Hi Alfie!
Sorry you're having trouble fitting your mudguards; it can be a struggle and sometimes plastic 'guards come from the package a bit distorted from shipping or storage; gentle application of a heat gun while massaging the plastic will help there. They can even be recurved to fit much smaller wheels. I successfully converted some 700C SKS 'guards to use on my 20in-wheeled recumbent with no problem.
I'll let you in on my little secret to lightning-fast, accurately-sized mudguard installations: Use spacers (later, as jags suggests, but also) *before* fitting and drilling.
Some years ago, I was pressed for time to install several bikes' worth of 'guards and knew I needed to speed things up a bit. I made a quick trip to the hardware store and bought some clear vinyl tubing, the kind used on fish aquariums. I chose the diameter needed for adequate clearance -- on the bikes in question (and this being 'Merka where metric measures were scarce at the time), this was 3/8in OD (nearly a centimeter at 9.525mm) and 1/4in OD (6.35mm). I simply used masking tape in several places to secure the tubing to the fully-inflated tire as a spacer, then used more tape strips to fasten the 'guard atop it.
The tubing held each 'guard centered and equidistant from the tire and all I had to do was measure the clearance between the 'guard and mounting points (chainstay and seatstay bridges). To do that quickly and measure the hole placement accurately, I just used a few single-, double-, or triple-chainring bolts and sleeve nuts, screwed apart as necessary to fill the gaps and serve as gauges. Placed under the bridges, a piece of tape fore and aft marked the centers for drilling and I just duplicated the standoff using 6mm nuts and washers, which nicely served as spacers on 5mm bolts.
With the mounting bolts and spacers in place, it was quick and easy to fit and cut the stays 'cos the tubing spacers removed all the guesswork. I would imagine most production mudguard fitting is done this way; it is much quicker and more accurate than fiddling with them a piece at a time, especially when dealing with multiple bikes.
Hope this helps.
Best,
Dan.
EDIT: Forgot to add: With the guards taped in place, just bolt the stays to the dropouts and mark the ends against the brackets, cut and reinstall with the supplied drawbolts and nuts for a quick, perfect job on the stay lengths.
