Rotax 9 Crankcase Oil Drain Adapter

[skippydiesel]

 Wanted crankcase sump adapter nipple;

 

Got this idea from Rotax Owners Forum https://www.rotax-owner.com/en/912-914-technical-questions/11526-do-i-need-a-larger-oil-return-line

For 912 owners, that may have high crankcase pressures - Replace crankcase oil exit Banjo, with less flow resistance, higher flow elbow.

 

I have a spare elbow  with swivel fitting PN 956580 Elbow (see below) need to find a connecting nipple from it to crankcase.

 

I believe the crankcase exit hole to be M16 x 1.5mm (?).

I don't know the elbow nut thread (M18 x 1.5mm?) or the angle specifications of the conical end.

 

I have a oil cooler fitting PN 956577 (sample shown) that fits the elbow but not the crankcase. I imagine the correct adapter will look similar.

 

• Do you have a suitable adapter nipple ?
• Are you able to advise on adapter specification description?

😈

Edited August 16 by skippydiesel

[skippydiesel]

This could be it;

Adapter M18x1.5 Female / M16 x 1.5

Edited August 16 by skippydiesel

[facthunter]

Definitely not. That type is for  joining a Pipe to a smaller pipe using thin tapered solderless nipples, which crush onto the pipe as the nuts are tightened. The nuts used have to be internally tapered also. It's an obsolete and not very satisfactory system. The SPHERICALseat on that elbow is one of the best set ups and seals on a narrow 45 degree seat on the correct  part with a thin line of contact achieving the seal reliably with some degree of misalignment tolerence. Nev

[skippydiesel]

Rotax 956655 ADAPTER M18X1.5/ M16X1.5. is $245 AU + delivery  Crazy price for a reducing nipple (in plumbers language).

 

Rotax 956655 appears to have the same specifications/dimensions as this $5 (including delivery) one from Amazon

I have ordered this one & will check it for fit before proceeding further. If it is compatible with my existing elbow (PN 956580) I am financially way ahead .

 

I will be using a copper washer (as per Rotax) & if space permits a locking nut for security.

 

My plan B  - I have found some AN ( - 8 & -10) fittings, (M16 adapter, swivel elbow & 1/2" barb ) that I think will do the job . All up about $50-55 AU

 

Plan C - Go back to original Banjo fitting and live with an oily belly.

 

So far no one has commented on my plan to reduce crankcase pressure;

• If it works, do you foresee any issues with lower pressure?
• Do you thing there is any chance that lower pressure may reduce oil being ejected out of the breather?

😈

 

 

[FlyBoy1960]

why do you think you have excess crankcase pressure ?    if you are getting an oily belly than I could quickly think of the following

 

worn out rings creating excessive crankcase pressures

 

over filling of the oil

 

why are you trying to invent a solution for a problem that probably doesn't exist if the normal engine installation instructions have been followed including the correct hose use and diameters

[skippydiesel]

37 minutes ago, FlyBoy1960 said:

why do you think you have excess crankcase pressure ?    if you are getting an oily belly than I could quickly think of the following

 

worn out rings creating excessive crankcase pressures

 

over filling of the oil

 

why are you trying to invent a solution for a problem that probably doesn't exist if the normal engine installation instructions have been followed including the correct hose use and diameters

Ha Ha.

 

FlyBoy,

 

A wee bit of credit, for not being a total idiot,  would be appreciated.

 

Didn't say I have excessive crankcase pressure

 

As a 15 + year operator/maintainer of Rotax 912 ULS, I know what the oil level should be (FYI: I have tried running the engine at less than 1/2 oil - get less oil out the breather but still more than I would like)

 

This is an almost new engine (240hrs). It's blown oil out the breather since new. Oil consumption within Rotax specified limits.  Engine runs like a dream.

 

My previous 912ULS (sold at 950hrs) did not require topping up between 50 hr changes, nor did it anoint the belly of the aircraft with oil mist.

 

A thread on the Rotax Owner Forum gave me the idea ie reduce crankcase pressure  may reduce (??) oil being ejected out the breather.

 

The Rotax Owner Forum thread was about relapsing, the crankcase oil return to tank,  Banjo fitting, as a possible restriction in the oil flow = higher crankcase pressures.

 

The replacement fitting,  a low restriction bend rather than the 90 degree of the Banjo, may be less restrictive, which may reduce the crankcase pressure, which in tern may reduce oil exiting the breather.

 

It would seem 912 operators, with perfectly good engines,  have mixed experince of oil exiting the breather. Some do most don't. The experts are unable to explain why. Speculate small differences in piston  ring sealing.

 

This is an experiment to see what happens. Hence I do not want to spend big $$ on something I may just return to standard (refit Banjo).

 

What do you think?

 

😈

[Thruster88]

I would guess any oil vapour from the crankcase is carried by the flow rate and not pressure. Reduced crankcase pressure may even increase the flow rate.

 

Carbon cubs have a neat way of eliminating oily belly, they direct the crankcase breather into the exhaust system.

[sfGnome]

Ok, dumb question (dumb, bearing in mind your extensive experience). Is there a hole in the side of the breather tube so that oil isn’t being pulled out by the Venturi effect?

[skippydiesel]

12 hours ago, sfGnome said:

Ok, dumb question (dumb, bearing in mind your extensive experience). Is there a hole in the side of the breather tube so that oil isn’t being pulled out by the Venturi effect?

Not a hole - It has a T piece with a tube going to an area of high pressure within the cowling - same/similar concept to the hole.😈

[skippydiesel]

14 hours ago, Thruster88 said:

I would guess any oil vapour from the crankcase is carried by the flow rate and not pressure. Reduced crankcase pressure may even increase the flow rate.

 

Carbon cubs have a neat way of eliminating oily belly, they direct the crankcase breather into the exhaust system.

Seems to me that this would exacerbate any tendency to excessive oil consumption via the venturi effect on he oil breather.😈

[skippydiesel]

15 hours ago, Thruster88 said:

I would guess any oil vapour from the crankcase is carried by the flow rate and not pressure. Reduced crankcase pressure may even increase the flow rate.

 

 

 

It seem to me that the more pressure, applied to a liquid, exiting a fixed hole, the higher the flow rate will be.

The Rotax system of oil return (crankcase to tank) is using the crankcase pressure as the "pump" to achieve this.

 

Speculation;

• I imagine the oil, combined with crankcase gas "spurts" (technical term) into the tank ie is not a nice continuous flow. This action will cause some oil to be suspend (droplets) as a mist, adding to the oil mist from the crankcase.
• The intention is for the oil to separate /drop out from the gas, within the tank. To a large extent this is happening.
• In my installation too much of the suspended oil (for my liking) is continuing past the tank and into the breather - from there deposited on the belly (& increasing oil consumption).
• If I can reduce the pressure, by improving the crankcase exit flow, less mist may be generated and more of the suspended oil may drop out in the tank.
• If I can do the experiment at minimal cost and it works, all good ! If it doesn't not too much has been lost.

😈

[facthunter]

The amount of blowby past the rings is the MAIN Factor. New engines have to be turned over slower to get the Oil to Burp. You can feel that when you pull the prop through.. Nev 

[facthunter]

IF you don't understand what I'm saying I will answer any questions. But Maybe not tonight. Late for dinner is not tolerated here. Nev 

[Thruster88]

On 18/08/2026 at 8:16 PM, sfGnome said:

Ok, dumb question (dumb, bearing in mind your extensive experience). Is there a hole in the side of the breather tube so that oil isn’t being pulled out by the Venturi effect?

 

22 hours ago, skippydiesel said:

Not a hole - It has a T piece with a tube going to an area of high pressure within the cowling - same/similar concept to the hole.😈

I believe the real purpose of the hole or T piece is to provide an alternative exit incase the end of the breather tube extending out of the cowl becomes frozen in very cold weather. Engines breath out water vapour. 

 

22 hours ago, skippydiesel said:

Seems to me that this would exacerbate any tendency to excessive oil consumption via the venturi effect on he oil breather.😈

It must work, these are lycoming powered factory built certified aircraft. 

[Thruster88]

Out of curiosity I googled the flow rate of the 912 oil pump, 16 litres per minute at 5500rpm. Seems like a lot.

 

I then asked Ai this hydraulic question about suction hose size.

 

I can see why rotax wants the oil hot before high rpm operation.

 

 

Overview

 

 

For a hydraulic suction line operating at a 16 LPM flow rate and a length of 0.8 meters, you require a hose with a minimum Inner Diameter (ID) of 3/4 inch (19.05 mm), also known as a Dash -12 size.

Sizing Analysis & Criteria

In hydraulic systems, keeping fluid velocity low in the suction line is critical to prevent pump cavitation. Industry engineering standards recommend a suction line fluid velocity between 0.6 m/s and 1.2 m/s. [1]

The fluid velocities for standard hose sizes at 16 LPM are calculated as follows:

Hose Size (Dash)	Inner Diameter	Fluid Velocity	Recommendation
Dash -8	1/2" (12.7 mm)	2.11 m/s	Too Small (Causes high friction & risk of cavitation)
Dash -10	5/8" (15.9 mm)	1.34 m/s	Borderline (Slightly exceeds recommended velocity)
Dash -12	3/4" (19.05 mm)	0.94 m/s	Ideal Fit (Perfectly within the 0.6–1.2 m/s target)
Dash -16	1" (25.4 mm)	0.53 m/s	Oversized (Safe, but unnecessary for a short 0.8m run)

[skippydiesel]

2 hours ago, Thruster88 said:

 

I believe the real purpose of the hole or T piece is to provide an alternative exit incase the end of the breather tube extending out of the cowl becomes frozen in very cold weather. Engines breath out water vapour. 

 

It must work, these are lycoming powered factory built certified aircraft. 

Rotax & Lycoming, both IC engines, have some every diffrent features;

 

I am sure the  breather "notch" /hole would assist in an icing situation however it's primary purpose is to reduce the chance of the breather acting as a venturi ie negative pressure

 

I  am hazy on Lycoming detail; (assuming a dry sump arrangement) I think it likly that they use an oil pump to take oil from the crankcase to the remote oil tank. The breather is connected directly to the crankcase removing excess pressure, combustion gas, oil mist & volatile fractions.  If I am correct there is likly no significant problem with the crankcase breather experiencing negative pressure via a venturi in the exhaust.

 

Rotax use piston compression, to transfer crankcase oil to the remote tank (no pump). There is no direct connection between the crankcases & the breather. The breather removes pressure, etc from the oil tank. If you create negative pressure in the breather you may draw more oil from the system than intended. Rotax specifically caution against this.

😈

Edited August 19 by skippydiesel

[skippydiesel]

1 hour ago, Thruster88 said:

 

For a hydraulic suction line operating at a 16 LPM flow rate and a length of 0.8 meters, you require a hose with a minimum Inner Diameter (ID) of 3/4 inch (19.05 mm), also known as a Dash -12 size.

 

 

As always Thruster, you deliver interesting facts on which to ponder.

 

I have never questioned Rotax hose sizing specification's - So far so good, in that to the best of my knowledge, the two 912 ULS that I have owned/maintained, have not suffer oil delivery / supply issues despite using 12 m (1/2") ID oil hose.

 

I don't doubt your theoretical analysis, but at the end of the day - what work, works.

 

Aside from satisfying curiosity (always good) how does this relate to my proposed experiment?

 

😈

[facthunter]

It's not piston compression used to Push the Oil up the return Pipe. It's what gas leaks past the Piston rings that Pushes it back to the Oil tank and all gasses pass through to the Breather pipe on the side of the Tank. High rate out of there means more blowby.  Also if you use a larger dia return pipe Gas will pass the Oil up one side and the oil won't go through the Pipe as intended. . Nev

[skippydiesel]

Attention ALL

Before I go ahead & install the, possibly higher flow crankcase drain elbow, does anyone foresee any negative issues? / problems?😈

[skippydiesel]

On 24/08/2026 at 10:22 AM, skippydiesel said:

Attention ALL

Before I go ahead & install the, possibly higher flow crankcase drain elbow, does anyone foresee any negative issues? / problems?😈

Well all the necessary bits are now on hand - Dies anyone have a word of caution/encouragement 😈

[Thruster88]

1 hour ago, skippydiesel said:

Well all the necessary bits are now on hand - Dies anyone have a word of caution/encouragement 😈

Make sure the oil return hose has enough lenth / route to allow the engine to move in it's mounts. It would be a shame if the B nut on the elbow came loose. 

[skippydiesel]

45 minutes ago, Thruster88 said:

Make sure the oil return hose has enough lenth / route to allow the engine to move in it's mounts. It would be a shame if the B nut on the elbow came loose. 

I will probably use the existing hose, that terminates in the Banjo fitting. It is likly a bit too long but better that, than short.

 

I intend to safety wire as much as possible.  Can't afford to have the new arrangement come loose in the air.

 

😈

Edited Thursday at 09:31 AM by skippydiesel