Oil cooler

[chumps]

The threads on my oil cooler drain plug are stripped. They have been for years, but I have managed to tighten it just enough to keep from leaking. I have gone beyond that point now. It leaves a 2 inch puddle under it if it sits for a couple of days. Is there a possible fix (rethread or such) or am I going to have to buy a new cooler? Or look for a good used one. 1971 289 SL Auto. Thanks, Chumps

[Tyler S]

The collar is made out of steel. You may be able to re-thread and use an oversize plug if there is enough meat on the collar. Or take it to a machine shop and have them weld a new bung/collar on it. Either way it needs to come out because it will need a really good flush after the repair to get rid of metal shavings.

[chumps]

Thanks for your reply Tyler S. I have the cooler out of the car. Do you or anybody else know what size threads are in the collar. There appears to be about 3/8 t0 1/2 inch of threads below where the threads are stripped in the collar. If I could tap out the first few threads down to the good threads and find a longer plug it might work. I figure that would be a good place to start. There are other things I could try if that did not work. If nothing works I would ultimately have to get a new cooler (ouch). Thanks for your suggestion. Chumps 

[Tyler S]

Most if not all Mercedes drain plugs have a thread pitch of 1.5mm  Take your drain plug with you and hit your local Napa. Should be able to get you the correct tap and match up a longer drain plug. Don't forget a new aluminum crush washer as well.

[wwheeler]

Here is an out of the box thought. If the drain threads just can't be fixed, could you just weld the drain plug in so it is leak free? Changing the oil and filter would get 85% of the total oil capacity out. Or you could loosen the bottom oil hose (messy yes) and drain what you can. Since the oil circulates around in the cooler, it is not like it is stagnant.

Crazy thought and I can't even believe I wrote this.

[ja17]

Probably a good thought Wallace. A lot of coolers of the era did not have drain plugs anyway. You could always unfasten the oil hose at the bottom to drain the cooler. However, any good custom fab shop should be able to figure out a permanent repair. Most likely a new ring with a new  set of threads could be brazed in place. Any repairs should not be done with solder since oil pressure in this unit can be fairly high. A good brass weld would work fine. A good shop could also weld closed the old drain, then drill and re-thread it.

[Tyler S]

Joe, the oil cooler is made from steel. They did this because of the high pressures you mentioned. A new bung would have to be welded in place. And im not sure if even that would work even though I mentioned it above because the fittings and drain plug collar are actually crimped in place. It may work though. Couldn't hurt to try but I would be concerned about burning a hole in the thin metal.

[chumps]

Thank you for all of your suggestions. I was able to find a tap with proper threads (1.50) and rethreaded opening. I have an inch long bolt that fits nicely in the threads. Is it okay to use a bolt with the proper threads and crush washer verses a 'drain plug'? If this fix does not work it sounds like the way to go would be to weld the plug in and use the lower oil line to drain the cooler on the future. Thanks, Chumps

[Mike K]

I had a similar problem, the other way around, when I did my first oil change about 2 years ago. I overtightened and stripped the threads on the drain plug. Replaced it with an M11 (7/16") bolt and crush washer. I've done 3 oil changes since then and no issues whatsoever.

The bolt I used was also a 1inch, it was necessary to shorten the bolt, it was way too long.
See pic below.

Best,

Mike

[ja17]

Yes, I know these coolers are steel. From the factory, the steel cores are actually brazed (brass welded) to the tanks instead of being steel welded or soldered. Brass welding requires less heat but is much stronger than solder. Brass welding (brazing) can be done on very thin steel since the base metal (steel) only needs to be heated to "cherry red" to make the weld. With steel welding both metals must be liquefied to make the weld and thin metals can be a challenge.  The biggest problem with these coolers these days is failure because of rust. The steel core corrodes becoming pitted and thin. At this stage, good repairs are nearly impossible since radiator shops cannot re-core these and solder repairs will not work.
Sounds like you have found a good solution. Definitely worth fixing in your case. These coolers are expensive these days.

[chumps]

The retap and new bolt and crush washer worked. I had the cooler thoroughly flushed out and tested. No leaks. I'm going to reinstall it today. Thanks for all suggestions and information. The knowledge on this forum is incredible and the fact that the people who know so much about these fine cars are willing to share it with us is special. Thanks again, Chumps

[Iconic]

That is fantastic, you are very fortunate.
Mine was like a sieve. As already stated, they are not repairable. I purchased a new one from MB Classic Center a few years ago.

You might think about shooting some paint on it to help preserve it.
Mark

[Benz Dr.]

Why couldn't you use a thread insert?

[Tyler S]

Benz Dr, I think the collar/bung is a bit too thin. There wouldnt be much left if you over sized it.

[Benz Dr.]

I don't remember it being that small but maybe it wouldn't work.

[perry113]

Worst case scenario.
I needed one for a 69 280SL a few years because the original was leaking. I bought a known "good" one on ebay that was nothing more than a leaking boat anchor when I sent it out for pressure testing.
I ended up buying a beautiful brand new one from Buds Benz.
They have some connection to source new ones that are reasonable in price.
Just so you know where to look if you need an option.
Regards
Peter