I don't think it would be too tight, but try the trans vent first of course . I have read the UK NOC forums and leaking countershaft seals have been repaired by Norton in the past. Will you be doing the work ? If so there may be 3 parts to look at buying. Find on the drawing of the transmission gear set Items 51 , 52 and 53. 51 - seal ring , 52 seal case , and 53 o-ring . All of these parts can be replaced after the countershaft sprocket is removed. Can you see the oil coming from the countershaft seal area ? Does it leak only after its been ridden ? Try the Trans Vent first..