Because the stock cap has two problems. Firstly they are unreliable as they often open at much less than the 1 bar (15psi) pressure specified. We’ve tested five in a week and one opened at 7 psi another 4 PSI! Opening early lets the coolant boil at a lower temp and coolant flies out even before the fan can cut in.
This brings us to the second issue. The coolant can only come out around the cap, then all over the engine bay then all over your tyres (maybe). Antifreeze is very slippery btw. The GTT tank utilizes a reliable Japanese style cap with choice of 14, 16 19 psi opening point. The GTT tank also allows any overflow to go to a container…so no mess, or potential hazard. Tank is baffled inside to eliminate airlocks, and fits original position with no modifications. Standard finish is mirror polished alloy.
Fits: Gen 1 Cooper S (also JCW / GP1)
Powder coated version Gloss Black, Red or White. £199+vat