Coolant expansion tank cap leaking?

The metal nipple on the expansion tank (yes Sean, that’s what Rotax calls it ?) excluding the flange barb, measures 8.2 mm Outside Diameter. So the 8mm OD hose specified in the Install Manual cannot possibly work. Am I missing something or am I the first person to catch this error in 35 years of Rotax installations? The overflow hose that came with my Zenith Firewall Forward kit that has served me well for four years and 175 engine hours measures 9.75mm OD and 8mm Inside Diameter. That’s what I’m headed to the auto parts store to buy.  

We've seen our share of mistakes in the manuals over the years. The line that most people typically use is a fuel line with an 8mm or 5/16th-inch inner diameter. We'll take note of this error and ensure that the Rotax factory is informed to make the necessary amendments in the next manual release.

Upon close inspection this morning I found a tiny lump of calcified (not sure if that’s the correct word- it was dried, hard antifreeze) sitting atop the upper shelf ring inside the expansion tank, this is where the rubber gasket on the bottom of the radiator cap contacts the tank neck shelf ring to seal the tank.  My expansion tank does not perch perfectly level atop the engine and this little lump had formed over four years of use at the low spot where coolant tends to gather. I had to remove it by gently scraping with a flat metal blade and wiping up the residue before it fell into the coolant. I then cleaned the inside of shelf and the cap, removed the overflow tubing, snipped off the ends and reattached the tubing, securing it carefully with homemade safety wire clamps. As soon  as the winds permit, I will go fly, bring it up to temp, then return and report back as to what I find.

Jim Isaacs wrote:

The metal nipple on the expansion tank (yes Sean, that’s what Rotax calls it ?) excluding the flange barb, measures 8.2 mm Outside Diameter. So the 8mm OD hose specified in the Install Manual cannot possibly work. Am I missing something or am I the first person to catch this error in 35 years of Rotax installations? The overflow hose that came with my Zenith Firewall Forward kit that has served me well for four years and 175 engine hours measures 9.75mm OD and 8mm Inside Diameter. That’s what I’m headed to the auto parts store to buy.  

Hi Jim,

In my 13 or so years enjoying working on my two Rotax 912 ULS, I have never taken note of the Rotax specification for the "overflow" hose.

In this time I have replaced the hose, at the 5 year rubber replacement, with a hose of a suitable ID (8mm) designated  automotive heater hose - never had a problem.

Always had a problem with using any form of hose securing clip/clamp at the radiator cap end (I refuse to call it an expansion tank) so like you have used a double wrapped safety wire - no problems.

I am intrigued by the build up of coolant solid in/on your system - are you using a glycol : water  mixture?

In my pervious engine I used  Castrol Redicool (green), now use Castrol Redicool PO (red).  In both cases I mixed the concentrate with purified water at the recommended 50:50 ratio (don't trust the premixes on offer), changed the fluid every 2-2.5 years - no build up of coolant solids (precipitate?)

Sean I’m using a common auto parts store national brand name antifreeze 50/50 premix. I’ve not had an issue in four years until now. Don’t plan to change the coolant until next year when I replace all the hoses at the five year change out when it all needs to drain out anyway.