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Fuel line fitting for MA-4SPA?

Loman

Well Known Member
I give up! I know there is a thread here somewhere with the correct information but I'm damned if I can find it again, despite using every conceivable search term.

What is the correct fitting to connect a fuel line to the fuel inlet/strainer on an MA-4SPA carb? Specifically, mine is p/n 10-3678-32.
 
Definitive answer from Marvel-Schleber

Hmmm. I am not so sure about that Ralph. I asked Marvel Schleber themselves and they came back with:

The 95-509 fuel inlet fitting on your 10-3678-32 carburetor is threaded at 1/4-18 dryseal NPSF.

I further asked what was the correct male thread to connect into that and they came back with a link to this page on a German database of standards.

It clearly says
The parallel, internal thread National Pipe Straight Fuel is coupled with tapered, external National Pipe Taper without use of a sealant.
and shows a diagram that has the straight thread of the female NPSF mating with the 1.47 degree TAPERED male thread of the NPT. So it seems that fittings such as AN816 (AN822 or AN823) -6D would be compatible as they have the NPT 1/4” 18 thread. As the nominal dimension of a tapered thread is taken half way down the run of threads, I would expect the fitting to ‘bind’ in the inlet when threaded about half-way in. This is probably the “dryseal” feature referred to and coincides with my own experience of test fitting an AN816

However, according to this information, the fuel pump fittings as sold by Van’s and ACS (among others) with a STRAIGHT UNF 9/16” 18 thread will fit because the two straight threads will mesh perfectly. However, the “dryseal” feature won’t operate. A proper seal therefore would depend on the O ring and some sealant.

As always, I am happy to be corrected on this
 
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Hmmm. I am not so sure about that Ralph. I asked Marvel Schleber themselves and they came back with:



I further asked what was the correct male thread to connect into that and they came back with a link to this page on a German database of standards.

It clearly says and shows a diagram that has the straight thread of the female NPST mating with the 1.47 degree TAPERED male thread of the NPT. So it seems that fittings such as AN816 (AN822 or AN823) -6D would be compatible as they have the NPT 1/4” 18 thread. As the nominal dimension of a tapered thread is taken half way down the run of threads, I would expect the fitting to ‘bind’ in the inlet when threaded about half-way in. This is probably the “dryseal” feature referred to and coincides with my own experience of test fitting an AN816

However, according to this information, the fuel pump fittings as sold by Van’s and ACS (among others) with a STRAIGHT UNF 9/16” 18 thread will fit because the two straight threads will mesh perfectly. However, the “dryseal” feature won’t operate. A proper seal therefore would depend on the O ring and some sealant.

As always, I am happy to be corrected on this

To add to this, the ORB (o-ring boss) fittings, that are used on fuel pumps, should only be used with mating surfaces designed for ORB. They are straight threads and the male side gets a precisely cut channel + o-ring/washer and the female side requires a precisely cut taper for the o-ring to seat in. Never install an ORB fitting into a female threaded appliance that does not have a taper cut for the oring to seat in.

While my experience is limited, I have never seen a carb that takes an ORB fitting (many FI servos do). Several carb bodies have a large, straight threaded port, into which an adapter fitting is installed, with gasket not ORB, and that fitting has the NPSF threading outlined above.
 
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Yep, went & pulled out 3 carb cores hidden in the back shelves to confirm the use of pipe thread fittings.
Loman, good call to go to the source (Marvel Schleber) & get the right information!
 
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