I recently resealed a 503 for a loyal customer. It was supposedly 'top ended' about ten hours before.
The first picture is of the shipping crate. I include this as many engines get destroyed by UPS and require another ship back for repairs. I've seen the fan tower cracked, the coil plate bentd, the coils broken off point engines, fins bent, shrouds damaged in several ways, etc.
It isn't a small matter and needs to be discussed just a bit. The crate shown here,,,,,,,,,,,,,

,,,,,,,does a pretty good job. Some problems did occur but since the shrouds were removed as well as the intake manifolds, there was little effect on the engine.

This shot shows the engine and the location in the crate. It had blocking across the box above the engine but no screws held it in place as they missed the blocks. Thus, the blocks fell down and didn't assist in locating the engine. The engine rattled around a bit on the lower cross pieces and beat them up a bit. I suggest drilling holes in them to match the engine mounting bolt pattern and using the mounting studs to locate the engine.

This shot shows the beating the cross boards got from a 1000 miles of UPS travel.

The reason for resealing this engine is to establish a known condition. The plane and engine are about ten years old and certainly ready for some serious TLC. The next three pictures show the visible leakage from the seals. Poor sealing can make the engine hard starting with a pull starter for sure, and place additional loads on even an electric starter. One pull manual starts using a primer are normal, and seal leakage can add several more pulls due to low crankcase pressure/vacuum during cranking. Without intake and exhaust valves, we rely on the crankcase to move the intake charge around the engine.

This shot shows the outside evidence of MAG seal leakage. The joint between the case and the fan tower seeps the oily mess and then dirt sticks to it. It will be much more evident on right side up engines as this one is on the lower areas. Inverted engines will show similar dirt but to a lesser extent as the oil travels by gravity to some extent. The leakage will still show this narrow ring of attracted dirt though.

This picture shows the bottom of the engine and it's possible this area has been wiped clean during maintainance or normal cleanups. There is hardly any way to keep it from coming back as more oily mess creeps onto the outside of the engine.
Also visible here is the damage to the coil plate from the engine being somewhat being loose inside the box. The coils got pushed against the side of the crate and they in turn bent the plate. The plate is fairly soft aluminum with little temper. It straightened easily and I saw no damage to the coil at all.

This shot shows the lower area of the stator bell housing. The oil has seeped forward getting into the electric starter area.

As more parts are removed , the oily mess gets worse. Even the slightest oil/dirt on the outside means much, much more inside.

This one shows the seal with the oily "beard'. It takes some time to accumulate this much oil and dirt. And a lot gets pulled away and thrown about inside the stator housing too. So what is seen is only what is left after leakage, seepage and what the 6000 RPM flywheel pulls away.

The back of the motor has exactly the same seal as the front, at least on the DCDI engine so leakage might be similar. But keep in mind, that a B box has oil right up against the seal. This flow of oil removes the "beard' as quickly as it forms. The seepage probably contaminates the gearbox oil a bit too. But if the seal is leaking out, it in also sucking in amd may be pulling gear oil into the engine, lowering the level in the gear box. This engine has a C box, with no gear oil contacting the seal. Evidence of the seepage is still there on the rear seal, no picture unfortunately, and around the back of the engine.

I pulled the fan tower, flywheel, cylinders and the next shot shows the pistons. They had little ring sticking although the ring/piston junction was sort of gummy. The carbon buildup on the crown of the pistons is more than I usually see, possibly indicating running rich on the average.

There is little blowby past the rings as seen below the lower ring. Typically there will be a small amount of blowby past the upper ring at its gap. The lower ring should stop this from going further.

Also typical of most engines I see, this one had a good seal from the head gaskets with no evidence of leakage. Also, the base gaskets looked intact with no leakage either.

I did see just a hint of scuffing due to cold seizure but not enough to damage the pistons or cylinders. I carefully smooth these areas with very fine emery paper, moving the paper circumferentially rather than along the side of the piston. Sanding removes the raised metal whch may cause further scuffing on its own, plus, the slight removal of aluminum may reduce the tendency for further cold seizure scuffing too.

The next picture shows the 31 rollers, wrist pin and the two protection rings. I remove the circlips and pitch them as they get replaced with new ones for sure. I carefully count the 31 rollers as this is the best time to realize one/some are missing. A good part of a proper reseal involves keeping track of the bolts and parts, especially the small parts such as the rollers during the teardown.

I've been asked several times to show the method of removing the rings. I start by pulling one end out of the groove and placing it on the top of the piston. I then spiral more and more of the ring out of the groove allowing the ring to further go on top of the piston.

The next two shots are of the ring and the successive spiralling of the ring. Note the good sized buildup of carbon inside the piston. This restricts the flow of combustion heat from the piston to the cool intake charge that swirls below the piston. It is removed during the piston cleanup.

The carbon inside the heads was about average for a good running 503, and maybe just a bit on the rich side.

This next photo shows the problem of mud daubers trying to fill all the little holes with mud. The locations can vary but the groove on the pulley looked good to at least one of them. This dirt can cause excess wear on the belt for sure, and if a point engine, can prevent the points from closing at all which halts spark production.
I also clean the surfaces of the pulley halves that contact the fan belt. The buildup of rubber and crud can influence the belt tension and possibly allow slippage and cooling problems. Note that there are two different thicknesses of pulley halves. The thick one is a stand alone half and goes on the shaft following the thick spacer washer. The second is rather thin and requires the support of the pully face. The one shown is a later one that has two small clips bent out which engage the plastic fan to assure they turn as a unit.

The next shot shows all the parts cleaned up.

Time to reverse the ratchet on the wrench and start reassembly. Here are all the case parts ready for assembly. Bolts are clean, all metal surfaces that have been honed are wiped with oil to prevent rusting. I run a flat file across the case faces. This removes any small amount of metal that may have been raised during the disassembly or the cleaning process.

This shot shows the sealants used, the file on the case half and the tools for this specific step. Having all the parts clean, counted and organized makes for a smooth assembly.

First step is to check the orientation of the seals. I always do this for each seal at both ends of the crank. I've got a history of seals and likely don't need to, but if the small tits on the seals change spacing/number, my method needs to keep up. Notice there is a small hole drilled all the way into the upper crankcase at each end of the case. The tits on the seal must be oriented so they don't cover these holes. Once the crank is installed, the tits can't be seen at all, so make sure the seal gets installed by using the portion that is visible. Mark them with chalk, crayon, or do as I do, note the position of the upper most tit.

These next two shots show the crankcase jusr prior to closure. seals are installed on the crank, the crank is dropped carefully into the case making sure the axial location rings drop into their grooves. Seals are orieneted such that the tits clear the oil passage, and a bit of white grease is on the crank area the seals sit on.

This shot does in fact show the minute amount of sealant used on the case halves. The total amount is about the size of a new pencil eraser, and spread along the face of the lower case. Use a patting motion to move the sealant, not a wiping motion. Wiping will wipe the sealant into the bolt holes and off the edge of the case. No need to have it there and make sure the coating is even. I often wipe my finger off on a rag if there is any too much sealant.

Also, get all the bolts cleaned beforehand so that the actual assembly can proceed smoothly before the thin layer of sealant dries. This picture shows how I roll the bolts to assure an even coat of antiseize on them. The antiseize should be on all the threads of the bolts.

Here is the case and crank assembled and torqued.

Next step is to install the pistons. All the engines for fifteen years have had the cageless roller bearings. And just as there is an exspensive tool that is difficult to use, I do it by hand and consider it the easiest way.
Lay the 31 rollers in five piles of five, and a pile of three, two and one rollers. I install them with just a thin coat of white grease on my finger which allows me to pick up all five and install them. I liberally grease the ID of the rod. I then place five rollers at the bottom of the rod bore. then five more on each side of those five. Then the dummy wrist pin, then five more along side of it on both sides.
Then install the three, then the two. The last single roller should be a bit difficult to get in as all the slack around the circle of rollers needs to be pushed to allow it in. This shot shows the first fifteen rollers installed.

The next shot shows the last roller installed. It is protruding just a bit right at the top.

The next shot shows the end retainer rings installed and the asssembly ready for the wrist pin and piston.

The next picture shows the wrist pin just about to push the dummy wristpin out the end of the piston.

This shot shows the parts aligned and counted for the second piston installation.

This next shot shows the lack of space to install a 32nd roller. Always check to see if you can get the 32 one in. If you can, somewhere one got dropped and it would be a good stopping point ! There should be NO room for an extra one.

This is the technique I use to hold the rollers for installation.

I use some narrow pliers to install the wrist pins. They are really, really hard so don't worry about damaging them. They should go in with a bit of difficulty and possibly require some rotation, maybe even heating the piston a bit.

I use the Rotax circlip tool as it does the install pretty easily. I installed them with a screwdriver for many years though and either method is OK. If using the screwdriver, always have some extra circlips on hand as they sometiomes launch across the room.

Cylinders are next and be careful of the base gaskets on the studs. They can easily be torn as the cylinders are installed.

This picture shows the rings being compressed by hand. Tools are available but require more time than this method. Gentle persuaion works best. Make sure to properly align the rings and the anti-rotation pins before trying to compress them.

Head gaskets and the heads are next. Make sure that the old head gaskets are removed. I've seen several engines where there were two gaskets. Compression suffers a lot when this mistake is made.

Here is a frontal shot with the fan tower back on.

And a shot of the electrical stuff reinstalled.

I put red locktite on the mating surface of the gearbox and the end of the case. this prevents the surfaces from moving due to prop torque.

And the final product. Shrouds were removed for shipping .

One question I get asked frequently is what tools do I need to do my own engine ? Well, here they are,,,,,,,,,,,
Starting at the left side, a Rotax flywheel puller, fan tool, still on the flywheel, fan tool, compression band for the rubber donut, a large very flat file, wrenches, side cutters for tiewraps, nut drivers for the metric buts and bolts, numerous small allen drivers, flywheel nut wrench, plug wrench, a soft mallet and a big hammer, bendy torque wrench and the circlip tool.

I hope you have enjoyed reading/watching this unfold. This engine was in great shape and required a minimum of new parts. A gasket set for sure, and four circlips were about it. The C box required a new gasket but no seals that I could see. The fan belt had about five shims and went back the same way. Plug caps were checked but still OK. Other items used are really small tie wraps for the primary CDI wires, white grease for the roller bearings, RTV for the case sealant and to hold the gearbox gasket in place, some dielectric grease on connections, some red locktite on the PTO flywheel bolt and on the MAG end flywheel nut, antiseize on the main case bolts, and a small dab of oil on the head studs prior to putting the head nuts on.