I have a 3.25 19 on the front and i want to put a new tyre on the rear should you run a wider tyre on the rear ie 3.50 .19 The tyre i have on the front is a Dunlop k70
M20s were originally fitted with 3.50 rear, but from contract S.2603 onwards, the records show 3.25" fitted. This change may have occurred sooner. It was presumably an economy measure and brought them in line with other WD motorcycles.
Nortons always had the 3.25 back and front and are not noted as steering worse because of it !
Period size is as stated, 3.25 x 19 front and rear however tyres are slighty different from almost 80 years ago. Your front K70 is a very good tyre but it's bit fatter than original fittment as its of a more rounded section and although it would be fine as a rear tyre any increase in section (3.50) might reduce mudguard clearance. My preference is for a Avon Safety Mileage 3.50 x 19 which isn't as fat as the same size K70 and will last longer. Were your to go to 4.00 x 19 you are very likely to have clearance problems, some people have had no issues with this size but on my bike is just couldn't be sensibly accommodated. Then of course there's tyre pressures!
Richard
