The Stovebolt Site Forums The Shop Area General Truck Talk Like new in-cab fuel tank w/M8x1.25 threads....

I recently traded for a like-new in-cab fuel tank to go into a 49 3100.

This new tank has an M8x1.25 threaded (metric) fuel tap on the bottom of the tank (that had a bolt in it)instead of the OEM sized 3/8x27 threads.
This tank came w/a like-new sendor that incorporates a screened fuel tap that routes fuel out from the top.

The OEM fuel line setup on the 49 taps to the tank from the bottom which uses gravity to help feed fuel to the pump. I am unsure if the OEM style fuel pump has enough suction to pull fuel up and out of tank. The tank was originally purchased for a 1950 3100 many yrs ago but never used.
A couple questions:
1) Did 1950 (or later) 3100 tanks use top fuel feeds and not bottom of the tank feeds?
2) Is there an M8x1.25 to 3/8x27 adaptor to adapt this aftermarket tank to OEM SAE fittings?
Dave

1948-53 tanks should feed from the bottom. My tank had the metric thread as well I ended up using J-B weld to seal it after no luck with an adaptor.

1) It started with '54.

2)Several people have claimed to have rigged up some kind contraption so they can feed from the bottom. It would be much easier to go to flaps and get a tube that will fit from the new sender to the old line underneath. Use a short piece of flex hose at the bottom end.

I believe the original bottom threads on the tank are 1/8 npt pipe threads. Not 3/8

Thanks for the input and correction

I've found a pdf indicateing that theres such a thing as an M8x1.25 (male) to 1/8 NPT (female) adapter but thus far I can't source one. I've called local and google search results fittings suppliers with no luck thus far. There is an oil pressure gauge kit that includes this fitting but do I want to spend $30 or more for it?

A machining question.
A 1/8 NPT tap requires an 11/32 drill bit which I have. I'm leaning to trying to drill out that female M8 nut thats spot welded in the tank and retap it to 1/8 NPT.
Is there enough metal in the nut to go oversize. Will the nut hold and not break loose in the tank?
Just weighing all the options.

Dave

Not sure but try for the fitting at a plumbing supply place.

have you confirmed the M8x 1.25 size?

why would they put straight thread there?

As stated before, it is designed to be a drain. It was designed to use the sender pickup.

It still has to seal

Another good reason to order the SPECTRA tank from rock auto.
Correct threads on the bottom and correct filler neck.

Why don't you just use the right size metric bolt with a sealing washer?

Good info.
How do know the thread size?
I used a thread gauge on the plug and a tap on the female spot-welded nut in the tank. the M8x1.25 fit both perfectly.

I haven't ruled out using the like-new sender w/the fuel outlet on the top. But my research has indicated a descrepency. The new glass bowl fuel pump I have was used from 37-51. (51 appears to be the cutoff between bottom & top fuel tank feeds).
There is a pump that is supposed to be capatible from 37 - 57 but it is not the glass bowl type I have. it is #82-212. There is yet another non glass bowl type that covers the same yrs as the glass bowl type and stops at 51. #82-211.
So it appears that the 82-212 is backwards capatible but the other 2 stop at yr 51, which I'm assumeing is when the tanks went to a top feed.
Thoughts?

I have been able to locate a source for the needed adapter. They say it will be an approximate 2 week wait for it to arrive from their whse once ordered and that their minimum order is $40. The fitting/adapter is $11.17ea
So now that $30 oil pressure tester kit that includes this adapter is back in play as is redrilling & tapping to 1/8 NPT.

I had my old tank boiled/cleaned out but both my pump bowl & additional fuel filter bowl are collecting rust/debris so I'm motivated to replace the tank (and while I'm at it install seat belts).
Stay tuned

I believe you are incorrect in your top feed year for tanks, 49-53 behind seat tanks feed from the bottom (originally).

If there is enough material in the welded on hex nut, I would drill and tap for 1/8" pipe, it will save money and time. All it has to do is hold a fuel line. Add some of the strongest Lock-tite when assembling and you should be good to go for years.

There are carburetor repair kits for the fuel inlet that are stripped out, you may look into one of those. 1/4" pipe is next size up isn't it? You could also solder in the new fitting if the threads don't look deep enough or hold as well as you like.

Joe

This topic came up a year or 2 ago and just for giggles I converted an M8 nut to 1/8" NPT being that 11/32" is slightly bigger than 8 mm so it will clean up. For best results use a pipe tap reamer to taper the hole before tapping which will reduce the cutting force required and prevent tearing of the threads. An interrupted (aka skip tooth) tap will also help.

1/8" NPT is quite easy to tap with a normal 1/8" NPT tap. Tapping the larger NPT threads can be easier after first taper reaming the hole and or a skip tooth tap.

Its fair to say my post referred to overall best practice and did not single out smaller, easier taper threads. In my experience all persons have different ability regarding positioning a cutting tool perpendicular to the workpiece so "easy" is a relative term. For this reason there is a benefit for those with little experience as the tapered hole will help the tap stay square due to increased contact when starting.

Good points and I agree.
On the other hand someone only doing this once can do so and only buy a tap. They don't need to waste money on a reamer for one job, also taper pipe reamers are harder to find locally than the tap.

I don't understand why a tank for a '49 would have the feed coming from the top? Did they send you the right one? Is this part of some new regulation? I mean it should work just fine, it just isn't what was there originally. That being said, as long as you have the top feed setup, it could conceivably be a bit safer, which is why they went to that sort of set up originally anyway. So unless you are dead set on originality...

Good questions.
I didn't purchase the tank new from a supplier. Long story short, the tank came with a veritable cache of very good used parts I purchased off of a fellow that stripped it all off his 50 3100 inorder to customize it into a show truck. He went to an under-bed tank on his customization and never used this new behind seat tank that came w/the truck when he purchased it from another. My guess is the tank wasn't used because of what I'm experiencing. I have no idea what supplier the tank came from but as Dads50 pointed out, there is a supplier that sells a tank without these fitment issues.

The tank has no internal nor outside rust and could be set on a shelf and sold as new. The bottom feed hole was plugged w/an M8x1.25 bolt and washer.
I'm not adverse to trying the top-feed sender but before I go through the trouble of swapping the tanks again, I would like to hear of anyone that has a fuel-bowl type fuel pump interfacing w/a top feed sender and not haveing any fuel delivery issues. I could go ahead and try it but don't want to modify my fuel line(s) & drill a hole in the cab unnecessarily.
Dave

I see absolutely no issue using the top suction line on a tank with any engine mounted fuel pump. Once there is fuel in the line the work on the pump is just the same as if it were draining out the bottom. Think of the pickup and first few feet of fuel line as a siphon, no work necessary to move fuel once primed. Those mechanical fuel pumps are dead simple and very reliable, it won't care in the least if you use a pickup tube or fitting on the bottom of the tank.

That said it sounds like the simplest solution is tap the metric tank fitting for 1/8" NPT and use the tank as if it were the original. Plug the extra suction line on top.

Grigg

Update:
Was finally able to spend some time on the truck and got a resolution to this issue.
I decided to drill out and tap an M8 nut to see if how it went before I tried it on the on the tank nut and afterwards I opted not to go this route. The new 1/8 NPT threads appeared to cut deep enough to make good threads in the drilled out nut but once the larger tapered part of the tap got into the nut the going got very tough, even with cutting oil. I had the nut secured in a vise and one time the tap twisted the nut right out of the vise. The thought occured to me that the tap could break the tank nut loose in the same manner so I punted.

With this in mind I went the route of using the top feed sender. I'm glad to report that the glass bowl type fuel pump had no problem at all pulling gas up and out of the tank, through a new 2' long dry gas line and on to the secondary filter and into the carb. Since the carbs fuel bowl was full it started right up and the glass bowl fuel pump immediately filled w/gas. This is good in that I had removed the bowl to clean it which had emptied it.
I ran the truck a good hr around my local highways and byways and I didn't have any fuel related problems.
I had a few other unrelated problems creep up but they are for another thread.

I do need to add that the M8 plug on the bottom of the new tank leaked on my overnight leak test.When I pulled it the teflon tape was nearly all useless. So I used some aviation permatex on the threads and between the flat washer and tank and this seems to have fixed that. I'm glad I decided to do the leak test w/the tank out of the truck. I've pulled the tank in this truck 4 times now and was getting tired of soing that
I've probably drained and discarded 8 gals of gas!

So in closing, the nearly new glass bowl fuel pump that is spec'ed for bottom feed gas tanks doesn't seem to have any problems pulling fuel from a tank via the later model top feed fuel sender. I'll update if it does fail if I think about it. This old truck will be getting alot more weekend usage as I get all the bugs worked out.
Dave

you don't use Teflon tape on straight thread you use a sealing washer, I would use a copper washer. But there are other options.