Fuel Tank Sump - Tank Thread - Parallel or Tapered?

[Bob.]

My S3 fuel tank sump thread appears to be 1 1/4" BSP and is parallel as is the replacement from one of the usuals. Snag is that the sealing face of the replacement is about 100thou further from the thread start and the sump tightens completely way before the sealing washer reaches the tank flange face. At first I thought the tank thread was dirty or damaged and was going to buy a tap to clear it, but I can see nothing obstructing it and now believe it is probably tapered. Can anyone confirm this please?

(Fuel tank sumps on S3 cars are much shorter than S1/S2 but the sealing washer is the same part number so I doubt the threads at the sump-tank interface are any different.)

[chuffer]

Bob, this is not an area where I have any knowledge, but looking at the picture of the old sump, is there a copper washer (say) at 15mm and then a spacer, which are possibly being held in place by the black paint?
Ray

[AussieEtype]

My series 3 fuel tank sump has a great big o ring to seal not washers.

Garry

[christopher storey]

The drawings for the 6 cylinder cars ( and I have no reason to believe that the V12 is any different ) show that the arrangement is that there is the sump, a washer , and a screwed ring . . It is that which appears to be missing from the sump shown in the left hand of your picture , but which is present in the right hand example - although because of paint it looks to be part of the sump . Maybe yours is stuck to the tank ?

PS looking again at your pic, if the right hand one is your old sump, then I think that what you need to do is to remove the screwed ring from it and put it on your new ( left ) one

[Bob.]

Thanks for the replies folks.
I had missed the screwed ring shown in the S1 parts list and now I have looked more closely I see it was used up to a certain point on the S2 as well, together with a fibre sealing washer (according to the SNGB parts lists), but the screwed ring is definitely not used on the one I found on my S3, where the sump casing comprises 3 parts permanently joined by what looks like silver soldering. There is a simple flange soldered onto the tank with no insert either as far as I can see.
The sump comprises a brass threaded flange (unlike the repro which is all steel), joined to a steel pipe section, then another flange, steel this time, which carries a 3/4"AF hex to tighten the sump with and within it the drain set screw. There are no removable parts on either sump shown in my previous picture.
The current tank seal (C26310), supplied by SNGB, is a flat rubber washer (they seem to vary from 2.5 to 3mm thick) and from the parts lists appears to have been introduced when the screwed ring and fibre washer were deleted. The S3 WManual specifies a torque of 25ftlb whereas I didn't find a figure for the earlier cars although I may have missed it.


S3 components

[71 V12]

Hi Bob,

Out of interest, what is the problem with the original part?

Regards,

Kevin

[Bob.]

Hi Kevin
Basically I couldn't tighten the sump sufficiently to stop it leaking even though the rubber washer seemed to show a good witness mark all the way round. The problem was twofold. It didn't want to tighten beyond a certain position - as if you had met a stop or had gone as far as possible into a tapered thread, hence my initial question regarding the tank thread form. As a PO had half mangled the hex it meant I didn't dare tighten it too much for fear of rounding off the hex so I bought the replacement sump so that I would have a good hex for tightening only to find it wouldn't go in as far as the original, due to dimensional differences.



Regards
Bob

[71 V12]

Hi Bob,

Whilst that hex looks a bit chewed I think you could salvage it. Firstly remove the drain screw and washer. The washer should be copper or fibre, I don’t remember which but yours looks like steel.

Then try a single hex socket to help give the best fix on the nut, and remove the sump. Clean the threads of any crud with a wire brush and check the threads for damage. If you have any damaged threads, and they look ok based on your photos, clean them up with a deep V needle file.

Clean any crud from the tank threads. Apply a bit of grease to threads and carefully screw in the sump to the tank. When the threads are engaged properly and you are sure they are not crossed, use the tight fitting single hex socket (5/8 unf? or nearest metric) and screw up whilst pushing on the back of the ratchet wrench to stop the socket from slipping.

If the socket still slips I would file the hex back until you get sharp edges, again to fit a single hex socket, at a guess 14-15mm.

Hopefully this works and you can return the part you bought for a refund.

Regards,

Kevin

[Bob.]

Thanks Kevin,
I had already refitted the original sump before seeing your suggestions. (BTW the drain plug washer is aluminium).
As a result of having previously tried to fit the repro item after cleaning the tank thread as well as possible without the correct tap, the original went in a futher 1/4 turn and using a hex socket, as you propose, I achieved the correct torque - although that might still be misleading due to thread binding. I am adding petrol progressively and am up to about 12 galls without leakage so far, only another 6 to go.
Regards
Bob