I started to think about the tank mould and quickly realised I had a problem, when making the pattern I had assumed that the filler bolted through the visible tank top surface. Stupid really as I only had to open the filler cap to see how it worked. The filler cap has no connection with the outer surface of the tank but rather is bolted to a recessed enclosure about 15mm below the surface. This would be impossible to incorporate into my tank pattern as it now sits.
I considered manufacturing a new plate to bolt to the top surface but this would have involved accurately cross drilling the surface plate to take the hinge pin for the cap itself.
I would also have to make up the neck that the cap locates into and seals against and this would have to be at the right height viz a viz the tank surface or the cap would not sit flush or more importantly seal. Looking at the cap itself there a number of critical dimensions measured from the tank surface, depth of mounting lugs, point of engagement of the filler cap in the neck, compressed depth of the sealing ring, depth of lip for engagement of locking latch. Manufacturing a new plate to hold the filler cap was not going to end well.
I therefore had to find a way of manufacturing the recess that I could bond into the tank after the top skin had been manufactured but before the tank base was also bonded to the tank top surface.
Scouring the garage for something suitable to make a mould from I came across a pot I keep all my chuck keys in which was exactly the same diameter as the filler cap plate.
I re-cycled some 3mm polypropylene from one of the belt cover moulds and I now have a reasonable facsimile of the original steel tank recess with a nice big flange to bond into the new CF tank when it gets made.
I cut the polypropylene with a jigsaw with a fine toothed metal cutting blade in. The polypropylene was melting with the passage of the blade then re-forming behind the blade so I had to go round each circle about 3 times before they parted company.