When thinking about a disc conversion for a pre-P largeframe, the conventional wisdom says put a PK fork in it. Which is fine if you live on the outskirts of Munich, Naples or Barcelona. For the rest of us it's generally a bit harder to find one. However, P forks are plentiful, and potentially cheap. So what's the difference? This table from a T5 forum has the dimensions:
My friend did a conversion with a PX fork and Scoot r s disc setup. It was great for braking but I never liked the too-tall look - too much air between the guard and the tyre. He did a pretty neat job creating a bulge for the top of the shock and it's mounting bracket, but it was clearly a bulge that wasn't meant to be there. Some people cut a hole, but I wanted something that looked a bit more "natural".
Eventually I got hold of a PX fork with a drum brake and everything on it (except the guard which apparently had been autographed by Manny from the Stone Roses!). According to the table above and a comparison with a spare Super fork, everything else was pretty much the same except for the lower section - 'Degree lower bearing seat to control arm pivot point' - was approximately 35mm longer than the original.
Crappy photo sorry.
So I was a bit apprehensive about cutting the fork... I went to a local automotive engineer who said if he were to do it and certify the modification it would cost me hundreds. But he also said if it was his, he wouldn't even think about not doing it... and put me on to his welder. The welder cut, sleeved and tig welded the fork by 35mm and ensured the original direction of the swing-arm was maintained. I was really pleased with his job, which he charged me $80 for.
Now that I had a shorter fork, I needed a shorter shock as well. I'd been browsing SCK one day and saw the nice YSS pro adjustables... I had a look at the others in the range and saw that the PK was exactly the same design, but 45mm shorter! And if that wasn't short enough, a Zip shock is shorter again! P/PX=255mm, PK=210mm, Zip=185mm.
If you fit this fork, you need to grind down the 'platform' that the guard sits on a bit, otherwise it will sit too high at the front. The other thing that I did was move the top of the shock mount inwards about 10mm, which means less bodywork on the guard to leave clearance.
After that I cut the hole bigger with a pattern from a P guard (thanks again Rob98801!) using the front bolt hole as the datum point. I drilled a new hole at the side. Then I beat out the rhs of the guard, making a gentle bulge along the top side section. Now there's maybe 5mm clearance from the bracket to the guard. You can tell it's a little different from one side to the other. Though with the bulging, moving the shock in and grinding back of the shock mount, I think I ended up with something that doesn't instantly look modified...except to everyone here!