Carburetor Fuel Inlet Fitting - MA4-SPA

krw5927 · #1 06-12-2013, 08:49 AM

For the engine and carb gurus here:

I have a Marvel MA4-SPA carb on my O-320E2D. The engine and carb used to be on a Skyhawk and will live out their useful lives on the nose of my RV9A - a far more worthy cause.

In trying to figure out what sort of fitting the carb fuel inlet (fitting with finger screen) would accept, the closest thing seems to be a standard AN6 threaded fitting with 1/4" NPT thread. However even lubricated with Permatex #2 the NPT fitting doesn't "feel" right going into the finger screen fitting. It gets quite tight only a single turn or so into the finger screen fitting.

I'd heard elsewhere (maybe here on VAF) that these carbs might need a NPSF threaded fitting. Not sure where to get one of those with AN6 on the other end. I did try the fitting with o-ring that fits into a Lycoming fuel pump and had no success. It's clear these are incompatible with the finger screen inlet threads.

Ideas? Am I being perhaps too gentle with a standard steel AN6 fitting with 1/4" NPT?

krw5927 · #2 06-12-2013, 04:04 PM

Quote:

Originally Posted by Low Pass

They're not NPT. They are a straight thread (not sure of the spec, SAE J514, straight thread oring boss maybe). Van's sells them at a reasonable price. But I *believe* a years back I bought the same steel fittings at Northern Tool (hydraulic fitting section). Prices were comparable as I recall. May find them at a hot rod shop.

Thanks, Low Pass! I looked at the Vans online catalog and had no luck finding such a fitting. Any chance you might be able to point me in the right direction?

Low Pass · #3 06-12-2013, 04:45 PM

Quote:

Originally Posted by krw5927

Thanks, Low Pass! I looked at the Vans online catalog and had no luck finding such a fitting. Any chance you might be able to point me in the right direction?

Don't thank me too quick - I've been steering you to a fuel pump fitting! Sorry about that. I believe you'll need a national pipe straight fuel thread (.250-18 NPSF). Where to find one cheap - not sure...

krw5927 · #4 06-12-2013, 05:24 PM

Quote:

Originally Posted by Low Pass

Don't thank me too quick - I've been steering you to a fuel pump fitting! Sorry about that. I believe you'll need a national pipe straight fuel thread (.250-18 NPSF). Where to find one cheap - not sure...

No worries, appreciate the effort. I'd actually bought one of the fuel pump fittings a while ago and it didn't fit. Anyone know what the proper fitting is and where I could buy one?

jrs14855 · #5 06-12-2013, 05:26 PM

I have a copy of an old Aviation Maintenance Technology article from when Facet supported these carbs. One picture clearly shows an AN822-6 fitting in this location. I am going to investigate this further but as of today I plan to use the AN 822. A 1/4" pipe tap bottoms out in the hole in the carb fitting so it may be a matter of manufacturing tolerance on the AN 822.
The gasket for the strainer fitting is all metal, a Marvel Schebler part number that is available from Aircraft Spruce.

airtractor8 · #6 06-12-2013, 07:43 PM

Quote:

Originally Posted by krw5927

For the engine and carb gurus here:

I have a Marvel MA4-SPA carb on my O-320E2D. The engine and carb used to be on a Skyhawk and will live out their useful lives on the nose of my RV9A - a far more worthy cause.

In trying to figure out what sort of fitting the carb fuel inlet (fitting with finger screen) would accept, the closest thing seems to be a standard AN6 threaded fitting with 1/4" NPT thread. However even lubricated with Permatex #2 the NPT fitting doesn't "feel" right going into the finger screen fitting. It gets quite tight only a single turn or so into the finger screen fitting.

I'd heard elsewhere (maybe here on VAF) that these carbs might need a NPSF threaded fitting. Not sure where to get one of those with AN6 on the other end. I did try the fitting with o-ring that fits into a Lycoming fuel pump and had no success. It's clear these are incompatible with the finger screen inlet threads.

Ideas? Am I being perhaps too gentle with a standard steel AN6 fitting with 1/4" NPT?

I have the same carb and setup. It is just a standard AN6 1/4 NPT fitting as you suggest. Nothing special at all. Sounds like the female thread on the carb fitting is damaged or out of round maybe?. The steel fitting is the way to go. Make sure you hold the strainer/carb fitting while turning it in or you can guess what may happen to the carb body thread.

Low Pass · #7 06-13-2013, 06:29 AM

Quote:

Originally Posted by airtractor8

I have the same carb and setup. It is just a standard AN6 1/4 NPT fitting as you suggest. Nothing special at all. Sounds like the female thread on the carb fitting is damaged or out of round maybe?. The steel fitting is the way to go. Make sure you hold the strainer/carb fitting while turning it in or you can guess what may happen to the carb body thread.

Despite what I thought was the case for many moons, the inlet fitting is not a simple npt. There's a flared sealing face at the bottom of the inlet fitting. Does npt work? Seems to. I used a steel npt fitting, with a little dab of sealant just for good measure, successfully for many years.

deek · #8 06-13-2013, 08:00 AM

This is a common issue. Although not recommended, an aluminum fitting will seat okay in the finger strainer fitting. I ran my steel AN6 fitting through a 1/4" NPT die and was easily able to take the fitting down enough to get what I thought was a reasonable number of threads engaged without worrying about bottoming out.

deek

krw5927 · #9 06-14-2013, 02:54 PM

Thanks, all! I took the finger screen fitting out of the carb, chucked it lightly in a vise, and torqued a standard steel AN-6 threaded elbow, with Permatex #2 sealant, into the fitting to the proper orientation. Re-installed the fitting assembly with a new crush washer into the carb body. It seemed to go in okay - we'll see in a few months whether it leaks or not.

Thanks again to all who responded!