Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board gas tap washer/seal

So, my 1971 T100R parts manual, shows the gas tap steel washer against the tank, and then the sealing washer, my 69 T120R parts manual shows the gas tap sealing washer against the tank and then the steel washer, I assume that its the sealing washer on first against the tank is correct?

Steve

Yes. I also put a small wrap of PTFE tape (yellow - petrol-safe) on the petcock threads before installing. Between the tape and sealing washer, I've never had to worry about a leak.

Steve

I have that tape here, so, its tape then the sealing washer.. thx for the heads up

I've gone to statoseal washers. They [so far] seem to have done a better job.

Hi Mark,


Uh-uh, the '71 T100 parts list has a series of misprints, not only the "Ref. No.'s" corresponding to the drawing but also the part numbers of the "Petrol tap seal", "Steel washer" and "Petrol tap nut".


Correct, the '69 650 parts manual also has the correct part numbers.

Note the "E7351 Petrol tap seal" are the Stat-O-Seal washers posted by "tridentt150v". Like him, I haven't had to use PTFE tape on tap threads.

Hth.

Regards,

Thanks for all the info guys, my sealing washers still look good, taps are functional, so I'll put them in as per the T120R manual, no tape, sealing washers first then steel washer

Hi Guys, Interesting about Teflon. I had to use Teflon on my stato seals that I got on ebay. They leaked instantly after install. I used several wraps of Teflon tape, which held perfectly for about 4 years. I then removed taps to repair leak in tank with Caswell tank liner.

I then got some from Rabers. They worked good for about 4 months or so, then started seeping. I'd prior seen better results from Raber's stato seal on other bikes.

I had bought 2 pairs from Raber's. Installed 2nd pair with Teflon tape. So far no leaks.

In the mean time I did some research. The original Stato seal had a different (smaller) ID on the rubber membrane as modern stato seal offered to us in USA.

So far as I can tell nobody world wide replicates the original. Back in the 70s I did lots of stato seal to perfect results. Now days replacements tend to leak.

I contacted Bay Seal Company Haward, California & talked to their engineer. He suggested what is being sold as stato seal is not for threads, but for under a bolt head with a smooth shank. That is really more like a Dowty washer. They are not too far so I rode over & he gave some samples to me.

Parker Seal makes a viton seal that has a smaller ID which grabs the threads much better. This seal is very California fuel & heat resistant 400f. Visually this looks like what we buy now, but smaller ID of rubber & thicker rubber overall. Not really recommended for threads. It looks like it might work good.

They also make a seal specifically for threads like our fuel taps have. This one is Buna N rubber. It has a much lower working heat range, but is still approaching 250f. He said it is very California gas resistant. This one has a thin triangular shaped membrane that is supposed to seal well. Looks like it will work good also.

They visually look like the normal stato seal we buy, but in addition they have a thin triangular shaped membrane extending into the ID of seal. Apparently this helps center washer on threads & the membrane follows threads such it seals better.

These seals are Parker brand & actually made for 1/2-13 thread bolts. However 1/2-13 has similar minor diameter as our fuel taps.

As I said these fit much more tightly around threads. This is with my trial fitting on a spare fuel tap compared to what is currently sold as stato seals.

Very carefully inspecting leaks of my old seals they were dry against tank & washer between seal & nut. The leaks were by threads 100%.

The original Stato seals from the 70s had a thin membrane extending from the thicker rubber ring bonded to the steel washer. These just worked & never leaked. We never gave it a thought. Just installed & was good.

I've not yet installed either of the Parker brand seals on my bike yet for testing.

I'm planning a 900 mile ride in a few months & don't have time to deal with stato seal experiments until after that. I have other things to get replaced & serviced for the ride such as tires, chain, trans sprocket oil leak & some carb mixture work. Plus new gaskets to make for air filter covers.

There are some UK suppliers of the stato type seals with the triangular membrane I've seen on line. Would be interesting to see what they have to say. They show sizes to fit BSP sizes. I didn't follow up on UK suppliers.

What I've done that seems to work well with Teflon tape is install nut all the way down on tap. Of course depending on thickness of tape, I put a good 5 wraps of tape from end of threads down to nut. Then install flat washer down to nut, then stato seal down to flat washer. Then add more wraps of tape. About 5 more. Just to where it will thread into bung & not bind up tape too much.

Then screw in tap until it lines up where I want it. Then tighten lock nut upwards against flat washer until it's tight.

That has been how I've done it on a few bikes & it worked good. Held good for 4.5 years before the tank reseal.

Just recently a friend had his stato seal spontaneously start leaking after a cold snap. These were the common seals we get from ebay or parts dealers in USA. Teflon wrapped as above fixed his too.

Just sucks seals that don't leak are not readily supplied.
Don

Hi Don,


I suspect the operative phrase here is "from eBay" ...

Risking stating the obvious, there are different Stat-O-Seals for, say, 1/4"BSP, M12 and M14, and possibly M13. An educated guess says you probably fit them to the taps a similar way to me - screwed on with a little lube, not pressed on without. Given the major diameter of 1/4"BSP, with the best will in the world, M14 still isn't going to seal, no matter what the eBay seller says they fit.

Hth.

Regards,

Hi Stuart, Thanks for the reply.

That's the thing, who knows what you are actually getting?

The Ebay seals are from the more prominent suppliers of Triumph parts that have brick & motor stores, plus workshops with good reputations.

What are the shops getting from their suppliers?

Another case in point is outer seal in primary case for alternator wire. These are the '69 & newer type. Sometimes you get the correct inside diameter hole, sometimes way too big. Wire flops about in hole.
Don

It has long been a problem to seal threads from gasoline, as gas is a solvent and will remove almost anything. I have found that genuine Hylomar M will work on petcock threads no matter what washers are used. It is a polyester urethane which can not be affected by almost any fluids (except acetone). I can say with certainty there is no other product like it. Hylomar remains flexible so vibration and temperature have no effect on its sealing capability. It's also great on drain plugs.
Hylomar is still made in Wigan, England under license from Rolls Royce and when used correctly is an amazing tool. My shop is the Southern California distributor for Hylomar and provides it to racers, engine shops, and for industrial applications.

Stagg also works, but its annoying to clean up if you ever have to disassemble. I'm [strangely] with Stuart on this one, all I use is a statoseal washer, no sealer or thread tape etc. We have an excellent Hydraulic shop in my town [for the timber and earthmoving industry] and they supply statoseal washers in a very wide variety of sizes. Kubota, Komatsu, Volvo [which can be a mix] and smaller CAT stuff etc is either Japanese or European so metric is generally needed. Larger CAT, Tigercat, John Deere, International and other stuff is made in the USA and is imperial....so the shop caters for both camps.

I can't stop the old ex plumber from within me taking over & using PTFE tape on any and all things threaded ,
Be it steam air oil gas liquid hot cold dry wet or maybe even just lukewarm ? ..
in my opinion a couple good winds of PTFE tape will leave all those fancy gimmick rubber / metal composite washers behind for total dead !!!

Gimmick washers are like having to wearing beach thongs to play football in because you can't lace up a pair of boots

Must admit that I have never had sealing problems when using the correct size Stat-O-Seal and compression washer.
You have to make sure that the gas tank surface is smooth and clean--no paint unevenness etc but if you take common sense precautions it seals good.
However all that pales into insignificance when compared with the mental image of Bodie playing football wearing nothing but boots and thongs!

Hi Richard,

You don't have any need to fear for your mental state, "thongs" is Oz for what you and I know as "flip-flops", not small pieces of nether underwear ...

Regards,

Oh--jeez--I guess I am not too good at languages.
Brought up in UK so I mastered English English --eventually.
Then I moved to US and after 17 years I am reasonably proficient in American English.
Now you tell me there is a separate Oz English.
Over the course of my life to date I have been to over 120 different countries and managed OK by just speaking English English.
If the other guy didn't understand then I spoke English English in a louder voice and everything worked out fine.
Think I will go back to that method.
Message to Bodie " I say, old boy, do you happen to know if your flip flops fit inside your boots rather well?"

Haaaa ! ..
I should have known i was living dangerous on a world wide forum with that comment , should have called em Flip-flops instead

It's no wonder now is it why i got banned from ever joining any pole dancing classes ?

Hi Richard,


Ime, Oz is more a subset of English, they adapt one word to different uses or meanings ...

... On one of my earliest visits, an Australian docker advised that a piece of equipment had stopped working, "S'cuse me, Mr. Mate, the f**kin' f**ker's f**ked."

... Or the common Oz phrase, "She'll be right" can refer to anything, not just of the female persuasion ...

Stat-O-Seals or Dowdy washers work on a smooth shaft. They are not going to seal against threads unless you use a new Nyloc nut to seal the thread.

Both Stat-O-Seals and Dowty washers were designed to seal two faces.
Certainly Dowty seals had their origins in the Dowty Company in UK sealing the hydraullcs on aircraft undercarriages.
However if you have one of these seals in which the rubber part is a close fit on the threads then by compressing the seal the rubber part of the seal is extruded into the threads--thereby sealing them.
If you think about it in our gas tap situation you have to seal three things:
a) The face on the gas tank.
b) The face on the compression washer.
c) The threads on the tap.
If only a) and b) are sealed there is no reason why the gas will not leak down the threads.
Just my two cents worth of course.

And the use of the word 'bugger' in its various subtle forms......gotta love it .................its English, but taken to another level [note I didn't say if it was higher or lower].

And btw, I didn't swear....in NZ and Oz bugger is used in prime time adverts on TV in one of its numerous and various forms.

I totally agree with you ,

Works pretty much along the well known principle to aid sealing "that leaky" gearbox sprocket on a T150 or Rocket 3 ..
You can fit all the correct purpose made 'o' rings in world on it if you so wish ? . "But" .. Unless you back it up with a good smear of silicon to seal the threads between the mainshaft & sprocket nut ~> IT WILL LEAK OIL <~ Horses for courses , seals n 'o' rings work well on flanges and other smooth surfaces but not threads ...

My personal take on it is ; if the Dowty washers are working for you then it's more by luck than any real design - maybe the crap fuel we get these days has swollen the seal to fill in the thread gaps ? who knows , For me it's PTFE tape - i'll target & seal the thread every time and just use the locknut & washer to secure the tap into it's final position.

On the gearbox sprocket, sealer should be put on the flange that sits against the face of the high gear. That way oil does not get to the splines and threads.

Hydraulic stuff has those type seals as on my Kubota tractor..........I have been using PTFE "teflon" white paste in the tube for many years with no washers and no leaks....The washers work nicely but I never have them when needed.....It's not like I think ahead to have them in stock...

Don, you often mention the "different" California gas....What exactly is in it that makes it different than the typical unleaded 10 percent ethanol used in other states? I'm sure the evaporation values are different, but what about various chemicals or solvents?

We have found the cheap Taiwan petcocks often leak past the threads as the threaded portion is often too small. They are really loose fit in the bung of the tank. The English made ones fit much better. You must also have a flat clean surface for seals to fit against, With painters putting on many coats of paint and clear coat it is often a source of a leak and the beginnings of the paint peeling up.
I can not think of one place on a Triumph motorcycle where we use any sort of Teflon tape.

Hi David,


With respect, simple observation says this isn't correct for Stat-O-Seals:-

. You've posted in the past that you can tell the difference between the 55-degree and 60-degree angles on threads. I can't but a correctly-sized Stat-O-Seal should be screwed on to the thread, because I can see the "extruded into the threads" described by Richard.

. Then, when the Stat-O-Seal is compressed by the plain steel washer and tap locknut, where's the seal going to extrude to, being constrained by the steel washer, the tank and the steel or ally of the washer surrounding the seal itself?

I've seven fuel taps that standard Triumph parts have sealed at least for well over a decade and a couple exceeding four. The vast majority of Triumph owners I know and have known would be surprised their fuel taps don't seal using standard Triumph parts. Certainly I've never had to go looking for a 1/4"BSP Nyloc nut, even if such a thing exists?

Perhaps it's as "RPM" has suggested, the OD of many taps sold in the US is too-much smaller than the 1/4"BSP major and minor diameters? Perhaps dealers are supplying correct 1/4"BSP Stat-O-Seals but these taps could do with M13 or M12 Stat-O-Seals?

Hth.

Regards,

Hi, Parker makes a seal specifically for threads such as our fuel taps are. It's called thredseal. 1/2-13 size is very close to 1/4-19 BSP of our taps. It must be wound on.

The also make a stato seal with a thicker rubber, meaning the ID of the rubber is smaller so it presses much tighter on threads. It indeed must be wound onto tap. This one is not obviously listed in catalog.

I have 2 samples of each in my possession, as well as the regular stato seal the Triumph. parts places in USA sell.

Here's a link to the Thredseal page.

https://www.industrialseal.com/threadseals-parker.html


Here is another type that is supposed to seal threads. This one looks much more like the original Triumph to me. But never seen one in real life.
Available in either viton or Buna. BSP sizes.

https://www.discounthydraulichose.com/Bonded_Seal_Buna_BSP_Metric_p/9500.htm

Don

Hi Hillbilly, I really don't know the exact differences in California gas.

Unless you go to race gas station which is still unleaded, the highest octane is 91.

We also have winter & summer fuel. The switchover dates are set by the regulators of emissions & it is set by the calendar, not ambient temperature day to day or month to month. It's on this day the fuel from refinery changes. So with a small selling station the fuel could be way out of season. The way the bikes start is different with each fuel. I tend to find in a warm snap, winter fuel starts harder. In stop & go traffic 105f the fuel can boil in float bowl & won't idle. You have to keep blipping throttle to keep it running. Takes only a few minutes of 20mph to cool carb, then ok. Hot starts often require a good 5 second tickle, then hold full throttle.

In many states the octane is 93.

I have no idea the why we have only 91. So far as I can tell we have a minimum of 10% ethanol in fuel. I have never seen non ethanol unleaded fuel sold in CA.

I know from personal experience with my Mercedes 2000 E320 I reliably get 2-3 more miles per gallon with 93 octane. If I go from 91 to 87 California fuel I will loose 2-3 miles per gallon. The owners manual states use premium fuel.

The runs looses noticeable power up hill 87 to 91. Oddly 91 to 93 power seems the same, but mileage is consistently better with 93.

Looking at actual values in laptop the knock sensors backs off timing about 14deg with 87 compared to 91. The motor sounds different & feels like it's straining with timing backed off. I don't know what timing does with 93.

I've never ridden my bike with 93 so I have no idea how it would start & run on 93. From my car experience I'd say better.

Again I don't know why we don't have 93 & what does out of state 93 have in it that gives better mileage? Obviously it has more BTU or something per gallon, but what?

I'm 66. Back when I was 10 years old or so the smog was so bad in Los Angles to Pasadena it looked like it was a foggy day. Except the fog had a yellow brown tint to it. Burned your eyes, nose, throat. You could not see road signs 1/8 mile away. This was not uncommon or usual back then. Now it's rather rare to have smog like that there. So whatever the gas has in it as well as the emission controls it's working pretty good. The thing is our old bikes don't work so well on it.

Don

Hi Don,


Nope, 'fraid nothing like it at all, because that's a Dowty.

If you look closely at any Dowty, the cross-section of the seal itself is basically C-shaped, with the ends of the "C" forming two tiny lips; in the original designed use as a seal on the telescoping legs of aircraft undercarriages, the lips seal on the inner shaft as inner and outer move relative to one another.

If a Dowty seals on a thread, that's luck, because the seal is barely thicker than the washer itself; there is virtually nothing that can be compressed/deformed/extruded if a Dowty is between, say, a tank and the steel washer on a standard lever fuel tap.


Images of Stat-O-Seals have been posted and linked in threads numerous times, both here and in other Britbike internet forums. The Triumph part number for the Stat-O-Seal washer is 70-7351, called "Tap seal" in the parts book for your '73 TR7RV. Enter that part number plus "triumph" into your preferred internet search engine and any number of images will be displayed; e.g. here they are displayed on The Bonneville Shop's website with the 83-0002 plated steel "Washer[s]" that fit between the Stat-O-Seals and the tap locknuts.

Two other things to note in the TBS image:-

. The correct 83-0002 plain Washers are the same 7/8" OD as the 70-7351 "Tap seal" Stat-O-Seal washers; afaict, the plain Washers were/are sourced specifically for this feature and application as standard 1/2" i.d. washers are 1" o.d. Any "Dowty" washer fitting the 1/4"BSP tap thread would have an obviously-smaller o.d. than the plain Washers.

. The apparent 'star shape' of the outside edge of the black seal itself is specifically characteristic of a Stat-O-Seal. Dowty washers are also known as "bonded washers" because the seal is bonded to the metal of the washer; otoh, Stat-O-Seal seals are not bonded to the metal of the washer, careless fitting or removal can cause the seal to detach from the serrated i.d. of the metal.

One other feature of a Stat-O-Seal not visible in that image, but visible in earlier linked images is the seal is relatively much thicker than the metal; otoh, a Dowty seal is pretty-much the same thickness as the metal - why I posted above, "there is virtually nothing that can be compressed/deformed/extruded if a Dowty is between, say, a tank and the steel washer on a standard lever fuel tap". Therefore, one useful specific property of a Stat-O-Seal is the metal part will not scrape tank paint when the tap nut is tightened; :bigt otoh, there isn't anything to stop a Dowty doing so.

Hth.

Regards,

On vehicles with modern electronic engine management and detonation sensors,the timing can be advanced on the higher octane...This is especially where a premium fuel is recommended.If the ECU retards timing for lower octane ,then power will fall off a bit, usually noticeable....Generally speaking,more advance give better fuel mileage at part throttle.....
You have a portable scanner that displays fuel trims,timing advance, etc? Plug it in and drive around for awhile to observe what goes on...

So, back to the original question, what is the best seal, best way, best of whatever to install the fuel taps...LOL

I suspect the use of Stat-O-Seals in this application was another of those later “what can we find that will just about do the job” solutions.

Quite why the relationship between tank boss and tap changed (most through history just fitted by winding in tightly, with a fibre sealing washer, and you could get the correct orientation of the tap by different tightening and/or different washer thickness).

For whatever reason (perhaps they didn’t want to waste time orienting the boss correctly when welding the tanks?) this adjustable system was employed, with locknut, washer and seal. Which introduces the thread sealing issue, which never existed before the apparent need for adjustable fuel taps.

Stat-O-Seals are not designed for thread sealing. They are essentially similar to Dowty washers, in that they seal surface to surface (eg bolt head to banjo in hydraulics). That the compression of the rubber may close-up on the (usually) smooth bolt shank is not important for its function. They are in effect, captive “O rings”, without the need for machined recesses for the rings.

Such seals have been used in hydraulics for many years, at 10’s of thousands of PSI, but never to seal threads, and I don’t know why we keep talking about aircraft undercarriages, that is but one application, and not in terms of sliding piston situation, an entirely different seal requirement.

If a Stat-O-Seal works in the petrol tap scenario, it does so by bodge, not design.

As has been described, it must be so undersized that it must be threaded on, and thus already half filling the threads, so that compression completes the filling of the threads (hopefully).

The star shape of this seal is more than just a star shaped rubber in a star shaped washer, the rubber is crimped between the metal layers to hold it securely in place (possibly the reason for its name?). One of its principal benefits is that it is less likely to come apart than a merely bonded seal.

If TR7’s suggestion of Thredseals had been available in the day, I think they would have been a proper solution, and maybe nowadays, if available in small numbers.

Dowty or Stat-O-Seals were never intended to be screwed onto their Fasteners, what a tedious production step that would be.

Yet I wouldn’t argue with the experts here that Triumph used them for this purpose regardless!

If something does the job it is not a bodge but a proper engineering solution.
Engineering is the art of making things work.
Just my two cents worth of course.

Hmmmmm? ,
Well with 45yrs + of sealing up boilers / gas & heating pipes and all thing else wet 'n' screwed .. i know what i'd be using
Looks like the perfect opportunity to prove how good those Doubty washers are and let us all know ? :bigt

Hi Dave,


You are confused. Whatever you're looking at, or have looked at in the past, it isn't/wasn't a Stat-O-Seal. Nothing on a Stat-O-Seal is "crimped", there aren't any multiple "layers".


But, if you're confused about what a real proper Stat-O-Seal washer looks like, you can't appreciate all the functions it performs in the position used by Triumph and BSA. Details of all those functions have been posted but you choose to ignore those descriptions?


:bigt


This is nonsense. The thread doesn't start in the same place on every tap so how would "orienting the boss correctly when welding the tanks" work?


This is nonsense. What do you suppose the fibre washer on the 82-1717 tap does?




Because, if you had read what had been posted previously, you would know that the "Dowty" seal is so called because it was invented by the company of that name when the company was developing telescoping aircraft landing gear suspension.


You have not understood what has been posted. There is no "must" about threading them on to the tap, particularly if you know you have a correctly-sized Stat-O-Seal and thread; I very much doubt Meriden or Small Heath workers threaded them on to the taps but that was forty-plus years ago. Today, especially where everything has been 'metric' for decades and cheap parts are bashed out without reference to standards, not everyone always knows they have a correctly-sized Stat-O-Seal and fuel tap thread; but we aren't on piece-work and a damaged Stat-O-Seal is more time-consuming than the precaution I described just because I happen to use it.


As I say, you are confused; whatever you're looking at, or have looked at in the past, it isn't/wasn't a Stat-O-Seal.

As I say, nothing on a Stat-O-Seal is "crimped". The metal part has a series of specially-shaped serrations around its i.d.; the "star-shape" I mentioned is the pattern made by the visible ends of the serrations in the image I posted; the ends of the serrations are not always visible on every Stat-O-Seal.

I suspect the seal itself is moulded around these serrations because, if the seal becomes detatched from the metal part, I've never been able to refit it, which suggests the seal isn't moulded separately and then pressed on to the metal part. Therefore, this particular property of a real Stat-O-Seal is of no benefit whatsoever compared to "a merely bonded seal".

Hth.

Regards,

Hi Mark,


In my experience, exactly what it says in the parts book:-



If you feel the need to wrap PTFE tape around the fuel tap threads, it's your bike.

Hth.

Regards,

Thanks Stuart, your remarks inspired me to look into the subject in more detail.
As I’ve only had cause to use “ordinary” fuel taps, I’ve not had to deal with this issue.

“This is nonsense. What do you suppose the fibre washer on the 82-1717 tap does?”

It seals the surfaces of tank to tap, it does nothing to seal threads. That is a conventional flange fitting tap, the type used for donkeys. Such a design has no need to seal the threads.

You are correct, I was wrong to think that there was some sort of crimping involved in the Stat-o-Seal, I must have been thinking of something else I’ve seen.

Though I suspect that the statement:

“f you look closely at any Dowty, the cross-section of the seal itself is basically C-shaped, with the ends of the "C" forming two tiny lips; in the original designed use as a seal on the telescoping legs of aircraft undercarriages, the lips seal on the inner shaft as inner and outer move relative to one another.”

is misleading, as Dowty seals were not the sliding seals within the hydraulic cylinders, but the static seals in hydraulic pipe unions. The intention was to provide a more reliable/re-usable washer than the usual copper or aluminium.

Both Dowty and Stat-o-Seals were designed for similar applications, where the compression of the rubber would force it against the plain shank of the bolt. Even this wasn’t essential, because the compression of the rubber between the surfaces provides a reasonable seal at moderate pressures, but to perform to design pressures it needs to be compressed against the correct size plain shank.

To stand a hope of sealing on a thread, it must be smaller than the nominal diameter of the thread to begin with, so that the compression can fill to the bottom of the valleys. I would agree with your technique of winding it on because of this straying from design intention.

I also hazard to think that the washer that goes with the seal has an ID closer to the thread diameter of the tap than any average washer may have, to ensure that it exerts pressure on all of the rubber without any “escape” gap.

"Therefore, one useful specific property of a Stat-O-Seal is the metal part will not scrape tank paint when the tap nut is tightened; otoh, there isn't anything to stop a Dowty doing so.”

I’m unsure what this means, both types should seal against flat, clean metal surfaces, not paint, and both are intended to be clamped metal to metal to metal.

Another’s comment:

“If something does the job it is not a bodge but a proper engineering solution.
Engineering is the art of making things work.”

I think is rather generous towards bodges. That several contributors have had problems with this arrangement, which should be a simple matter, does suggest that it lacks a certain “properness” as an engineering “solution”.

As I mentioned, PTFE paste in the tube.... You can see it on the gas tap fittings and where i wiped my finger on the fuel tank on my race bike, NPT threads made up wrench tight, no leaks.. ..Of course it's not pump urine but leaded race fuel..

IMHO all that's needed to seal petrol taps to the tank is a simple fibre washer that has the same internal diameter as the tap threads.

What is important is to ensure that the tank outlet flange is flat to ensure a good seal and that no paint present and that the tap has a nut fitted to tighten against the fibre washer.

Previously I have tried Dowty washers but never had any success. The problem as pointed out above is that Dowty washers are intended to seal against an internal smooth surface not a thread, furthermore the steel body does not easily crush and seal against the tank outlet flange and therefore is prone to leaks.

if you stop fuel at the threads the washer is doing little, plumbers tape is SOP for me, after that a fibre washer and a thicknessed steel washer so the tap clocks in where you need it. Select steel washer to clock in. Chances of a new cheapo tap fitting an old tank or new tank well are slim, even a decent quality item will be a bit undersized just to make sure they screw in.
i dont consider this a bodge ,its std practice for most hydraulics especially for this type of fitting, plumbers tape is good stuff. Dont go mental and keep it neat .
have used "Dowties" in the past and they work just the same as fibre washers, look a bit neater ,