Pressure Fittings

[Ara Dedekian]

        What determines the proper size of the exit hole in a pressure fitting.   

        I installed a pressure fitting on a Fox silver body muffler. The 4-40 retaining screw was replaced with a Fox 4-40 threaded pressure tap. I have Fox 35 Stunt mufflers that have both sizes shown in the photo. The small tap has a .028 hole and the larger 6-32 threaded one has a .052 hole. Fox holes range from .052 to .057 on my stash of the larger Fox muffler taps. DuBro pressure taps are .057, K&B 40 tap is .087, Enya 35 is .071 and OS ranges from .065 (muffler 762) to .071 (muffler 892). These are all from .35-.40 sized engines.

         Somewhere in my memory is an explanation, possibly by Duke Fox in his engine instructions, of the reason for the .028 hole size on the smaller tap.

        Ara

[Chuck Matheny]

The larger hole is meant for muffler pressure...the smaller holed tap is for crankcase pressure to pressurize the fuel tank.


Crankcase pressure is my favorite way to run non-throttled engines and I've done it with .049 to .50 sized engines.
The hole size of the CC pressure tap is not critical.
You can accidentally flood the engine with either the large or the small hole.
A very fine thread needle valve [in the 128 thread per inch range] with a "locking" collet will give you the most precise and consistent way to handle the higher fuel pressure.
Dub Jett Mfg. should still be open for business and he sells such a NVA [needle valve assembly].
Super Tigre sold such a NVA that you can still find on ebay.
MECOA might also be of help.
The old OS .10 NVA worked well with either bladder or CC pressure.

[Dan McEntee]

  Having the pressure fitting at the bottom of the muffler will cause you some issues. Castor build up when running will block it off or at a minimum restrict it. If you look at most if not all factory muffler that have a pressure fitting, it's not installed anywhere it will be on the bottom whether the engine is upright or inverted. You do want to avoid this.
  Type at you later,
   Dan McEntee

[Rusty]

  Having the pressure fitting at the bottom of the muffler will cause you some issues. Castor build up when running will block it off or at a minimum restrict it. If you look at most if not all factory muffler that have a pressure fitting, it's not installed anywhere it will be on the bottom whether the engine is upright or inverted. You do want to avoid this.
  Type at you later,
   Dan McEntee

I know a guy who in the 1980s would put his pressure fitting on the bottom of tuned pipes and mufflers because the wanted the oil to be drawn back into the tank and not drip all over his stuff.  Well, Dan has it right.  All them plugged up and he was constantly cleaning the vent line. 

[BillLee]

Using crankcase pressure:

If the passage between the crankcase and the tank is too large, it can cause a fluctuating run. As the pressure increases in the tank (engine running fast) causes a richening of the mixture (engine slows down) which causes the pressure in the tank to decrease (leans the mixture) which causes the engine to speed back up which increases the pressure in the tank which ... and so forth.

The solution is to put a small diameter restriction in the pressure line (or: a small hole in the pressure fitting).

Maybe Duke knew a thing or two! :-)

Regards,

Bill Lee

[Dan McEntee]

Using crankcase pressure:

If the passage between the crankcase and the tank is too large, it can cause a fluctuating run. As the pressure increases in the tank (engine running fast) causes a richening of the mixture (engine slows down) which causes the pressure in the tank to decrease (leans the mixture) which causes the engine to speed back up which increases the pressure in the tank which ... and so forth.

The solution is to put a small diameter restriction in the pressure line (or: a small hole in the pressure fitting).

Maybe Duke knew a thing or two! :-)

Regards,

Bill Lee

   Of all the old engines that I have had come into my possession that had a pressure tap in the cranks case, and might have been used is a "high performance mode" type of operation such as combat , speed or racing, the fitting was usually soldered up and a small pin hole drill instead. In my meager experiments with running crank case pressure over the years I have found it doesn't take much at all to over do it and get a result t hat is not satisfactory!!
   Type at you later,
  Dan McEntee

[doug coursey]

BACK IN THE 50'S AND 60'S WE USE TO SOLDER AND DRILL A SMALLER HOLE IN THE FITTING ON OUR COMBAT AND RAT RACE FITTINGS...

[BillLee]

Making a small hole in a piece of brass tubing (or in a pressure tap that will solder):

Get a piece of solid wire, like a piece of solid control line wire. Stainless is better but not essential.

Use a kitchen match to get the wire dirty with soot.

Stick through the brass tubing/pressure fitting.

Fill the hole (in the tubing/pressure tap with the wire sticking through) with solder.

After the solder solidifies, twist the wire and pull it out.

What's left is a hole the size of the wire through the tubing/pressure tap.

It is awfully hard to actually DRILL a small hole such as is needed for this application, and this technique works well.

Regards,

Bill

[Howard Rush]

Making a small hole in a piece of brass tubing (or in a pressure tap that will solder):

Get a piece of solid wire, like a piece of solid control line wire. Stainless is better but not essential.

Use a kitchen match to get the wire dirty with soot.

Stick through the brass tubing/pressure fitting.

Fill the hole (in the tubing/pressure tap with the wire sticking through) with solder.

After the solder solidifies, twist the wire and pull it out.

What's left is a hole the size of the wire through the tubing/pressure tap.

It is awfully hard to actually DRILL a small hole such as is needed for this application, and this technique works well.

Now you tell me. I broke a lot of little drill bits trying to drill solder 60 years ago. Think of how much money that is with compound interest.

[Dave Rigotti]

Bill,
Just ball park it for me.....Asking for a friend....What size hole would you use in a 1/2A speed engine, Profi for example?

Using crankcase pressure:

If the passage between the crankcase and the tank is too large, it can cause a fluctuating run. As the pressure increases in the tank (engine running fast) causes a richening of the mixture (engine slows down) which causes the pressure in the tank to decrease (leans the mixture) which causes the engine to speed back up which increases the pressure in the tank which ... and so forth.

The solution is to put a small diameter restriction in the pressure line (or: a small hole in the pressure fitting).

Maybe Duke knew a thing or two! :-)

Regards,

Bill Lee

[Gerald Arana]

Now you tell me. I broke a lot of little drill bits trying to drill solder 60 years ago. Think of how much money that is with compound interest.

Sounds like around,.. eh,.. er,... a dollar two eighty maybe?   

Jerry

[Dan McEntee]

Now you tell me. I broke a lot of little drill bits trying to drill solder 60 years ago. Think of how much money that is with compound interest.

    As a kid, I couldn't afford some real tools like drill bits. I had a small, Stanley had cranked drill that a neighbor gave us. I was readying an article is American Modeler magazine about building a rubber powered helicopter. In that article, it called for some small holes in some metal pieces for the rotor, and since the article was directed towards kids, suggested using short lengths of music wire, cut square with side cutters. The resulting chisel shaped end would drill through wood, brass and aluminum. Takes a bit of practice to get it just right. One of my first great discoveries as a 10 or 12 year old kid in the world of model building!!

   Type at you later,
    Dan McEntee
 

[Steve Helmick]

We did the soot/wire/solder thing in the early 1960's for crankcase pressure. For muffler pressure, a much bigger hole isn't a problem, because the flow is limited by the amount of fuel removed from the tank during the run time. I like muffler pressure, and have never had any problems with carbon blockage. As I recall, the smallest wire we had was .008", but with the soot and all, I wouldn't swear that the resulting hole was only .008". 

The CF tuned pipe that came with my .46VF had a pressure tap (blocked off), so I tried that...big fail, as mucho fuel siphoned out of the tank pre-flight, resulting in short tank syndrome (STS). Steve

[BillLee]

Bill,
Just ball park it for me.....Asking for a friend....What size hole would you use in a 1/2A speed engine, Profi for example?

Not sure what would be appropriate, but since the restrictions are so easy to make, I would suggest making a range starting with some really small wire for the hole, and then some larger. Fly...find the one that works.

Bill

[bill bischoff]

Implied if not stated, make up different size restrictors with short pieces of brass tubing using the described soldering method. The restrictors can be installed in the crankcase pressure hose. Pressure restrictors are not limited to the hole in the pressure fitting only. You can also make the restriction in the pressure inlet tube of the tank. This is helpful if your pressure line is very short or very curvy.

BB