Wasserboxer Bug - Splitting header tanks

[Moog]

Here's a puzzle for you guys...

I have a 2.1 fuel injected WBX in my Bug. Pumps out around 130hp.

I have split 2 header tanks / expansion tanks (whatever you want to call them) in the last two weeks. The tank is a standard Mk2 Golf item with Golf cap. I put the first failure down to a dodgy expansion tank, but 2 in a row tells me not. I'm now thinking it is the cap, but the valve in the cap seems to work, and it was all 100% new when i built it all, including the engine.

I have new cap and expansion tank coming Monday (2 actually, just in case!), but any thoughts on whether there may be something else amiss. Cooling system is modelled on a Mk2 Golf. Coolant temp never got over 95-100°C both times it split. Coolant temp does get up to 103-105°C in traffic queues before the ECU cuts the fan in, but it has done this many times and hasn't split the tank. First time the tank split the temp was barely 95°C on my way home from work (5 miles). Second tank split was 150miles later, after i had it at 100-110mph for 5 mins (much to the suprise of the Corrado i shot past who then tried to tail me...). First tank (origin unknown) split above the seam alng the back, second split along the front seam (Italian made).

Way i see it, the cap would always limit the pressure, so it must be the cap. But... any other thoughts? I'm an professional engine designer of many years experience (Ricardo), so not a novice to these things - i'll admit that coolant system design is not my top subject, but i don't believe I got it that wrong.

Rich (Ricola) or anyone with waterpumpers - Any thoughts? Did you have any teething probs like this?

Ta for any help guys...

Moog

p.s. Here's a pic...