There's a jet in there?
I did not know until now that there's a jet in most (all?) group heads.
Enlighten me - why is it necessary? What does it do? What difference does changing the size make? How do you determine the right size to use? How many different types are there?
I can't speak for all groups, but in the e61 the jet serves to slowly let water through into the pre-infusion chamber. The nett flow needed through the puck is not huge, so that water path through the 0.6,7,8 whatever hole is quite sufficient.. You basically don't want a vast volume of high pressure water to blast down onto your precious coffee puck, so some kind of flow restriction is very desireable. Think of filling your glass from a kitchen tap compared to a 64mm fire hose
There are also jets in the thermosyphon loops, to control the rate of heat exchange. Like most things coffee machine, they aren't called jets but Gilceurs (the french word for - jet!).
Does that help?
Pretty well most Group designs use a jet for both the thermosyphon (if applicable) and to limit the flow rate of brew water through the Group.
This is especially important in machines that have a Rotary Pump fitted, because if left unrestricted, the rate of flow will be the maximum that the pump can deliver - Think upwards of 50L/Min....