Oil Cooler Drain
 

Only the very early 107 SLs had an oil cooler, but seeing some sedans did too, maybe someone here can help.

I just changed the rad on my 72 350SL, but re-used the existing oil cooler that mounts on one side of the rad.

I drained the cooler before removing and put the drain plug aside. When I went to re-install it, I noticed it had no seal washer. I checked EPC and they don't show a washer. (drain plug #17 in EPC diagram is part 0019971530 )

I measured the drain plug and it does seem to have a very slight taper. It gets tight when abut 2/3 in.

Anyone have any idea how this plug is supposed to be installed and sealed?

Some pics (don't ask why epc shows the drain at top! It is at bottom)
https://www.dropbox.com/s/4v4whm1d04...nplug.jpg?dl=1
https://www.dropbox.com/s/7tbvlnvtxd...drain.jpg?dl=1
https://www.dropbox.com/s/fizck209cn...20rad.JPG?dl=1

It looks like BSPP ( British Standard Pipe Parallel ) but could be a DIN / ISO / German equivalent. Go to Discount Hydraulic Hose and look at their thread ID PDF.

On the plug, where the threads hit the under head hex, there should be a slight taper from the hex to the threads. ( this is a reverse of a tapered seat spark plug. ) The outer sharp edge bites into the soft aluminum cooler boss. This is typical for BSPP in soft metals. If used on steel, a sealing washer is used.

This should plug screw in easily until the under head hits the boss so you have a thread issue that needs addressed.

Doubt Behr would have used a British thread! Looking at that DHH pdf, it looks like my screw plug may be designed to have an o-ring. But I don't know if it is an original screw plug MB part A0019971530. No pictures on net that I could find. I am able to screw the plug all the way in. I am sure it had no crush washer on when I removed it. It may not do any harm to have an aluminum washer plus a little sealant. Just went out and picked up two washers. Probably use flat one plus some Permatex HT sealant.

New rad and cooler now in. Job complete. No leaks yet!

It is completely acceptable to use a soft crush washer or rubber bonded washer for what you are doing.

Don't count on Germans not using BSPP , I recently speced out our 4th stamping press hydraulic clamp system and the clamp cylinders use BSPP. The system is made by HILMA Römheld GmbH Schützenstraße 74, 57271 Hilchenbach, Germany

What looks to be an O ring receiver on the plug is likely the tapered area that bites into the cooler boss creating a seal. On harder materials, the tapered area also deforms to complete the seal.

Generally, for an o ring type seal ( Like Boss SAE o ring ) there is a tapered area above the female threads that will capture the o ring so none of it is exposed. This type of fitting will almost seal on it's own without an o ring.

Cutting the plug only is possible but not typical practice for most standards.

As it turns out, others who have same car have confirmed that, as with my car and epc, the oil cooler never had a seal washer on the drain plug. Presumably the aluminum boss is soft enough to make a washer unnecessary.

By the way, what little cone there is under the hex head is same diameter as the threads, so that is not how it seals.