possibly if you were to change all the pivot bolt, bearings and bushes back to the 125 size or alternatively bore the frame out which may lead the the bigger arm interfering with the rear of the engine but once that's done you'll have to get to work on the rear end and fit RM400 sized bearings into the 125 wheel as the 400 runs a bigger axle so technically , yes it is possible
a total woftam ('waste of f*ckin' time and money' for our vintage alien friends) but if one really wanted a 400 arm on a 125 then it's not out the realms of possibilties. Nothing is!! Is the cup half full or half empty? actually I ask the same about my head sometimes but I wonder if that's not just a total void as I don't really have the capacity to think that logically
do you have a 400C arm caps
