Monday, June 27, 2022

Complying With Rotax’s 5 Year Time Limit For Rubber Coolant Hoses

In section 05-10-00 of the Rotax 912 Line Maintenance Manual there are considerations for time limits for various Rotax engine components … most notably, the table for all the rubber components which Rotax suggests replacing every 5 years. As can be seen below, Rotax wants a lot of stuff replaced.

The Rotax 5 year time limit table for rubber parts.

Highlights of some of the items that require replacement are: All the rubber hoses used in the cooling system, fuel and oil hoses that are not Teflon, carburetor mounting sockets, carburetor diaphragms, carburetor and fuel pump vent hoses, plus the fuel pump.

This post only covers replacement of the rubber coolant hoses. The decision was made to follow the path quite a few RV-12 owners have taken … which is to NOT replace the rubber coolant hoses using rubber hoses from Rotax. Instead, silicone hose will replace the rubber hose because, unlike rubber hose, silicone hose does not harden or crack and will last a very very long time … so 5 years from now, the silicone hoses will not need to be replaced. (As a side note for fellow RV-12 owners, silicone hose is great for coolant … but definitely should NOT be used for fuel or oil applications). I was able to locate some high quality silicone hose in a green color which color coordinates nicely with the green paint on the engine’s valve covers and the green color I panted the fiberglass air shroud on top of the engine. The silicone hose was purchased through HPCouplers (.com) and is Flexfab 5521-062 for the 5/8" hose going to and from the cylinder heads. Flexfab 5521-038 was purchased for the 3/8" hose going to the expansion bottle mounted on the firewall. Flexfab 5521-100 was purchased for the two 1" hoses to and from the radiator.

Work begins by draining the coolant from the cooling system. The Rotax manual suggests draining the engine coolant by removing the lower bolt from the water pump. This is much easier said than done on a completed RV-12 … and I can tell you with all honestly, I will NEVER remove that bolt again to drain the engine coolant. It is just very difficult to get onto with a wrench … and once you do get a wrench on it, there really is not enough room to turn the bolt. After fighting with the lack of access and trying various combinations of tools to remove the Allan head bolt, I got to the point where I could break the bolt lose but not able to do much with it after that. I finally gave up and waited to remove it when the engine was jacked up while completing the nose gear leg replacement service bulletin … (which was being completed at around the same time). The only tool combination that seemed to work well for removing the lower water pump bolt was using a wrench slid over the shaft of a hex socket as shown in the photo below.  Hindsight being 20/20, it is truly far easier to just pop off one of the lowest two coolant hoses going to either cylinder #1 or cylinder #2 … or just remove the lower hose on the radiator.

While there was more clearance from having the engine hoisted and the lower radiator hose removed, a metric hex socket and a wrench was used to remove the lower water pump bolt to drain the engine coolant. Prior to hoisting the engine and removing the lower radiator hose I figured this bolt was a no go because access to the bolt just plain sucks on a RV-12.

The single ply rubber hoses leading to and from the cylinder heads on the Rotax 912ULS are 17mm … good luck finding that size in a silicone hose here in the USA. The closest fitting silicone heater hose is 5/8" which equates to 15.88mm … so the silicone hose is stretched out a little to fit onto the existing coolant fittings. Also of note, the outer diameter of the 5/8" silicone heater hose is slightly more than that of the 17mm rubber hose Rotax uses … this creates challenges, because the Rotax constant tension hose clamps just barely slide over the silicone hose once the hose is slid onto the coolant fittings attached to the Rotax 912’s engine. Some owners have opted to order slightly larger hose clamps from McMaster-Carr … but if you are really patient and lube the hose with coolant, the Rotax clamps will get the job done.

The coolant hose replacement began in earnest by replacing all the lower coolant supply hoses between the water pump and the cylinder heads. For easier access, the muffler was removed from Rotax 912ULS to gain unrestricted access (that’s laughable) to the lower coolant hoses. Prior to removing the old coolant hose, a red Sharpe was used to mark the current position of each coolant hose on the cylinder head fittings so the new silicone hose could be placed in the same position on the cylinder head coolant fittings.  Next, each rubber hose was carefully removed and used as a template to measure the replacement silicone hose.

The tool used to make all the necessary hose cuts was a Craftsman Handi-Cut. It makes nice, clean, smooth cuts. If you have access to a Handi-Cut or similar tubing cutter, use it.

Photo of the Craftsman Handi-Cut used to cut the silicone coolant hose.

Reinstalling the silicone hoses on the right side of the engine went fairly smooth. The inside of the silicone hose and the outside of the coolant pipe fittings were wet down with a little coolant to make them a little slick so the silicone hose could be positioned onto the fittings easier. Once the silicone hose was in position on the fittings, the constant tension clamps were slid into position on the hose. Discovered it also helps to wet the outside of the silicone hose with a few drops of coolant, so the constant tension clamps slide into position easier … this is because the silicone hose is slightly thicker, so the clamps tend to drag over the hose, especially at the bead end of the fittings.

Replacement lower silicone coolant supply hoses for cylinders #1 & #3 attached to the water pump.

Replacement lower silicone coolant supply hoses for cylinders #1 & #3 attached to the cylinder head coolant pipes.

Replacement lower silicone coolant supply hoses for cylinders #2 & #4 attached to the water pump.

Replacement lower silicone coolant supply hoses for cylinders #2 & #4 attached to the cylinder head coolant pipes.

A few years back, the DOG Aviation procurement department purchased a special locking cable tool made for the type of constant tension hose clamps used by Rotax. This tool works well for the most part … just as long as there is plenty of clearance to use it. However, there are a few locations where there just simply was not enough room to use it. For the really tight locations I needed to contact Chad, the local A&P mechanic at the airport, who graciously allowed me to borrow his Tool Aid hose clamp pliers. Thanks Chad!! After using Chad’s tool, I immediately ordered a set for myself.

The special locking cable tool made for constant tension hose clamps works well to expand a constant tension hose clamp and hold it expanded, as can be seen here … just so long as there is plenty of access room. The head of the tool takes up quite a lot of room, so there are numerous locations where it just didn’t work well.

This is one of the two Tool Aid 19750 hand tools for constant tension clamps borrowed from Chad. This tool was used at locations that did not allow good access to use the cable tool … it has cups on the end of the tool that capture the clamps. The cups rotate, so the tool can be used either horizontally or vertically … or any angle in between.

After using Chad’s clamp pliers, the DOG Aviation procurement department ordered the Tool Aid 19750 hose clamp pliers set. The pliers with the large cups worked great for the Rotax constant tension clamps on the DOG Aviation RV-12. Have also recently used the other pliers on smaller hose clamps.

The Tool Aid 19750 hose clamp pliers set. The upper pliers in the photo worked quite well on the Rotax hose clamps and the lower pliers work well on smaller sized hose clamps.

All and all changing the four lower water hoses was not too big of a deal … other than receiving a NASTY DEEP cut on my thumb (darn that hurt!) when the hose clamp pliers slid off one of the clamps when my hand was over the pliers (dumb) guiding the clamp between the lower engine mount triangle and onto the cylinder #2 water pump fitting.

Moving on to the upper hoses … NOTE: I have two suggestions for those about to replace the upper cylinder head return coolant hoses. First, because this is a mandatory 5 year rubber replacement, the rubber carb sockets will also need to be replaced … meaning the carbs will need to be removed also. So, make life easy for yourself and remove the four hex bolts on each intake manifold so the intake manifolds can be removed to allow for easy access to the hose clamps under them for the #3 & #4 cylinders (don’t forget to order 4 new O-Rings for the manifolds, Rotax part number 230-910). Second, my other suggestion is to remove the water tank, attach the four new silicone hoses while at the work bench, then fish the tank back into position … trying to slide the hoses onto the tank when it is on the engine is very difficult because there is very little room for hands and tools. Just remove the tank, you will be glad you did.

As work began on the upper hoses, things got a lot more interesting and much more difficult. While working on replacing the four upper coolant return hoses that connect the cylinder heads to the water tank, I ran into two roadblocks … the first being Rotax uses a close to 90° pre-molded rubber coolant hose between the coolant tank and cylinder #3. So, the DOG Aviation procurement department purchased a pre-molded 5/8" 90° silicone hose to use at this location. Decided to begin the fun by changing the #3 cylinder’s hose first figuring the pre-molded hose will set the position of the coolant tank. For starters, it was very difficult to slide the pre-molded hose onto the coolant fitting on the #3 cylinder head and it got worse from there. Even after removing the fitting from the cylinder head so I could hold the fitting in my hand, it was still a fight to slide the hose all the way onto the fitting … but eventually it did go on.  OK … so far so good. This is where the wheels totally fell off …. the pre-molded 90° hose that was purchased has very little stretch because of the 4 layers of reinforcement fibers used internally, and it absolutely just REFUSED to slide onto the coolant tank. I measured the beads on the tubes coming out of the coolant tank and they measure 20mm … obviously beyond the stretching ability of the four ply pre-molded hose. 

I tried all the tricks too, removed the coolant tank from the engine so I could get more leverage on it and lubricated the pre-molded 90° hose and tube with coolant … nope. Lubed the hose with liquid BoeLube … nope. Tried slippery CorrosionX .. nope. Even brought the coolant tank and the pre-molded silicone hose home so I could put the hose into boiling water in the hopes of getting a little more flex … nope. That four ply pre-molded 90° hose was just not going to slide onto the coolant tank pipe … period. Sadly, being furious about all the screwing around with the 90° pre-molded silicone hose, it was tossed into the trash before I realized no photos were taken. Can't say for sure but suppose if the 90° pre-molded hose had been one or two ply, it may have worked out OK.

Moving on, after checking the actual bend radius of the Rotax pre-molded hose and comparing that to the specifications for maximum bend radius of the silicone hose, discovered the necessary bend was well within the bend radius limits given for the 5/8" Flexfab hose … so just installed a straight hose to cylinder #3. As an insurance policy, although not necessary, a Goodyear E-Z Coil was also installed over the #3 cylinder’s hose as an insurance policy to prevent the hose from kinking … and a bed of silicone was placed under the E-Z coil to prevent it from wearing into the air shroud under it.

The second roadblock I discovered involved trying to slide the Rotax constant tension clamps over the 20mm bead on the end of the coolant tank pipes. The Rotax clamps were slid onto the hose far back from where the pipe bead will end up … then the hose was slid onto the tank’s pipe. Even with the Rotax clamps expanded as far as they would go, the clamps refused to slide over the beads on the coolant tank. The very time-consuming solution was to place the clamps onto the coolant tank’s pipes first then slide the hose over the bead up to the clamps and then slowly work the clamps over the hose in stages … slide hose on a little, slide clamp up a little and repeat until the hose was fully seated and the clamp in its proper position. I was doing this by myself so it took FOREVER … but, had I had a helper, it would only have taken a few minutes. This chore REQUIRES a helper if you want it to go smoothly using the Rotax clamps. Slightly larger hose clamps are available from McMaster-Carr and this is one area where they may have been the way to go … but I really wanted to use the Rotax clamps to keep the parts original except for the hose itself.

Coolant tank with all the new silicone coolant hoses installed ready to be fished back into position on top of the engine. Note the use of the Goodyear E-Z Coil to prevent the hose from kinking when it takes the 90° bend to cylinder #3.

To add another level of difficulty, the cylinder head coolant return pipes for cylinders #3 & #4 reside directly under the intake manifolds. Getting good access to those hose clamps requires removing the carburetor/ intake manifold assemblies … which is what was done. Not saying that the manifolds must be removed … however removal is easy, so why not just remove the four easy to access hex bolts and the manifold is off in a minute (don’t forget to replace the two rubber O rings under each manifold Rotax # 230-910). It is hard to photograph this area and get a good photo … but as an example, looking closely at the photo below of the intake manifold, one can see the new green silicone hose and black clamp tucked under the manifold.

Looking closely at this photo, one can see that the coolant return hose runs under the intake manifold. Getting to the hose clamp is tough so removing the intake manifolds makes accessing the hose clamps for cylinders #3 & #4 much easier.

Looking closely, the Goodyear E-Z Coil can be seen installed over the coolant hose for the #3 cylinder. The coil will prevent the silicone hose from kinking along the bend.

To complete the coolant hose replacement job, both the upper and lower radiator hoses were replaced with 1" ID Green Flexfab 5521-100. After both original hoses were removed from the radiator, the coil springs inside each rubber hose need to be extracted and inserted inside the new 1" silicone hoses (lower hose is 30" long and the upper is 20" long). The hose between the coolant tank and the expansion tank mounted on the firewall was also replaced with 3/8" Flexfab 5521-038. I elected not to use traditional radiator hose clamps on the 1" silicone hose because they tent to cut into hose. Instead, a form of constant tension hose clamp was used that is smooth on the inside along the entire area where the clamp rides on the hose and it has Belleville washers that allow the clamp to expand and contract with radiator temperature while keeping a constant tension on the hose … the clamps were purchased from McMaster-Carr.

Close-up photo of the new constant tension hose clamps with the Belleville washers used on the 1" silicone hose. The Belleville washers allow the clamp tension to remain the same as the clamp expands and contracts with coolant temperature changes.

Because the nose wheel gear leg assembly was replaced with Van's newly redesigned stronger version, the mounting for the lower radiator hose is done differently. The original gear leg normally has a hole drilled into it where a single Adel clamp is mounted to support the lower radiator hose. The new nose wheel gear leg assembly no longer has that mounting hole ... now the lower radiator hose is supported via two Adel clamps, as shown in the following photo.

As can be seen in this photo, the new Van’s nose wheel gear leg supports the lower radiator hose via two Adel clamps.

The coolant used to refill the cooling system was Dex-Cool mixed 50/50 with distilled water. Dex-Cool is on the Rotax list of recommended coolant for the Rotax 912ULS engine. All and all except for the nasty gash to my thumb, replacing all the coolant hoses was not all that hard once all the challenging aspects of the task were figured out … but it was a royal pain in the derriere to be sure.