Since the plastic tank radiators of the 80's are notorious for breaking off their plastic necks (can ruin your day and your engine!) it is best just to replace the old radiator with new or reinforce the top neck of your current radiator.
Problem & Solution
Common among these chassis:
This can be catastrophic if neglected. If your top radiator neck breaks off when you are cruising at highway speed you can loose all your coolant very rapidly. If you are not religiously watching your gauges, your engine will overheat and may even seize up. Recently I had one customer come in to my shop to have her 1983 300SD checked. While she was standing there, I was pointing to a few items in the engine compartment. As I explained to her about the upper radiator neck, I grabbed a hold of it and gave it a light tug. To both our amazement it came off in my hand. I said, â€œMam, this is your lucky day." She had no idea what could have happened to her engine had this come off while she was driving!
I believe there are two reasons for this failure. Mercedes has stated it is important to only use their engine coolant. It has some special additives in it that control Ph factor in the water. When the Ph factor gets way off, the coolant can actually cause deterioration of the plastic in your radiator. I have personally seen this happen. On the neck that came off in the above illustration it looked as if the plastic had actually rotted out. It was very brittle and crumbled easily.
The other reason for failure can be the use of the narrow Mercedes hose clamp. If that clamp is over tightened (easy to do) it will put undo stress on the neck. I often put on the wider U.S. made hose clamp. I believe this type of clamp can spread out the load and help prevent premature neck failure.
As a preventative measure I epoxy a metal (aluminum or copper) sleeve inside the neck. What gave me this idea is I discovered some of the newer radiators already have the metal support sleeve installed. They have also responded to this common problem! If your Mercedes is dear to you and you plan on keeping it for some time, I would recommend you make this modification yourself. I sell a kit that includes the pipe, epoxy, and instructions.