Old British motorcycles have fasteners built to the inscrutable Whitworth system. Metric isn't as perfect as it might seem. German metric is different from Japanese metric. For example large diameter Japanese fasteners tend to be finer thread than their German counterparts. The only use I've found for my 18mm wrench is on a BMW. You might as well throw the 16mm and the 18mm away. Craftsman's old combination wrench set came with a useless 16mm but no (useful) 17mm. Ten bucks buys you a 17mm. Don't even get me started on bicycles. English bikes used to be their own thing, "cycle thread". Then you had French metric, Italian metric and Japanese metric. Don't try to mix Campagnolo and Shimano bicycle parts.