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Rob'Z sent me my first pair of over-bored Z1 TB's last week and I've only now just been able to inspect them. They left my possession as scrap metal, but many call them Z1 58mm upgraded throttle bodies. The butterfly valves stuck, the bore in each throttle body wasn't circular, and I wasn't happy with my idle. In order to prevent the sticking issue, the valve stops had to be set too aggressively causing the valves to not close fully and allowed too much air leakage. My post is not about Z1's contribution concerning this part and it's short comings, rather what Rob has been able to do with them. They have now been over-bored to 61mm. A new set of butterfly valves were cut and installed as well. After the travel stops were adjusted properly, there is no sticking, even when the valves are allowed to snap shut with the full force of the return spring. The amount of leakage when held up to the light is far less than in it's original 58mm state. My OEM's still seal tighter in that respect, but I'm much happier with the aftermarket pieces now that they've been modified and will be having my second set repaired as well. Oh, did I mention they flow more air too? Can't wait for spring to get them reinstalled. The pictures are a little unclear, but it was difficult to hold them to the light and not shake the camera while still getting the proper white balance. To show how minimal the light leakage is, I've included an in focus picture of the first throttle body. It doesn't emphasize the leak, but does a better job of illustrating how small the gap is.
Passenger 61mm (blurry)
Passenger 61mm (in focus)
Driver 61mm
OEM control
WARNING: I lack industry backing
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Warning: crappy photos of my repaired Z1 TB's - 
