The late model T25 / T3 / Vanagon header tank and expansion bottle system are ideal for use with a Subaru engine conversion, as long as all the components are good quality. This is important, as there is major issue with many of the unbranded aftermarket header tank caps. Note by unbranded, we mean on the cap itself – some of the dodgy caps are sold in boxes printed with the branding by of a very highly regarded aftermarket make, yet the cap inside is unbranded and can be faulty.
Make sure the cap you use is either genuine VW blue cap 025 121 321 B or its Blau aftermarket equivalent. There is a very big problem with a lot of the unbranded aftermarket caps (especially those supplied with a new header tank), and it is often misdiagnosed by those not familiar, often only after having replaced, hoses, radiators and / or heater matrices which have been killed by the misdiagnosed dodgy cap. The good caps are inexpensive – just use one rather than chance it with an unknown cap. The genuine caps say VW Audi on them, and the Blau ones say Blau in the centre: