I tightened the cap firmly, just a hair more firmly than normal, and the next time I opened it, the inner part broke free from the outer, and required quite a bit of force to remove.

Looking at the design of the cap, I'm led to believe they have a bit of a torque spec, if you will.

Now, I certainly didn't tighten it much, just nice and firm (I don't want it too loose!). Is the expansion tank cap a special treatment item? Have yours broken apart through routine use?

I'm definitely carrying a spare cap in the car going forward, as ridiculous as that sounds.

 

Mine did the exact same thing on my XK8 and I simply removed the broken pieces, and filed the bottom of the cap, making sure the surviving O ring was in good shape, and it has been working fine.....take care...Mike