Oil Pressure Sender Advice

[planesmaker]

Skippy, it appears someone has forced screwed the speco in, I would probably source another speco as going back to original thread would do more damage to the thread and maybe not seal.

 

 

[Downunder]

Bstrachan...

 

"post 4" is 100% correct. A 1/8npt male may well screw into a 10x1 but that does not make it the correct thread.

 

Oil Pressure Sender - Rotax P/N 456-180

 

 

[onetrack]

I'm with bstrachan. Every single item I have ever purchased from America or Canada has NPT threads on any piping locations - and I've purchased very substantial amounts of American and Canadian equipment over more than 50 years.

 

Every once in a while, you will find an occasional product from the U.S. or Canada, which has been built specifically for the Australian market, and it will have BSP pipe threads. However I reckon I could number these items on one hand.

 

Re "remotely locating" the sender - while this may sound like a good idea for sender protection - by doing so, you're introducing another failure point, in the form of the piping or flexible hose that is needed to connect the sender to the engine.

 

You could mount the sender remotely elsewhere on the engine, secured to the engine, and use steel or copper piping to connect it to the oil gallery outlet - or you could mount the sender remotely on the airframe, away from the engine, and connect it with a braided flexible hose. The latter solution is going to remove engine vibration as a source of potential damage to the sender - but it is adding the potential extra failure point of the flexible hose.

 

A remote mounting elsewhere on the engine, is only going to remove some potential engine vibration damage, not all - but then you still have the added potential extra failure point, of the steel or copper oil line.

 

I would run a 1/8 NPT thread tap through the engine sender mounting hole and then check the cleaned-up threads very closely for damage.

 

If there's substantial thread damage, then I would use a Helicoil or Recoil thread insert to repair the damage.

 

If the damage isn't too bad after clean-up with the thread tap, and the bulk of the thread form is still intact, then I'd install a new sender with Loctite to ensure a good seal with no leaks.

 

I'm of the opinion that someone has forced a 1/8" BSP threaded sender into a 1/8" NPT thread in the Rotax.

 

In 1/8" size, NPT is 27 TPI thread pitch - BSP is 28 TPI.

 

27 TPI equates to 0.94mm thread pitch. 28 TPI equates to 0.9mm thread pitch.

 

NPT threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form) - BSP threads have a 55° included angle, and have rounded peaks and valleys (this is a Whitworth thread form).

 

 

[bstrachan]

I'm with bstrachan. Every single item I have ever purchased from America or Canada has NPT threads on any piping locations - and I've purchased very substantial amounts of American and Canadian equipment over more than 50 years.I didn't consider the possibility that some ham-handed idiot might have screwed something into the oil pump housing that didn't fit, but I guess these things happen. At any rate I am prepared to assert without further proof that the female thread in an UNMOLESTED 912ULS oil pump housing is 1/8-27 NPT. 'Nuf said.

 

Re "remotely locating" the sender - while this may sound like a good idea for sender protection - by doing so, you're introducing another failure point, in the form of the piping or flexible hose that is needed to connect the sender to the engine.

 

VDO recommends it. I've replaced enough of these things to go along with them. These senders are not designed to withstand the vibration resulting from hard mounting to the engine.

 

You could mount the sender remotely elsewhere on the engine, secured to the engine, and use steel or copper piping to connect it to the oil gallery outlet

 

I wouldn't recommend it. Been there, done that, regretted it.

 

- or you could mount the sender remotely on the airframe, away from the engine, and connect it with a braided flexible hose. The latter solution is going to remove engine vibration as a source of potential damage to the sender - but it is adding the potential extra failure point of the flexible hose.

 

True enough. Neither of the external oil hoses on the 912 is under any kind of pressure.... but braided stainless steel Teflon hoses are meant to cope with high pressure oil.... you pays your money and you takes your chances.

 

A remote mounting elsewhere on the engine, is only going to remove some potential engine vibration damage, not all - but then you still have the added potential extra failure point, of the steel or copper oil line.

 

Bingo!

 

I would run a 1/8 NPT thread tap through the engine sender mounting hole and then check the cleaned-up threads very closely for damage.

 

If there's substantial thread damage, then I would use a Helicoil or Recoil thread insert to repair the damage.

 

If the damage isn't too bad after clean-up with the thread tap, and the bulk of the thread form is still intact, then I'd install a new sender with Loctite to ensure a good seal with no leaks.

 

There are other ways, but I'm not going out on a liability limb by suggesting them.

[onetrack]

"VDO pressure senders are not suitable for use in aircraft."

That's interesting. So that cuts out VDO immediately as a source for oil pressure senders.

 

Pressure Sensors & Switches | Senders and Sensors | VDO Instruments and Accessories

 

The other interesting point that VDO make, is that the use of Teflon thread tape or pipe sealant, will interfere with grounding and produce false pressure readings.

 

They also point out that VDO senders should only be used with matching VDO gauges. So the VDO/Speco mismatch that Skippydiesel has, is an immediate potential source of problems.

 

Note the variety of threads available in the VDO senders - NPT, NPTF, Metric, and BSP.

 

 

[bstrachan]

That's interesting. So that cuts out VDO immediately as a source for oil pressure senders.Lawyerese BS. Every 912ULS I've ever seen (except Rex's brand new one) has a factory installed VDO sender. The warning is a CYA. This cute little tag is packaged in with the new sender where you can't miss it. You figure it out.

 

And before somebody pounces on me, I mis-spoke earlier. The 912ULS oil cooler IS connected to the engine by pressurized hoses, and on my airplane they aren't even the stainless braided Teflon variety.

 

Pressure Sensors & Switches | Senders and Sensors | VDO Instruments and Accessories

 

The other interesting point that VDO make, is that the use of Teflon thread tape or pipe sealant, will interfere with grounding and produce false pressure readings.

 

They also point out that VDO senders should only be used with matching VDO gauges. So the VDO/Speco mismatch that Skippydiesel has, is an immediate potential source of problems.

 

Note the variety of threads available in the VDO senders - NPT, NPTF, Metric, and BSP.

 

 

[cscotthendry]

Re "remotely locating" the sender - while this may sound like a good idea for sender protection - by doing so, you're introducing another failure point, in the form of the piping or flexible hose that is needed to connect the sender to the engine.

You could mount the sender remotely elsewhere on the engine, secured to the engine, and use steel or copper piping to connect it to the oil gallery outlet - or you could mount the sender remotely on the airframe, away from the engine, and connect it with a braided flexible hose. The latter solution is going to remove engine vibration as a source of potential damage to the sender - but it is adding the potential extra failure point of the flexible hose.

 

A remote mounting elsewhere on the engine, is only going to remove some potential engine vibration damage, not all - but then you still have the added potential extra failure point, of the steel or copper oil line.

I'm glad someone brought this up. I was going to mention it, but you beat me to it.

There is significant pressure and temperature in the oil system. If you look at how Rotax engineered their oil system, you can see that they have done what they can to keep the pressurised passages inside the engine. The lines to and from the oil tank are the only oil transports outside the engine and they have zero pressure differential between the inside and outside of the hoses.

 

By remotely mounting the oil pressure sender, you are creating a high pressure pathway outside of the engine that could rupture. With that kind of pressure in that line, a rupture will empty your oil system in a matter of seconds.

 

 

[cscotthendry]

The other interesting point that VDO make, is that the use of Teflon thread tape or pipe sealant, will interfere with grounding and produce false pressure readings.

Which is exactly why tapered thread (read "self-sealing") fittings are used for this application.

 

 

[skippydiesel]

I'm with bstrachan. ........................................................I'm of the opinion that someone has forced a 1/8" BSP threaded sender into a 1/8" NPT thread in the Rotax.

In 1/8" size, NPT is 27 TPI thread pitch - BSP is 28 TPI.

 

27 TPI equates to 0.94mm thread pitch. 28 TPI equates to 0.9mm thread pitch.

 

NPT threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form) - BSP threads have a 55° included angle, and have rounded peaks and valleys (this is a Whitworth thread form).

Thanks Onetrack - I agree with your analysis - the "builder" of my aircraft seems to have a little bit of a history of a "she'll be right mate" approach to aircraft fitting. Got to say my Speco oil pressure indication system has stood the "test of time" (18 years 800 hrs & still working without leaks) and it's only the slow degrading of the read out that has shown up the bodgy sender situation.

 

I think the sender must be a Speco unit - how it comes to have the male thread type it has, is a mystery. (current Speco senders are all 1/8 NPT)

 

I will purchase a 1/8 NPT fitting this week & try it in my 912 - hope it fits.

 

As an aside: VDO senders suitable for 12 volt automotive type applications (potentially light aircraft) seem fall into two categories:

 

"Pressure Senders - Single Terminal - Australian applications"

 

These seem to be at the low cost end and are expected to be direct to engine mounted. No reason why they could not be remote mounted but little benefit likely with the potential weaknesses mentioned elsewhere.

 

"Pressure Senders - 2 Terminal Above Ground - 2, 5, 10, & 25 Bar"

 

 

Higher cost (double +) intended for remote mounting and have two terminals (second for warning light). Could easily be engine mounted (same fitting system) but may shorten operational life due to direct engine (higher frequency ?) vibration.

 

 

[skippydiesel]

Have purchased a 1/8NPT fitting (part of a mechanical oil pressure kit). IT FITS !!! and seems willing to tighten up, despite some damage to the female threaded port.

 

I am speculating that the old sender, with parallel thread, has only damaged the innermost part of the port, leaving the outer threads in usable condition.

 

Having had such good service from the Speco gauge/sender, have ordered the same again (with 1/8 NPT fitting).

 

2" Street Series, 0-100 psi, Gauge & sender $70 (inc GST) through Bursons.

 

Far cheaper than VDO (sender only $57 -160) & perhaps the reverse (of VDO) operating system confers a more durable component life - we will see.

 

The sender will be fitted/secured with Loctite as per Rotax requirement.

 

Will report back when new system fitted & test flown.

 

 

[skippydiesel]

In the interval much has happened -

 

Installed Speco sender - ground run - strange very high readings on existing Speco gauge - only noticed when engine shut down (electrical power still on) gauge needle swung hard to right. Clearly there is an incompatibility with new Speco sender and old Speco gauge.

 

Installed new Speco gauge - ground run - TERRIFIC ! - have what appears to be good oil pressure readings from 60 psi on cold start @ 3000 rpm down to 45 psi warmed engine @ 2000 rpm and consistent 55 psi @ 3000 - 5000 rpm.

 

Test flown this morning - new gauge & sender working perfectly - nice consistant readings from healthy engine. No oil leaks.

 

Despite the sender tightening down successfully, I am concerned about the condition of the female threaded socket, so in addition to using the recommended Loctite, I have safety wired the sender using a suitable hose clamp, around the body of the sender, as the anchor point. Cant do any harm and gives me peace of mind that here will be no sudden failure at this junction.

 

 

[nomadpete]

Might be worth noting that VDO have two different catalogues. This can cause confusion, depending on where you source your sender.

 

In USA, a 360-004 sender has a 1/8 NPT

 

In Australia, a 360-004 has a 1/4 NPT.

 

It seems that in Australia, a 360-002 has a 1/8 NPT thread (1000kpa or 150psi full scale)

 

Check carefully before committing to using anything on a Rotax.