone else has already done the brass fitting repair. I am sure it served well for years, but by the time I got it, it was leaking.
Here is the fitting that was in my radiator:
it is a brass 1/8" threaded by 1/4" barb fitting.
The repair has developed cracks and was leaking coolant.
It dawned on me that brass (fitting) and plastic (radiator) are not very good materials to glue together because the brass would expand a lot more than plastic would, and this is a radiator so this is routinely going to get to 200 F or so. The uneven expansion of brass and plastic is going to put stress on the glue joint and eventually will crack it.
A better "match" would be plastic glued to plastic. So I set out to find a similar plastic fitting. Even if the plastic that the fitting is made of is not the same as what the radiator top is made of, it would still have thermal expansion properties closer to the radiator plastic than brass!
I was able to find a similar fitting at Ace hardware:
I shortened the threaded part because it does not need to be very long (the radiator walls are only 1-2mm thick). I want to be able to thread it in the hole, with glue already on it, and then tighten it to the point where it 'bottoms out', before the glue starts to set.
I sanded down the barbed end until it fit inside the rubber hose tight but not so that it would break when I try to put the hose on it.
Another nice thing about gluing plastic to plastic is that glue that is specially formulated for that can take advantage of the fact that plastics can be softened by the glue thus increasing the bond strength). Here is the plastic-specific glue I used:
Here is the fitting glued in place:
This glue job isn't like a JB Weld deal, where you would see a bead of glue (or, in some cases, a big blob of it). The glue is clear and the bond is made by altering the plastic surfaces by slightly dissolving them, so no bead or blob of glue here.
I have not yet done any kind of pressure test, but the repair seems to be very solid.
The barbed end flexes better than brass did, so any external stress put on the fitting does not transfer to the glued part entirely; the flex in the fitting takes on a lot of that stress and does not pass it to the glued joint.
I hope this helps someone who has a similar problem with their stock Behr radiator.