The way of Saviour Machine

This is an example of what I do.  It is a petrol or gas tank cap from a BMW R65 from the mid-eighties in need of some attention.  Caps like these and variants thereof are fitted to many eighties BMW motorcycles.  They have a problematic design flaw that afflicts every one of these caps sooner or later if they are used in the way that they were intended, i.e. used without having to worry whether it is going to break every time it is closed.  The problem is the ratchet mechanism that activates when the cap is tightened to the required degree and prevents over-tightening.  The ratchet mechanism fails, the cap cannot be undone and remains locked in place.

There are a few possible solutions once yours fails.  The first is to buy another, but they are well over a hundred quid new.  There are no aftermarket reproductions available (*note to self - design and engineer cheap aftermarket BMW airhead twin gas cap*) so I don't fancy that.  You can drill a hole in the top of the cap and insert a screw or bolt so that it permanently connects the upper and lower part of the mechanism, but it looks daft with a screw sticking out of the top.

So I've decided to do away with the lock mechanism altogether, join the upper and lower halves so it operates as a basic screw cap and then re-finish it.   First you need to fill the hole where the lock mechanism has been removed.  I then bolted the top half to the bottom half with a 45mm long bolt so it is effectively one complete unit.  The bolt needs to be recessed slightly into the top of the cap so that filler can be layered on top.