What can you tell me about caring for my fuel filler caps? Fuel cap repair? Fuel cap O-rings?
(Search strings: gas cap. gas cap repair. filler cap repair)
Technical Editor:
Cap wear is seldom limited to one portion of the cap. Most wear occurs in five places:
– The center axle, including corrosion
– The axle bore in the upper body, due to axle corrosion and lack of lubrication
– The lower pivot corners on the lift tab, due to lack of lubrication
– The cap surface where the lift tab pivots up and down, due to lack of lubrication
– The locking ears on the lower body, due to angularity during rotation because of a loose bore, or over-tightening
While not directly related to cap wear, surface corrosion on the adapter ring will make it impossible for the outer O-ring to seal, will shorten the life of the O-ring, and will make it impossible to stop leaks. Prevention of adapter ring corrosion is easy, repair is difficult, and replacement is very expensive (and messy).
Lift tab wear, on the tab itself and the mating surface of the upper body, is what causes the tab to remain partway sticking up, rather than lying flat. Parker will not sell small parts for their caps; and there is no approved fuel cap overhauler anywhere, that I have ever found. If you stretch the inner spring in an effort to get the tab to lie flat, it won’t work. You’ll just generate more wear on the bottom of the tab.
There are several reasons why the tab won’t lie flat. Wear in the cap bore, wear and rust that have reduced the effective diameter of the tab axle (center pin/shaft), and wear on the underside of the tab where it abuts the cap. The tab relies on going ‘over-center’ in order to lie flat. With the cap disassembled and cleaned, and the axle well-polished, you can put the tab-axle into the cap bore, then pull down on the axle threads (while holding the cap body). If the bevel on the underside of the cap is such that the center-line of pull (through the roll pin) falls toward the main edge (lifting edge) of the tab, pulling will tend to pull the tab flat. But if axle wear and bore wear are allowing the axle to cant sideways under spring pressure; or if the pivot area on the underside of the cap has worn to the point that any pulling force is now trying to unlock (lift) the tab rather than holding it locked, raising spring pressure will only make the tab stick up more. The only effective solution, if the axle and bore wear are not excessive, is to re-file the flat on the underside of the tab, to shift the pivot point back past the pin centerline (toward the short end of the tab). This takes a fair bit of precision filing (initially with a coarse file), with lubricant to keep the file teeth from clogging. If the axle can wobble around in its cap bore, it is unlikely you can shift the pulling angle far enough (via filing) to overcome the wear. And tightening up the spring beyond what is needed will just make things worse (and will accelerate tab wear).
There are two things that ruin these caps. The first is rust on the center axle, which kills the inner o-ring and grinds out the cap bore. The other is over-tightening the axle in a fruitless attempt to stop water leaks, instead of replacing the inner and outer o-rings. On an outside plane, the outer ring needs replacing every year, unless it is kept coated with LPS-2 after each refueling. The center axle should be sprayed with LPS-2 after each refueling anyway. If you put a squirt on the outer cap perimeter as well, it will migrate around the ring; and will protect both the o-ring and the filler neck adapter surface from weathering and rusting.
The cap tension should be adjusted such that it requires only slight effort to pop up the tab, using the tips of two fingers. If it has to be forcefully pulled up, or even pried up, it is adjusted much too tight. That will do nothing to halt leaks through fissured or worn o-rings; and it will rapidly wear out the cap.
Axle corrosion and bore wear chews up the center -010 O-ring, and allows the axle to rock in the bore. When the axle rocks in the bore, the lower body will not remain square to the upper body, while being locked and unlocked. The result is chewed up locking lugs on the lower body, along with operator frustration from being difficult to get positioned and locked.
Over-tightening due to continued use with a bad bore and bad inner and outer O-rings, in a futile effort to stop leaks, makes everything get worse even faster.
At the risk of repetition, there are four preventive measures that should be taken to extend cap life and aid easy operation (assuming that it isn’t too late, and the cap is still good):
– Keep a small squirt of LPS-2, Boeshield T-9, or LPS-3 spray on the center axle and the lift tab rubbing area, from the top. Also put a brief squirt on the junction of the outer cap O-ring and the cap adapter ring. It is best to squirt these spots after every refueling (at a minimum). After squirting on the spray, remove the cap, spread the oil film around on the adapter ring a bit, and reinstall the cap. Do not use common light lubes such as LPS-1 or WD-40; they won’t work well enough. Don’t use heavy oils or more common (cheaper) lube sprays, as they will probably contain solvents that will attack the rubber o-ring. The Boeing Boeshield T-9 is probably the best choice (developed by Boeing), but is also the most expensive and can be harder to find. If it cannot be found at a local Marine Supply, it is available from Aircraft Spruce as Boeshield T9, PN 09-01623, $11.89 for a 12-ounce spray can.
– Make sure that the adapter ring flange is kept parallel at the edge of each of the three cut-outs. If the edge of a cut-out has even a slight curl up or down, due to cap mis-positioning during removal or reinstallation, it will adversely affect cap operation and locking lug wear. You can use small offset-jaw channel-lock or Knipix pliers to bend the edges back straight.
– Make sure that the O-rings are kept fresh, pliable and leak-free, , by replacing both inner and outer rings every two years or so; more often if the plane is tied down outside and the cap is not being lubed. Lubed cap o-rings on a hangared plane may well last longer than two years.
And you thought that filler caps were simple! If you follow these practices, the caps and adapter rings will have an indefinite life. List price is now upwards of $600. Which is why it pays to take care of them.