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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Gas Tank Fittings and Leak Test

Can someone confirm that the gas tank fittings for a TD are 3/8" BSPF (British Standard Pipe Parallel)? The thread size is 0.656" at 19 threads per inch. I believe the threads are the straight threads and not the British Standard tapered threads (BSPT).

Also has anyone done a leak test of the gas tank after repairing the sensing unit gaskets and threads? I would hate to replace the tank and have to put enough gas in the tank to leak test with gasoline. If it still leaks you have all that gas to remove and then removal of the tank.
Roy Challberg

Hey Roy. I have my gas tank off for painting, and bought a cutoff switch at my local hardware that is a direct fit. I believe all the brass fitings can be purchased at your hardware store. With the cutoff in place, you can test for leaks in the tank with out having to connect the fuel line, and would make for an easy drain of the tank with out getting it all in your face. Also, the cutoff is a good antitheft deterrent. Hope this helps. Larry
Larry Brown

Forgot to mention mine is a TF, but I don't think there is a difference in the fitting. Take one of yours off and take it to the hardware store and check it out. Thanks. Larry
Larry Brown

Roy - your concern about leaks at the sending unit gasket and threaded holes is very valid. Those units may leak at the large round gasket, through the machine screw holes or through the rheostat cover plate - or all three. Not fun after the tank is back in place on the car and your spare tire carrier is all bolted up.

I would avoid the black rubber looking gaskets that are often supplied and use the cork gaskets for both the cover plate and the gasket - the round one- for sending unit itself. Test your screws in the threads and see how loose the screws are. If they are loose at all, they will of course leak.

The base of the sending unit also becomes warped when over tightened and this will cause you much grief. If the base is very distorted and you are not able to repair it, then find a better sending unit - or just fit a cover plate - and dont use the sending unit.

I imagine others have better knowledge about the proper permatex type sealer to use - I just cant remember what I have used in the past. Just know that, unless you are very careful with this assembly, it will most likely leak.

I have often wondered how long it took for these to leak when now. I think the gaskets were of a much better material originally.

Good luck
J Delk

Larry-
I'm a little confused. What type of "cutoff switch" fits into a brass fitting?
When I hear cutoff switch I picture some type of electrical switch.

Got a picture or sketch?
Thanks,
Roy
Roy Challberg

Larry -- What hardware store did you find fittings with 19 TPI? All US fittings are 18 TPI. While if the US fittings were machined loose enough they may screw on they will bugger up the threads when tightened.
Cheers,
Bob
R. K. (Bob) Jeffers

Roy and Bob. I should have said ball valve instead of cut off switch. Although I have owned my TF for over 25 years, it is entirely possible the po may have changed the fittings at some point in time. I took the fitting out of my tank and went to my local hardware store and found everything I needed to fit up. The line going to the fuel pump on mine is copper, so it would have been very easy to change this fitting. Since my fittings must not be original, I apologize if I have mislead you. I should have known nothing is easy on our cars! At any rate, it worked on mine. Larry
Larry Brown

Roy - You are correct, the original fittings are 3/8 BSP, not BSPT. As for checking for leaks before putting gas in the tank, most leaks are going to be on the bottom of the tank. I have a small drop light that I can drop down in the tank through the fill neck. I do this in the basement and then turn the basement lights off and check the bottom of the tank for light leaks. Repeat, putting the light in through the sender opening, which will get the center section of he tank and the right side. I found pin holes this way that were so small, I couldn't see them withthe lights on. I had to repeat the procedure with a center punch and hammer in my hands and with the light off, I would position the center punch over the pin point of light and punch a dimple in the tank, then with the lightes back on, I brazed the holes closed (the tank had been dip stripped prior to doing this).

I agree with J. Delk regarding the sending unit. I would use the thinist cork gasket material I could find to make the gaskets out of and liberally coat both sides with Peratex 2B Aircraft sealer and coat the screw threads. Don't over tighten the screws and you should be ok.
Good luck - Dave
David DuBois

Hi Roy,

Regarding a sealing compound,you could also "Three Bond" Gasket Sealer which is gas(petrol),oil,and water resistant.I think this might be an American product.Also,some MG suppliers sell tiny gaskets to go under the head of the setscrews to assist sealing.

Cheers
Rob. Grantham
Rob Grantham

Tighten each screw a tiny little turn, then the next... I went around and around about 4 times...no leak and I used an oil/gas black gasket I found in one of those gasket variety packs!
gblawson - TD#27667

Dave: Where can I order Peratex 2B Aitcraft sealer? Thanks.

Stuart
Stuart J. Ramos

Stuart-
I believe that should be "Permatex 2B" and should be available at most well stocked auto supply stores. Permatex 2B is a non-hardening sealant in their "Forms a Gasket Sealant" line.

I noticed that on their web site they have a 98D sealer called a "high tack gasket sealant". It is in a small can with a brush in the top. They say it is resistant to gasoline and to be used for fuel pumps and carbs. Anyone use this product?

Dave-
I like your method for checking for small pin hole leaks. I'll try that. I thought I might plug both holes in the bottom (3/8" BSPF) and install the sending unit with sealed threads and all. Then I would use the fill neck with an expandable plug to pneumatically test the tank for leaks (gaskets and all). The problem with this method is you need a large tub of water to check for leaks. Or I guess you could use some soap solution leak checker. The other method would be to give the plugged tank a hydrostatic test with water. This method would require the careful drying out of the inside after the test to avoid any rust.

Comments?

Roy
Roy Challberg

Good Morning Roy.
I have replaced the sending unit gasket three times on my current TD. First with Moss motors gasket (cork) it held for about 5 years. Next with the black one Gordon talks about. I t leaked between the two faces of the gasket within two weeks. What did end up holding was a gasket purchased from Abingdon Spares. It is the same material as the gasket material provided with a SU fuel pump rebuild kit. You can look a lot of this information up in the archives as it has been an ongoing problem. One word of caution if you have used the cork gasket as I did you probably over tightened the screws , in an attempt to get it to stop leaking. When I replaced it the last time I had to straightened the sending unit flange by tapping it lightly on the face of the vice anvil with a small bald peen hammer. I also used Permatex 98H high tack gasket sealer which is gas/ alcohol resistant. It comes in a little can not tube and is available at NAPA Auto Parts. Cover the screws with sealer before threading them into the gas tank. Next snug up the threads as Gordon has suggested, not to tight. If like me, it will still leak a little. Watch it for a week, if it leaks snug it up a little more. If you over tighten it the first time you will have to start allover.
If you want to seal the leaks in your gas tank, as long as they are just pin holes you need to use an aviation grade sealer. Bill Hirsch sells a slouch/etching /cleaner kit which includes aviation grade sealer. http://www.hirschauto.com/ You can look up additional information on this process in the archives. I know the hard way why you need aviation grade sealer. The biggest thing is it is alcohol resistant, which softens up the regular gas tank sealer. Hope this helps, John
John Hambleton

Roy - Thanks for corrrecting my typo on the permatex - my fingers get to going faster than the brain at times. The pressurization of the tank will work for finding leaks if you don't have a small light that will fit inside. I like the light method because it doesn't require a big water tank and it doesn't get everything wet. When I did it, I was surprised at how well it worked, it definitely caught all of the pin holes as the tank have not leaked since doin it. Cheers - Dave
David DuBois

For final comment and future reference---

The bottom drain plug and the gas feed plug are both 3/8-19 BSPF (British Standard Pipe parallel not tapered pipe thread)

The screws holding the sending unit in place are 3BA (British Assoc. thread)

The gas tank side panels are held in place by 1/4-26 BSF.

I made a leak test fixture using a 1 1/2" expandable test plug with a hollow threaded shaft for pressization using air. Testing to about 2-3 psi should be more than enough. I used a soap solution type leak test fluid at the flanges and fittings. I also checked the seams while pressurized.

I know there was an earlier thread debating the size of the sending unit base flange screws. I checked three different tanks and 3BA is the thread.

In pressure testing I noticed that the seal for the sending unit terminal is the weak link and hard to seal.

If anyone needs the air pressure test fixture let me know. I can send a sketch or you can borrow the actual fixture.

Roy
Roy Challberg

"If you want to seal the leaks in your gas tank, as long as they are just pin holes you need to use an aviation grade sealer". I have to say a resounding NO to aviation sealer, it does not stand up to the formulation of today's fuel and it will dissolve over time. When it dissolves and gets into the system, it is very effective in gluing the check valve disks to the seats in the fuel pump and the needle valves to their seats in the carburetors. You ask how do I know this? I just got my tank back from the painter after having had it dip stripped to get all the aviation sealer that I had put in 25 years ago when I restored our TD (Aviation sloshing compound was the only thing one could get in those days). Additionally, in order to get all the sealer out of the tank, it was left in the striping solution so long that most of the drain and outlet fitting was eaten away, so I am now faced with trying to build up the bosses with JB weld and then tapping and installing helicoils in the fittings to restore the threads. If you want to seal the tank (and I strongly recommend doing so unless it has been treated with a zink phosphate coating), then get the stuff that Moss or Eastwood sells. Cheers - Dave
David DuBois

This thread was discussed between 16/10/2006 and 31/10/2006

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.