Subject is metal and has a center disc that appears to be made of brass or similar. What function does this center disc serve? Is this disc supposed to vent air and coolant? I ask because my cap’s center disc will vent bubbles of coolant from the edge if I plug my overflow bottle vent and squeeze the bottle, forcing coolant to flow from the overflow bottle back up the tubing and into the expansion tank with the cap installed. I did this squeeze test because for the first time in four years, the morning after a flight my 912UL expansion tank was nearly empty and my overflow bottle was nearly full.  It’s normally the other way around. I suspect I need a new cap?

  • Re: Coolant expansion tank cap leaking?

    by » 6 months ago

    The expansion tank is sealed with a pressure cap that includes a pressure relief valve and a return valve (brass disc with a spring). When the coolant heats up and expands, the pressure relief valve opens, allowing the coolant to flow through a hose into the transparent overflow bottle at atmospheric pressure. When the coolant cools down, it is drawn back into the cooling circuit.

    It's probable that the hose connecting the expansion tank to the overflow tank has become pliable and is collapsing under negative pressure when the coolant is drawn back into the expansion tank. While it doesn't encounter any issues pushing coolant through the hose, it collapses when drawing coolant. It would be advisable to consider replacing both the expansion tank pressure cap and the connecting hose between the two tanks.

    The part number (P/N) for a new Rotax 1.2 BAR pressure cap is 922072.

    Rotax doesn't supply the hose that leads from the expansion tank to the overflow bottle, but they do provide specific requirements for the hose:

    • The hose from the expansion tank to the overflow bottle must be rated for vacuum/suction for a minimum of 125°C (257°F), ensuring it can withstand high temperatures and vacuum/suction during the cooling down period.
    • The hose should have an outside diameter of 8 mm (3/8") and a maximum slip-on length of 15 mm (9/16").
    • To ensure proper operation, it's essential to keep the hose to the overflow bottle as short as possible.
    • The suction height between the overflow bottle and the expansion tank should not exceed 250 mm (10 inches).

    For more information, please refer to the Rotax 912 installation manual available here: Rotax 912 Installation Manual and consult Rotax Service Instruction SI-912-020R10 and SI-914-022R10, which can be found here: SI-912-020R10.

    Thank you said by: Jim Isaacs

  • Re: Coolant expansion tank cap leaking?

    by » 6 months ago

    Thank you! Odd that the IM calls out an outside diameter for the tubing, I’m used to seeing inside diameter specifications…guess I’ll have to go pull the tubing off and measure the nipple diameter on the expansion tank.

  • Re: Coolant expansion tank cap leaking?

    by » 6 months ago

    Hi Jim,

    Certainly sounds like you may need a new cap (may be cheaper from your local Ducati motorcycle shop).

    On the subject of Rotax naming/terminology;

    The so called "expansion tank" over the top of the crankcase is not an expansion tank. The expansion tank is the overflow bottle usually mounted on the firewall.

    How the system works;

    In my youth radiators were only filled to just above the fins. This left a large air space above the coolant surface, to accommodate the expansion of the coolant, as it was warmed by the engine. If the air space was too small (to much heat or coolant) the coolant would be vented by the cap (steam!). When the system cooled, a significant internal vacuum would be created. This often caused radiator hoses to collapse, hence the use of internal spring or wire reinforced hoses.

    Rotax cooling systems are as for modern vehicles - that is a zero coolant loss system, using an external expansion tank (overflow bottle) to accommodate excess hot fluid.

    The function of the radiator style cap is three fold;

    *Increase the boiling point of the coolant in the system by initially preventing hot coolant from escaping. As the coolant warms, pressure increases, this raises the boiling point of the fluid. The cap contains a calibrated spring/valve which will allow fluid to escape when reaching a predetermined pressure.

    *Prevent excessive pressure built up in the cooling system, by releasing hot coolant into the overflow bottle (expansion tank).

    * When the coolant temperature drops (cruise/descent/ taxi/shutdown), a partial vacuum is created within the system. The cap valving then allows coolant to return, from the overflow bottle, into the cooling system. Rotax coolant hoses should not collapse.

    A correctly functioning system should not have air within ie when you open the radiator cap (cold engine), the coolant level should be at the inner lip of the fill point. Air in the system can only be caused by a leak or incomplete filling when coolant changed.

    The modern cooling system is both lighter and smaller than the old (good for aircraft ).

    The overflow bottle cap should have a small hole, to allow it to be at atmospheric pressure, air to escape/return, as fluid level rises & falls and if excess fluid enters, for the fluid to be vented.

    The overflow bottle should have a cold coolant level of approximately 1/4- 1/2 bottle volume. This will increase with a hot engine so always leave a generous space above the coolant level. Should the bottles coolant level drop (using coolant) to the bottom, exposing the return line , air will be drawn into the cooling system reducing its efficiency.

    In my experience, the Rotax cooling system should not need topping up between services and even then the amount (into the overflow bottle) should be very small.


  • Re: Coolant expansion tank cap leaking?

    by » 6 months ago

    (just lost my reply - second go)

    Sorry RO - I must have been writing my long winded response as you were replying far more concisely.

    On your theory;

    "It's probable that the hose connecting the expansion tank to the overflow tank has become pliable and is collapsing under negative pressure when the coolant is drawn back into the expansion tank."

    I can not agree.

    Assuming that there's an adequate (not too small/blocked) hole in the overflow bottle cap, there will only be a very small posative/negative pressure influence on the return line. The return line would need to be of very soft or degraded material for it collapse under such a small pressure change.

  • Re: Coolant expansion tank cap leaking?

    by » 6 months ago

    Hello all

    Just an observation.  I have seen, admittedly only one time, the plastic overflow bottle had the fill-line hole plugged at the bottom of the bottle. (factory Rotax bottle)  In checking more it was due to overheated fluid melting the plastic and closing off the line at the nipple.  It was just like you said, full bottle but nothing being sucked back on engine shutdown.  You can pull the cap and take it to most motorcycle shops and they can test your cap for leakage and to see if the vent valve to allow return of the fluid on a cooling engine is working.  


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