When I fitted Yamaha 600 forks to my 920, I wanted to keep the trail the same as the original 850 yokes, & found that late model BMW K75s yokes would fit the bill. These have the 41mm bore to suit the forks & a 2 1/4" offset, which combined with the 28 deg head angle & 19" wheel gives around 4" trail. Handles fine.
We are a bit off track in this thread - I will jump on the bandwagon.
By using forks off a 1980's BMW R65 with stanchions off a R65GS, I wanted to improve my Mk3's front end. The objectives were:
* Saving weight over the rather heavy Norton fork. Stanchion diameter is a modest 36 mm, just a tad larger than Norton's stanchions.
* Stem diameter of 25mm matches that of the Commando.
* Obtaing a wider fork provides space for a double disc hub and calipers;
* Ride height at the front remains the same, thus frame geometry is unaltered;
* Bushless fork design means better slider support and low wear,
* Springing is assisted by compressed air, resulting in a highly progressive effective spring rate. (Wilbers aftermarket springs offer an initial spring rate 6.5 N/mm. not far from that of the Commando.)
* In combination with a 18" front wheel, a trail of 103mm is achieved, which is identical to the trail of the Mk3 according to my calculatiions.