You will need to use this adapter when installing either a VNT17 or KP39 turbo on an ALH TDI. The stock discharge pipe has a smaller diameter adapter crimped to it as the turbocharger outlet is smaller on the stock turbocharger. You will be able to reuse your stock discharge pipe after removing the stock crimped adapter. This adapter is also compatible with any 1.25” ID discharge pipe. CNC machined from solid billet aluminum .