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@Brommers
The standard QR seat clamp on the Brompton, which came with the main frame, is good for two reasons. Firstly, it stays in place and doesn't spin round due to the fact that there's a slot in the steel seat clamp which forms part of the main frame and a matching ridge in the QR seat clamp. Secondly, it fits and is designed to work with the steel band brazed around the seat tube.
But it's a bit boring and fugly, so I decided I'd replace it with a Hope QR seat clamp. However, the Hope seat clamp doesn't have the ridge to keep it in position, and the shaft on the QR lever is too short. So the first thing to do was to disassemble it.
In order to keep the QR lever from spinning round when it's undone, I got a spare bit of brass tube out of the offcuts bin, machined a recess in one end on the lathe which the brass bush from the Hope QR lever fits into, and milled the locating ridge on the other end. This is part way through the machining process on the lathe:
I also needed to make a longer pin for the QR lever, with 5mm threads at both ends but a 6mm diameter in the middle (to match the rear triangle release catch), so I used a long stainless 6mm bolt and turned it down to suit. I also had to machine down the Hope aluminium nut. Reassembled, it looks like this.
And in place it looks like this:
Took hours to do, of course. And the Hope sign on the QR lever is still upside down...