My old E39 kept having problems with antifreeze leaks, mainly water neck and radiator. I got tried of it and since I live in a area where the temp pretty much never gets in the triple digits (Seattle), I removed the 2 bar (29 psi) cap and replaced it with a 1.4 bar (20 psi) cap. After I changed the cap I never had a leak again. I believe the 1.4 cap was actually for a 4 cylinder 3 series.

Now I have a E60 and would like to do the same thing but the cap design is different. Anybody know where I can find a lighter cap for E60?

Why - is your E60 leaking coolant too? If you reduce the amount of pressure that is allowed to build in your cooling system you increase the temperature. That's kind of dicey with an all aluminum engine IMO.

No, it's not leaking.....yet. It's mostly plastic, so it eventually will I'm sure.

Water boils at 212. For every pound of pressure, you raise the boiling point by roughly 3 degrees. 2 bar is a little over 29 psi. 29x3=87, 87+212=299. That means the stock cap would prevent boil over to 299 degrees! A 1.4 bar (20psi) cap the math works out to 272 degrees.

I know on my E39 the temp light came on a around 220, according to the service manual (beware, pulling from memory). I would guess the E60 will tell me around the same time, at which time I'll shut it down to find the problem. I will never get anywhere close to 272, much less 299!

So why does BMW use 2 bar? I guess it's for the people that think the high temp warning is a 'suggestion'. Or the, "It's OK, it'll be fine til I get home" people. Another point might be that there are parts of the world that temps of 130 degrees are common, but not here. In the 20 years I've lived here the temp has never officially gotten to triple digits, well until 2 days last year, but I was in San Diego at the time so I'm not counting that.

I've got a nicely restored '71 Chev 1/2 with a mildly built 350/700R4 and I run a 7 PSI (233 degree) cap for the last 10 years, no problem. I've even towed a 4000lb boat and trailer over a mountain pass several times, no problem.

If you are looking for the expansion tank cap for your E60 I do have an extra new one I was about to return to racepages.cocm.. I have a 2004 530i with the plastic expansion tank.

How many bar is the cap?