fuel cap silverline boats model engineering mikes-models

One of my other pastimes involves the ownership of half a fishing boat with
my younger (he looks a lot older though!) brother Stephen. It is moored in a
marina on the lovely river Dart in Dartmouth, Devon. From here we fish the
local area which, we are fortunate enough, has close access to the famous
Skerries bank amongst literally hundreds of wrecks with 10 miles of shore,
thanks to two world wars! It is a sobering thought every time we fish a
wreck, to think of the tragedy that befell it. The climate around the south
Devon area is very mild, indeed I have a lovely palm trees in the front of my
garden! Anyway back to the project! We had the misfortune to loose the
fuel filler cap overboard when filling from the fuel barge on the river Dart just
after we bought the boat new. Because Diesel and salt water (or any water
for that matter) don't mix and whilst we support the RNLI we didn't want to
call them out 10 miles off shore to rescue a couple of dopes. So we covered
the filler as best we could and gently motored to the nearest marina. There,
we found that we could not just buy a replacement cap but had to have the
aluminium filler tube as well!

Since starting the Mills 1.3cc diesel engine I needed to learn how to cut
threads using the lathe, but at different diameters than found with taps and
dies. Having 'had a go' I thought it would be useful if we had a 'spare' filler
cap on board. Why not enhance my skills or experience levels with making
a new cap? I had the female threaded aluminium filler pipe (photo 1) so set
about trying to make the replacement. The first problem was to see how
many threads per inch I needed as well as the outside diameter of the new
cap.

Having never used a thread gauge before this was the first new skill to
master. It seemed easy enough to find the right profile, but what did 11 5/8
g mean? Well I guessed the 11 could be threads per inch (tpi) but how to be
sure? Easy! Just draw around the outline and extend these to coincide with
the length of one inch. Took seconds to write but about 20 minutes to think
of it! Sure enough there were 11 tpi. What the 5/8 g stands for I don't know
but I am sure some helpful soul out there will enlighten me, then I will pass
the information on to you.......

So I know knew the tpi and measured the internal diameter easy enough but
thought I would have a go at making one in plastic first, and who knows if it
turned out ok then that could be a spare.....

Using the Myford ML10 I looked at the chart inside the gear cover and saw
that I needed to have the following gears.................Once changed (using a
piece of paper between the sets to ensure the right clearance) I was really
surprised at the speed the cross slide moved! All right if you know these
things but it certainly needed very quick reactions, something I have a little
trouble with sometimes...another story...

Placing a bar of some form of plastic in the 3 jaw chuck I thought I had
better support it using a revolving steady in the tailstock. There I practised
away a couple of times until I felt ready to have a go for real. I do not have
a thread dial indicator on my little Myford so reverse the motor to drive the
carriage way back to the start, allowing the next cut to be made.....every so
often checking the turned nylon for fit in the aluminium filler tube. I cut the
slot or grove in the top by using an involute gear cutter I have. May not be
the best use of the tool but it 'worked' for me.

This nylon 'practice' piece will now be taken down to the boat for
emergencies and I will now have a go at making a nicer cap either in brass,
aluminium or if the bar I have is big enough, stainless steel. However, any
metal cap I produce will have its centre hollowed out to make it lighter. I
need to do this because it will sink easy enough when Stephen drops it
without helping it go to the bottom!

Please let me know if you want any more detail regarding the manufacture of
the cap. I am just pleased to have completed another 'unexpected' project
with some success. It will all end in tears no doubt but at the moment this
little 'one offs' seem top be working.