Sunday, 17 March 2019

Fuel Tap Leaks

Updated from 2017

Following on from the replacement fuel filter I made here, I wanted to show you how I have renovated the double plunger taps fitted to my Ariels.

The plungers fitted to these taps are a simple design with a few variations - prewar, the plungers are adjustable and of about 3/4" diameter; post war, they reduce to about 5/8" and lose their adjustability. There is the plated knob, which may be round and knurled or hexagonal; the knob has a short shaft which has a milled groove, aligning with a peg in the tap body (to stop it coming out), and this groove is usually straight, but may be milled with a dog leg such that you can turn the knob to avoid inadvertently shutting off the tap. Fitted inside the knob is a brass inner shaft, of about 5/32" diameter for most of its length which is provided with a short section of larger diameter, to fit inside the tap body and to carry the cork.

Lastly there is the cork - a cylinder shape fitting tightly inside the tap body and around the inner shaft.

Here's a selection. These are all post war (so about 5/8" diameter; the hex is 0.600" across flats), and came to me from John Mitchell of the AOMCC.


You'll notice how some of these plungers have a straight slot for the retaining screw, and some have a slot with a 90 degree bend. These are the 'turn to lock' type, which are later than the ones with the straight slot:


Now, these taps are pretty reliable if you know what you are doing when you set them up. The corks wear, and they dry out leading to leakage but if you keep them in good condition they work well. If you have a leaky tap, first consider what you have been doing with the bike. If for example you drained the tank over winter, or the bike has been out of service for a while, or you just bought the tap off eBay try letting the plunger soak in a puddle of petrol for a while, or immerse it in a pot of boiling water for a few minutes to rejuvenate the cork. This approach has restored many taps to working order.

If you have to replace the corks, you need to remove the inner shaft using a punch - a 3 mm or 1/8" punch will be fine. Locate the mushroomed end of the inner shaft and place the knob, shaft down, in the jaws of a vice.


Tap the mushroomed end of the inner shaft with your punch and drive it out


You should have the plunger in two pieces:


Replace the cork. Check that the new cork has a hole that is on-centre all the way through, and that the ends are square - a lot of them are poor quality. Check that it is solid and does not have any dark grooves in the cork that are going to cause leaks.

Check also that it is the right length. The short length of MIG wire in the picture below has two bends in it - one to indicate the open end of the tap and one to indicate the location of the fuel port drilled through the tap - the port that the cork will shut off. You can see that the short cork fitted to the tap has no hope of covering up the holes, but the longer new cork will cover the holes nicely.


Reassembly is simple - fit the new cork over the inner shaft and press the tap together in the vice. Wet the cork in petrol and try it in a tap - it should slide but be quite stiff. Up end the plunger again, and use your punch with a hammer to slightly swell the small end of the inner shaft, retaining it in the tap.

Prewar, the tap plungers were made so that you could adjust the corks and the reassembly sequence is a bit different. This is a prewar plunger, though not in it's own tap; the inner shaft has a 5/32" BSW thread (yes, really) retaining it in the knob, and the hex is about 3/4" across flats.


The shaft has a screwdriver slot and a lock nut, so when you slacken the lock nut you can turn the shaft and knob independently and compress the cork, improving the seal. The lock nuts are often missing, but early Meccano nuts are identical and readily available from specialists; they are a 5/32 BSW thread, but the hex is designed (or maybe just fits) 4 BA spanners.

The shafts are sometimes too short - the screwdriver slot is easily broken if you try to adjust it without removing the locknut. In this case, the only option is to replace the shaft or the whole plunger. Replacement shafts are not available but can be made on the lathe, as I show in this post.

There is a note of caution here though. If the locknut is absent, the thread damaged or just not tight, my W/NG is perfectly capable of loosening the tap cork after a fast run. The result of this discrete self-destruction is a strong smell of petrol in the garage and a drip tray curiously full... The moral of the tale of course, like most things with old bikes, is that nothing is 'fit and forget'.

If after replacing or adjusting your plungers, your tap is still leaking there maybe excessive wear or damage to the tap bore; you may have had trouble adjusting the cork to get it to slide evenly. It's a good idea to remove both plungers and inspect the bore - if you suspect it is worn, run an adjustable reamer through it to skim the surface and restore a consistent diameter through the tap. The bore is normally about 11/32" ID - I use an 11/32" - 3/8" adjustable reamer. Your aim is to remove the minimum amount of material, but to true up the surface:


When you have passed it through, look and see if you have cleaned the whole surface of the bore:


But what if you have no plungers? If you have a lathe you can make them quite easily, as I show in this post. Indian-made plungers are readily available on eBay and the ones I have seen use long M4 screws for the inner shaft. These are again of highly variable quality - the ones I have are plated very well, and look good - but of the five I have, only three have the inner shaft hole in the centre and the corks are way too short...


Usefully though, they can also be made adjustable. I made this one using a long brass M4 screw, with a slotted head filed flat and an M4 lock nut.


Once refurbished, these taps are very reliable. Attach them to the tank with a Dowty sealing washer:


Here's a dual plunger tap beloved of Ariel, fitted with two pre-war (so large size), adjustable plungers.






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