Rotax 503 Overhaul:

Rotax recommends an overhaul on 300 Hours or 5 years of operation.

After 435 Hours and 7 years of perfect operation I decided to open up my 503 and document the process.

(Click on any Picture for a larger image)

My motor has 435 Hours on it and it has never skipped a beat. It always starts after priming on the first or second flick of the prop by hand. Some say I’m Crazy to open up a perfectly good running motor and that these little 503 motors are bullet proof and can easily do a 1000 Hours!

Well here are my reasons: My motor is 7 years old and I’m starting to see that some of the rubbers showing their age (See Picture). The first owner did very little hours in the first 2 years of the motors life, so it sat around with infrequent flying perfect to get corrosion. In addition it has never been opened and inspected before. I initially wanted to sell the plane, but I made a decision to keep the plane for another 3 years. Rotax recommends an overhaul on 300hours or 5 years what ever comes first. So adding all these factors together I decided to open up and check it out.

I borrowed the flying schools bearing and flywheel puller and started to strip the motor. The first thing I noticed after I pulled the cowls is the amount of corrosion that formed on the cowl and heads. Keeping in mind that my plane was always hangered except on the occasional fly-away. During those fly-aways I can only remember it standing in the rain under its cover once or twice. In addition I can only remember flying in the rain once or twice as well. The corrosion wasn’t visible on the outside on the motor, and it was kind of a shock to me because I typically keep my motor very clean. Now I can imagine what motors look like standing outside, or even at the coast!

Corrosion that formed on the cowl cylinder heads outside.
The next picture shows close-ups of the heads and piston tops. A fair amount of carbon formed burning a nice muddy brown. The camera flash makes it look lighter than it is. The experts tell me this is a sign of good burning motor, the jetting is just right.
Next I inspected the rings. Both the top rings were nice and loose, but both bottom rings was stuck on 50% of the diameter of the piston side.

Then I noticed potential problem #1. The magneto side piston had a slight score line right down the centre in the middle of the intake manifold side. It is almost as if something very small went down the intake manifold and scored the side of the piston. It is just visible and one can hardly feel it, but it is there. I checked the cylinder walls and there is no scuffing. What ever small this was it did nut hurt the walls. Below is the intake manifold side view of pistons is on top with the corresponding exhaust side below.
Next I turned the motor upside down and removed the circlips from the piston wrist pin. You can see where you insert a pick to remove the circlip shown on the table.
Next I pondered a while how to best remove the wrist pins, since I do not have a wrist pin puller…and then Eureka I got an idea that worked very well. I used 2 deep sockets and a big clamp. The small socket is smaller than the wrist pin and pushed it out into the bigger socket. I used 12mm and 19mm deep sockets and the wrist pin slid out without putting a lot of force on the clamp. Very easy, but pictures speak a thousand words.
A View of how to push out the wrist pin.
After removing all 31 needle pin bearings and the 2 rings I noticed problem #2 on the motor. Notice the 2 rings on the wrist pins. If you look at it closely you can see it is “blue-ing”, the ring is dark blue in colour which means the metal got hot and ‘burned’.
It was evident on both wrist pins and occurred exactly where the needle pin bearings run as the pictures below depict. This means that some time during the motor’s life that the wrist pins did not get enough lubrication and hence the metal got hot and ‘burned’.
After a bit of cleaning and showing the scratch on the piston to some experts it is clear that this scratch could actually be a crack and it is quite deep. That means this piston is kapoet, I will need to get a new one. The 2 expert guys I showed this to are still scratching their heads as to how this could have happened, since there is no evidence of any foreign objects. Hmmm good thing I decided to inspect my motor, who knows how long it would still have lasted…
Any case I started to measure up the components to see how they compare to the Rotax wear limits. First the pistons, you measure 90 degrees to the wrist pin 18mm up from the bottom.

Next I split the crank case, and with the crank still in the lower halve check the crank for out of round with a dial gauge. Check both the PTO (Power Take On) Gearbox side and Mag Side it should be less than 0.03mm
Next I checked the axial play, with a feeler gauge between the conrod and the trust bearing, it should be less than 1mm.
I took out the crank and inspected the 5 roller bearings. Firstly spinning them freely to see if I can detect any anomalies. Then I put some force on it with my hand and turning them through 360 degrees to see if I can detect any anomalies. Then you want to move them back and forth to see that they do not have excessive play on them, keeping in mind ball bearing do have some play.
Next I inspected each and every ‘ball’ in the ball bearing that you can see, to check for pitting. So far everything is clear.
The last crank check is to rotate the conrod and inspect the rollers inside as best you can for pitting and or blue-ing.
You have to measure the cylinders in 6 places to check out of round and conicity.

You use this funk tool to get the distance, then measure it up with a vernier.

Take of one ring and put it in the sleeve and measure the ring gap 5mm from the top of the cylinder.
Here is the table with measurements. Everything is still within wear specification dimension limits, but I need to replace some components which show signs of out of normal wear patterns.
The heads clean up nicely with the aid of a soft brass brush, just be carefull not the scratch the outer rings where the head seals! Do not use a metal or stainless wire brush it will scratch the Heads!

Spot the new piston ;-). The old one cleans up nicely with a lot of elbow grease and green scotch bright, just ensure you clean in the same direction as the oil groves on the side of the piston. I weighed both pistons and they are within 2 grams of each other, my scale is only accurate within 2 grams so they are OK!

The needle bearing come in a housing like this, I grinded of the shoulders on the inner ring on the right hand one. This then allows one to push it out the piston side during the installation of needle bearings and wrist pin.
The 2 crank halves joined with gasket maker, and pistons back on the conrods
Put the cylinders on with gaskets and finger tight the heads, then use your exhaust manifold to align the cylinders before torqueing the heads down.  The Rest of the rebuild is straight forward
I Decided to overhaul the carb as well. Here the entire carb stripped into its components
Here is the new parts for a carb overhaul.
This shows the wear and tear on the needle. The top one is new the bottom is the old one. You can see the silver area where the wear occurred. The numbering is gone and there is definite shoulder in that area that is not evident on the new needle.
The Needle wear was traced to my needle not having the little O-ring on top. The little O-ring presses against the insert on the plastic top and ensures the needle does not vibrate. Rotax came out with this update a couple of years ago, I guess mine was from before then.
I also decided to overhaul the fuel pump. Here is a view of the pulse pump
Here is it in all its components
Here you can see a close-up of all the gunk that gets passed by the pulse port.
This picture shows the 2 transfer vavles. Pitty but Rotax South Africa does not sell the components to service these little valves. They have the rest of the components but not these. The valves work the hardest to that is kind f stupid. I you overhaul it buy a Mukini overhaul kit, it includes these 2 little valves as well as all the big gaskets for about $15 complete.