Basically what you have to do is remove the pivot first. Then you remove the welded on piece on the rear brake pedal. Then you have to either need a scrap of metal to cut to shape and weld in place or you need an st90 or similar brake pedal. Then you cut the piece off the donor pedal and weld it to the atc90 brake pedal. You can't really take a pedal off another machine and just bolt it on as the atc90 brake pedal has certain bends the other brake pedals don't. You may have to move the new welded on piece around once or twice so only tack weld the piece into place and get it situated where it works best for you. If you don't get it in exactly the right place then you will need a piece of metric "all thread" and bend it so it works with your new pedal. That should pretty much do it. I hope that makes sense for you.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
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A+ Inc Suspension 70
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Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90