I am building a cafe/bobber special with a '78 G5 frame and all running gear, including 40mm forks, from a '93 Cali 3. The forks have just had a total refurb by TWS Suspension. I have acquired the conversion brackets to allow me to fit 4 pot Brembos to front forks (the OE calipers that came in a jumble of boxes need total refurbishment and rather than spend money on these I would rather upgrade to 4 pots). The issue is the Brembo calipers will only accommodate disc thickness up to 5.5mm. EBC do 5 bolt stainless discs but they are 5.8mm thick. The OE discs that I have are like new but they are 6mm. The supplier of 4 pot Brembos suggested the pads could be sanded down to match the EBC or OE discs. My question is, is it advisable in the case of the EBC option to take say 0.2mm off each pad and if so should I take these to an engineering shop rather than hand sanding? Or, am I missing a trick here - plenty of other members must have faced similar issues - is there in fact a 5mm disc that fits the 5 bolt hub? I know I could just have the OE calipers refurbished and stick with the OE discs but the 4 pot option seems too good an option to pass up especially as I have the conversion brackets (I am delinking brakes and fitting 17mm front master - already fitted 12mm to rear). Any advice would be appreciated even if members wish to share their reasons for sticking with OE calipers and discs.
In the past I have had brake discs skimmed, you could always do this to reduce the thickness
I have had cast discs skimmed as well 
I would go pads route and get a couple of mm skimmed off the pads on a milling machine, i have had discs skimmed and they pulsed thru the lever afterwards due to not being perfectly matching for both sides.
Well, it seems money is not an issue for you, but just answer yourself, with the help from pen and paper, if you prefer to shorten a lifespan of disc - or of a set of pads(or two). I would go pads. If you fit four of them to the thick plywood, and sandpaper, say, 120 grade, to the other surface like table mat - what can go wrong??? Maybe your marriage:)
Hi Chris - Thanks for input. I have read previously that disc skimming has created problems for some people. I am now leaning towards the pad milling solution. My bike will be a low mileage 2nd bike so early wear on the pads is not really an issue.
Reading on various forums, some people have experienced issues with having discs skimmed. I didn’t want to spend £350 on new discs and take a chance that things might not go as planned.
I would try a dry install to make sure that the 6mm disc is a problem - it may be within installation tolerances, in which case - no problem!
If you’re using Brembo p4 calipers then a 17mm master cylinder will likely feel very wooden at the lever, with lots of power but no feel. 16mm would be better, as used on mid 1990’s Ducatis. (IIRC early 90’s models had a 15mm mc which was perfectly adequate but soon upgraded to 16mm).
Thanks for the advice - thankfully I have yet to purchase the front master cylinder!
Look for an engineering shop with a disc grinder.
This is a surface grinder with a rotary magnetic table, they will reduce your existing discs, and, or remove any wear marks and shouldn’t cost much