help identifying a restrictor fitting

[prkaye]

The fitting on the left (the black one) is a restrictor fitting i bought from ACS for the oil pressure port on my O-320.
The engine came from Aero Sport with the fitting on the right. That fitting does not have the obvious pinhole restriction on the pipe-thread end like the black one does, but it does appear to have somewhat of a restriction on the flared end. Are these just different types of restrictor fittings, both equally suitable for the oil pressure port?

 

[sjhurlbut]

both

Yup I've seen both. Same function.

 

[Bavafa]

The left one is standard that is used for manifold and fuel pressure. The right is standard that is used for oil pressure.

 

[rvbuilder2002]

The left one is standard that is used for manifold and fuel pressure. The right is standard that is used for oil pressure.

???

Never heard of any standard or difference between oil and fuel.
I personally wouldn't use one with an orifice any bigger than the one shown on the left. The one on the right might be the same because it looks like it has a threaded in restrictor that was installed using an allen key.

 

[prkaye]

is there something unique about RVs and these restrictor fittings, or are they always installed on the oil pressure port?

 

[Freemasm]

The types of pumps utilized create pressure "ripples". The restriction dampens that out to give a steady reading and prolonging the related instrument's life.

Edit = To better answer your question, its nothing specific to RVs

 

[planenutz]

is there something unique about RVs and these restrictor fittings, or are they always installed on the oil pressure port?

This is not something unique to RV's - all engine installations should have them installed on the oil pressure line. The fuel pressure line should also have a restrictor in it. The basic idea is to prevent significant fuel or oil loss in the event the hose bursts or comes loose. In the fuel line it also helps dampen out pressure oscillations associated with the mechanical fuel pump operation. Just make sure you don't put the restriction on the main fuel feed to the carb/servo - like was found to be a contributing factor in a recent RV accident.

If you don't have one, I recommend installing one.

Cheers,

 

[rvbuilder2002]

If you don't have one, I recommend installing one.

I agree.

A recent local incident (of which I only have 3 hand info so it may not be 100% accurate) of an RV-6A fortunately was only an incident and not an accident.

Current owner was not the builder.
Builder had used a solid metal line between the firewall and the oil pressure port on the engine, and a restrictor fitting was not installed.
Airplane had quite a lot of time on it but the inevitable finally happened and it failed in flight.

At the drop of oil pressure and immediate turn to an airport was made. Estimate of time from the oil pressure indication dropping to zero, to the engine making bad sounds internally was 5 minutes. They got the airplane onto a runway but the engine is likely a total write-off.

If a restrictor fitting had been installed, cleaning up a big oily mess might have been the only post flight task (along with replacing the pressure sense parts with what had should have been used in the first place).

 

[Freemasm]

Have heard this and not disagreeing. The restricted fitting on a lot of older aircraft (Mooneys at least) was the bulkhead fitting on the FW connected to the instrument sense line. Because of type certificate restrictions, it was probably that way until remote sensing became the standard. That tribal knowledge at the time was to limit cockpit flammable fluid ingress in the event of instrument physical failure; diaphragms being the weak point.

The weak link is not the sense lines and failure of these lines are no greater possibility than the much larger/high flow process lines. A modern remote device that is internally orificed won’t benefit. Does the subject restricter really reduce risk especially with sensing devices not installed in the cockpit? Probably by some almost negligible degree. Cheap mitigation but it doesn’t completely solve any safety problems. That said, I’ll still utilize the subject restricters where I can.

 

[HFS]

Set Screw With A Drilled "Orifice" Hole

I've been using this type of restrictor (if there is, in fact, a small orifice hole drilled through the base of the set screw) for a long time.

The basic benefit, if there is one, is that you can more easily match the orifice size to your needs. Just disconnect the mating tube/hose, remove the set screw (with the hose/tube installed it can't come out - safety feature), drill it to a larger size, or replace it with one drilled to a smaller size, reinstall, reconnect and you are in business.

Also easier to make as a DIY project - only threading & tapping required.

YMMV - but this does work.

HFS

 

[PaulvS]

If making as a DIY project, what is a good way to hold the set screw for drilling? And any hints for drilling the tiny hole (.040 I think) in the hard steel?
Thx
Paul

 

[Tommy123]

If making as a DIY project, what is a good way to hold the set screw for drilling? And any hints for drilling the tiny hole (.040 I think) in the hard steel?
Thx
Paul

Drill slowly, use lube.

 

[HFS]

DIY Restrictor Fittings

I use Brass or Aluminum set screws - a lot easier to "work" and not used outside their performance specs.

As far as holding them - Make any "fixture" you want - wood, PVC sheet/bar, aluminum, steel ..., drill and tap a complementary hole, squared up in the base, screw in the set screw, and you (should) have a suitable base to use for holding/drilling.

HFS

 

[PaulvS]

I use Brass or Aluminum set screws - a lot easier to "work" and not used outside their performance specs.

As far as holding them - Make any "fixture" you want - wood, PVC sheet/bar, aluminum, steel ..., drill and tap a complementary hole, squared up in the base, screw in the set screw, and you (should) have a suitable base to use for holding/drilling.

HFS

OK thanks. I've only seen steel set screws with the recessed hex head but it should be easy enough to make one from a brass screw by cutting a straight slot, similar to the jets in some carburetors.

 

[TS Flightlines]

.02 cents worth--Back in the day of rubber hose, where the original hose ends were reusable, restrictor fittings were common, just incase of a hose failure, or fitting blow off. Yep---weve seen some where you could pull the fitting off because the liner had deteriorated to the point of not gripping the fitting stem. With the advent of teflon hoses, that problem has pretty much gone away. Now enter modern day avionics and very sensitive pressure sendors. A restrictor fitting helps to dampen the pressure pulses, especially on MAP systems.

On the custom restrictors we make ( the straights and 90*s) we use a stainless allen set screw thats pre drilled to .040. I install it in the Flare side. The NPT side hole is alittle large for the threads (10/32). AND with the threaded version the restrictor is physically captured by both the fitting internal bore, and the hose end. In the NPT side, it could become carbon clogged, or if it were a pressed in restrictor, high temps 'could' loosen it from the fitting and it be sucked into the cylinder. Certainly dont want that.

Again, my .02 worth,
Tom