Mac Bowden Posted January 21, 2012 Report Share Posted January 21, 2012 Was carrying out my 500 hour checks and noticed that the engine was able to be rocked relative to the small engine mount. Discovered that 2 10mm bolts diagonally opposite each other were slack! When all was tightened up I noticed that the slight harmonic vibration, more like a grumble, I experienced at about 4100 revs had changed to about 3500 revs and the engine seemed slightly harsher at idle 1800 revs. Have re-balanced carbs etc but I suspect it's the rigidity of the mounting system. However, I notice that my propellor spinner is now a lot higher than before with little spinner boss showing below the cowling and yet the down angle and off-set are the same as before, 5 degrees down and 2.5 degrees right. When I look through my album of CT pix I see quite a variation as to the alignment of the propellor to cowling.Is it just a case of engineering tolerances? Link to comment Share on other sites More sharing options...
Roger Lee Posted January 21, 2012 Report Share Posted January 21, 2012 Hi Mac, Three things to do, but it will be the last one that contributes more to the sag. First tighten those 4 engine ring mount bolts to 30 ft/lbs. One in each corner of the engine mount. It's an 8mm Allen head. Two back under the carbs and two down below behind the coolant hose. They are easily accessible. FD had these originally torqued to 22 ft/lbs and they all started to come loose. Then they put out an SD dated June 2005 which they increased the torque to 26 ft/lbs (35NM) and up to 30 ft/lbs. Rotax recommends 30 ft/lbs (Part Manual, section 28 page 28-3). See attachment and item #2 on the drawing and the second page item #2. Technically these are supposed to be checked every inspection. I have always used 30 ft/lbs and since torquing them to 30 ft/lbs I have never had one ever loosen. Now tighten the 6 engine mount bolts that go through the fire wall (3 down each side) to 200 in/lbs. After these are done then the last thing that is usually the cause can't be fixed unless you pull the engine. This is one of the big reasons I tell people to pull the engine at hose change. Taking shortcuts doesn't pay. The rubber engine mounts that can't be changed with the engine on get crushed and cracked under the weight and these will contribute to the engine misalignment. Always change the 16 engine mount rubber isolators during the hose change and torque those bolts to 200 in/lbs. Change one set at a time and remember that the aluminum tube that runs inside each set of rubber isolators is a different length to help with the engine alignment. If you pull them all out and mixed them it would be a major issue. If you did manage a major screw up then the correct pattern and lengths can be found in the FD parts manual. p.s. The only other thing that can cause engine misalignment is a really hard landing and a slight and maybe unnoticeable tweak to the engine frame. p.s.s. Every mechanic should have all 5 Rotax maint books as they all have info they need and some have info the other may not. If they haven't attended a Rotax school most would not know to look in the Parts Manual for special torques on special items. p.s.s.s. So long as all the nuts and bolts listed above are tight and an engine mount isn't bent then that little bit of sag since it was new won't hurt a thing. You still have good clerance. Engine ring mount bolts.pdf Link to comment Share on other sites More sharing options...
Mac Bowden Posted January 21, 2012 Author Report Share Posted January 21, 2012 Roger, thanks for the PDF file , I'll add it to the collection. My situation is the opposite of sag, in a different time and place that sounds like boasting, the mount seems about 15-20mm too high and there's definitely no slack in the firewall mounting holes to allow for any vertical movemment to lower the engine slightly. Maybe it's just a cowling issue. Link to comment Share on other sites More sharing options...
Roger Lee Posted January 21, 2012 Report Share Posted January 21, 2012 Hi Mac, Until proven otherwise all nuts or bolts are considered hand tight, but wrench loose. Any shift in the engine position can be caused by the items I listed, but also a hard strike at landing. A tiny bit of movement or slack that is unnoticeable in each one of these will be multiplied in distance out at the prop side where you can actually see a difference out by the prop. I would still put a wrench on all of these. Link to comment Share on other sites More sharing options...
Mac Bowden Posted January 21, 2012 Author Report Share Posted January 21, 2012 Hand tight wrench loose, I like the term. I have a purpose made trestle for under the belly of the airplane and when sitting on it with the front wheel assembly off the ground i'll loosen the bolts to take out any cumulative slack. Watch this space. By the way, HARD LANDING, what's that ? Link to comment Share on other sites More sharing options...
Al Downs Posted January 24, 2012 Report Share Posted January 24, 2012 Hi Mac, Three things to do, but it will be the last one that contributes more to the sag. First tighten those 4 engine ring mount bolts to 30 ft/lbs. One in each corner of the engine mount. It's an 8mm Allen head. Two back under the carbs and two down below behind the coolant hose. They are easily accessible. FD had these originally torqued to 22 ft/lbs and they all started to come loose. Then they put out an SD dated June 2005 which they increased the torque to 26 ft/lbs (35NM) and up to 30 ft/lbs. Rotax recommends 30 ft/lbs (Part Manual, section 28 page 28-3). See attachment and item #2 on the drawing and the second page item #2. Technically these are supposed to be checked every inspection. I have always used 30 ft/lbs and since torquing them to 30 ft/lbs I have never had one ever loosen. Now tighten the 6 engine mount bolts that go through the fire wall (3 down each side) to 200 in/lbs. After these are done then the last thing that is usually the cause can't be fixed unless you pull the engine. This is one of the big reasons I tell people to pull the engine at hose change. Taking shortcuts doesn't pay. The rubber engine mounts that can't be changed with the engine on get crushed and cracked under the weight and these will contribute to the engine misalignment. Always change the 16 engine mount rubber isolators during the hose change and torque those bolts to 200 in/lbs. Change one set at a time and remember that the aluminum tube that runs inside each set of rubber isolators is a different length to help with the engine alignment. If you pull them all out and mixed them it would be a major issue. If you did manage a major screw up then the correct pattern and lengths can be found in the FD parts manual. p.s. The only other thing that can cause engine misalignment is a really hard landing and a slight and maybe unnoticeable tweak to the engine frame. p.s.s. Every mechanic should have all 5 Rotax maint books as they all have info they need and some have info the other may not. If they haven't attended a Rotax school most would not know to look in the Parts Manual for special torques on special items. p.s.s.s. So long as all the nuts and bolts listed above are tight and an engine mount isn't bent then that little bit of sag since it was new won't hurt a thing. You still have good clerance. Roger is there a time you would be available to discuss a similar problem I have on my CTLS? Let me know and I will give you a call. Thanks Al Downs Link to comment Share on other sites More sharing options...
Roger Lee Posted January 24, 2012 Report Share Posted January 24, 2012 Hi Al, I will be at the airfield the next 2.5 weeks, I have 3 hose changes and firewall blankets to do. Better to call me after 1730 hrs Tucson time when I'm home. Link to comment Share on other sites More sharing options...
Al Downs Posted January 24, 2012 Report Share Posted January 24, 2012 Hi Al, I will be at the airfield the next 2.5 weeks, I have 3 hose changes and firewall blankets to do. Better to call me after 1730 hrs Tucson time when I'm home. Thanks Roger I will call you this evening. Link to comment Share on other sites More sharing options...
Mac Bowden Posted January 25, 2012 Author Report Share Posted January 25, 2012 Result of today’s maintenance. Front of plane lifted onto trestle placed just aft of firewall leaving the front wheel 4” off the floor. Cowlings removed and all 6 off 8mm X 60mm mounting bolts slackened resulting in hardly a millimetre of drop with the engine assembly! Other than the engine assembly moving forward whilst the bolts were slack nothing else moved. The metal bushings bonded into the firewall are so well matched to the diameter of the bolts that there is no room for vertical movement. Torqued everything back up and the spinner to cowling position is exactly where it was before I started. Typical German engineering. Lesson learned? Leave it well alone and go flying instead. Link to comment Share on other sites More sharing options...
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