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I've found those switches to be quirky, test good, but not work. Also if plugged or restricted, what water is coming thru the radiator can be cool, but just not much of it, so the aux fan won't come on. High speed shouldn't come on very often, mostly in stop and go with A/C on. Lastly if you go the E28 route you need a 533/535 radiator, they're the M30 cars. 528e or the Eta is an M20 and the radiator is completely different. As far as fittment, 533/535 fit in the space just fine, but you need to find a way to mount it and I think the E28 upper hose and maybe the water discharge/thermostat cover. Probably remove the drivers side rad mount too.

The E28 rad is a nice upgrade, will fit, but it's not a straight bolt up, some work is required.

Quick Question: After draining the coolant through the rear plug, I realized that I need to change the old rusty block plug. But I cant find the darn plug in RealOEM. It takes a 19mm socket. Anybody know this part number?

Ok. Radiator was inspected and determined to have very good flow; so no "apparent" problem here. Two questions at the stage of the battle. FIRST QUESTION, is the one from my previous post. Does anyone know the part number for the 19mm drain plug at the block. Am still shocked to see half as many barnacles as you would otherwise find on any mile long sea wall in Miami flow out of that hole. Where can I find this plug? SECOND QUESTION: I want to flush out the block before closing up the patient. Can I simply attach a garden hose somewhere up high and hope that the flow sends remaining barnacles out the drain hole? How best do you suggest I accomplish this? I could fashion a smaller hose to fish up into the hole to create flow back out the hole, or go in at another spot after the thermostat, no?

Yes, lots of stuff can come out of that drain hole, don't worry about that. I've found it so plugged I had to work at it with a screwdriver to poke a hole to start the purge of debris. And I looked but couldn't find the part on Realoem and I usually have pretty good luck there, I don't know where they hid it. But I think it's just a regular short bolt, even one from a hardware store should work as long as it's not too long. Slather it with anti seize if you're worried about corrosion.

But... That's not your problem. Very little cooling takes place at the bottom of the bores, the aluminum head does most of the heat transfer and the top of the bores next. You have to figure out if the problem is too much heat being generated or not enough being dissipated. I know you're comfortable about the head and HG, but you might want to pick up one of those kits that test for a blown head gasket or worse a cracked head.

Long shot, but if the timing is too far retarded it can cause running hot, but if it's that far retarded it will usually run poorly.

Could the plug be 22x1.5 with a 22x27 washer, 11 12 1 257 379? It is shown in 11/9 which is cylinder head though.

What does the oil on the dipstick look like? If a "chocolate milk" like substance is evident you may have a head gasket or cyl head issue like Mike says. It's not that rare for even a a replaced cylinder head on these cars to crack and cause overheating. Just sayin'.

I bought an all aluminum drop-in replacement for the E12 radiator for my Euro 528
restoration and it's working great. Made-to-order out of Arizona, but pricey...
I take my radiators seriously here in Austin, TX. My 530i runs an all aluminum version
of the E28 radiator, also from here.

http://www.rondavisradiators.com/

canada karl wrote: ↑Fri Jun 18, 2021 8:32 am What does the oil on the dipstick look like? If a "chocolate milk" like substance is evident you may have a head gasket or cyl head issue like Mike says. It's not that rare for even a a replaced cylinder head on these cars to crack and cause overheating. Just sayin'.

Sorry for disappearing from my own thread. I have been traveling and then I threw out my back. No Tech FAQ for that. Anyway, oil is fine and I found the correct screws and washer/gasket (07-11-9-963-200). See images.

Tomorrow, I will be removing my old rusted plug and replacing it with one of these. This plug is used in several places on the block, incidentally. Too bad the parts diagrams do not reveal this location as it is best one to drain the fluid, as I understand it. Then I will run the car to see how the new Thermo-switches work out... see if the car will run cooler. If not, I will definitely be looking hard at the aluminum units Bondi is touting. The fact that he is in Texas heat carries a lot of weight. (Thanks Bob!). I'll keep you posted as the journey continues and until the overheating problem is resolved. Many thanks to all of you who have contributed.