This is a cutaway view of both the tank and the cooler. The studs are inside the cooler. They accomplish two things. First, as you rightly suspect they increase the area in contact with transmission fluid. The second reason they are there is to disrupt laminar flow, as turbulence improves cooling by causing more fluid to impinge on the studs.

If you slice open a radiator tube on a modern core, you would find that the tube is corrugated for the same reasons. Modern fins are also corrugated, and some are slit, because the same considerations apply to the air side.

Sooo...how much does a jam nut and a washer cost (Genuine Mercedes, that is)?

C$14.26 for two sets. In US$? About US$5.35/set in Canada at dealer. No shipping costs!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

That's a lot of dough for a nut and washer. Couldn't you find some at the hardware store?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I don't think you would find the original design with the integral shoulder/face. The new style with separate washer and thin nut - maybe, but you would probably have to cut a regular nut in half after burning a couple of gallons of gas driving around to find it

I was quite happy with the price and that the OE parts came in a day.

Installed on one side today. No easy way to use Teflon tape with the thin nut. Used Permatex thread sealant instead. Old one looked like it had something similar on it. I did smear the washer face with some high temperature Permatex gasket maker. Letting it sit overnight before refilling.

Cleaned and inspected other side - It looks fine, so leaving good enough alone

Presently draining transmission.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

My cooler nozzles have some type of white sealant on the thread. Did a search to see if anyone knew what was best to use and came across an interesting but long link from BMW forum.

E32 Cooling system -- Radiator Replacement

These Beemers have rads with tanks on sides and from that link, it seems the transmission cooler connections are a problem for them. (Keep scanning through the link - parts on transmissions nozzles are in several places.)

Even new Nissens rads failed at top cooler nozzle after very short life. In their case, the plastic tank cracked where the nozzle exits. Behr appears to now have reinforced rads to solve that BMW problem.

Could be that on BMW because of side tanks, the upper nozzle gets more stressed?

I am about to go out and install either the one or both nuts. But I WILL check closely that the plastic tank does not have cracks! And if I ever have to buy a new one, if the new type is re-inforced. From on-line pictures of Nissen and Behr for W123, it doesn't seem rads have been reinforced.

My Behr radiator nozzle exits look a lot like the old BMWs. You can see the old and new types below.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

This BMW appears to have the transmission cooler on the hot side, which is odd. Probably done to reduce fluid viscosity on cold days. The problem is that the tank would run really hot under load, which must cause degradation of the plastic and I bet more transmission problems. Mercedes is more typical in having the cooler on the cool side, so fewer problems. W201's and W124's have crossflow radiators like the BMW, but with transmission coolers on the cool side. You don't hear about tank problems, other than the occasional broken neck problems.

You can dress the connector with teflon tape or pipe dope to make a tight seal. Mercedes or no, it's just plumbing.

I know the prices for nuts and bolts at the dealership seem high but you are gonna get something that fits and something that is the correct specification.

Whilst I have lots of other nuts and bolts about the place ready to be used (a whole rack full of spares) if you buy a single large nut or a single large bolt and have to have it posted the dealership sometimes works out cheaper.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Got car back on road today despite being under the weather with a bad cold. I am sure lying on my back on cold garage floor didn't help!

Installing the new cooler nozzle nut and washer was easy enough.

After cleaning up the other side and making sure I could get the atf union nut loose, I decided against messing with it. If that aluminum nut breaks, I now have a spare!

While car was raised, I changed the transmission filter and ATF. I had not done this before. Easy enough job, but for someone in mid-70s with car on jack stands, not much fun. Not much labour, so next time I will get garage to do it! Mind you, I bought a 1/4" drive in-lb torque wrench - may have to find another use for it!

Filled with coolant and ATF (in steps) then went for drive. Everything seemed good. Parked car and noticed red drip from rad area Surely not my repair to rad leaking ATF?? OhOh other side! - I had loosened other side cooler union just to be sure I could - but had not re-tightened it!

Anyway, I think I am now done with 300D for the moment. Time to move on to other things.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Did you clean out the transmission cooler per Sun Valley's instructions? Seems like a good thing to do given the construction of the cooler with all those closely spaced pins which could trap debris and decrease cooler efficiency.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

No, but you are right that it would have been a good idea. Just ran out of steam and wanted to get job done. Lot's of other jobs to do on cars and house before winter sets in!

The transmission pan was spotlessly clean and I didn't like messing with those hose connections - Brass union nut onto aluminum threaded nozzle seems kind of place where threads could easily strip. On the side I fixed, the thread was already slightly damaged, but I used the new steel nut as a die and was able to get past the bad thread.

It would be interesting to confirm what metal Behr and Nissen use for the cooler and it's nozzles on replacement rads. I know it looks like brass, but so did the cad plated OE nuts and washers I bought.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Unable to edit post#1 of 10-21-2015, but because pictures are not showing up, reposted here:

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Use a split nut held with vise grip pliers on the exposed threads of the nipple to provide a counterhold for the flare wrench on the cooler line fitting.

Spray penetrant beforehand, and let it soak for a good 15 minutes or more..

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022