GROHE Faucet Thread?

I have a GROHE bathroom faucet that has seperate hot and cold twist nobs. The faucet has a single pipe that splits into two like an upsidedown T. Does anyone know what kind of thread comes on these?? I have tried 3/8 compression, I have tried 1/4NPT, it seems to fit better to the NPT however the thread is finer, so it still leaks!

Should have been supplied with the flex tubes that make the connection between the faucet and the spout, use what was supplied.