Bulletproofing the 503
By: Jim Showker

The following article describes a procedure that is experimental in nature and is presented for your information. The Challengers 101 group neither recommends nor discourages our readers use of any information or procedures presented in the article.

The use of an anti-seize sealant/lubricant in engine assembly is used by many mechanics but is not a universally accepted practice.

-Advisory Committee

This article is the culmination of about 10 years of experimenting and trying out things on various two stroke engines. We didn't develop any of the work or processes described here. I think what we did was come up with a combination of hi-tech add-ons and machine work that can be done to the 503 to make it a much more reliable, stronger, and cooler running engine.

What am I talking about? First and foremost is the reduction of CHTs that we have achieved on 3 different engines, every one virtually the same. Imagine running your 503 and having a constant reduction in CHTs of 80-100F less than what you have now. This is such a radical change that, even though we have told many people about it over the years, most people, outside of a circle of friends that have seen the engines run, do not believe it.

I think if I were to advise something simple to do at your next rebuild that would reduce CHTs by 25 degrees, you might believe me and you might try it. But 80-100F less? It sounds impossible.

We just built another 503 using these simple mods, and we got exactly the same results again. At first when I would tell people about it, and they would scoff and tell me my EIS was off or something similar, I doubted it myself. But not after three engines. This really works.

What is it? It is simply the implementation of hi-tech coatings to the engine internals and cooling fin externals that cool the engine off along with some crank strengthening and balancing. These steps will also allow the engine to make a little more power. At first I was very conservative about this, and thought maybe 3% or so more power. This is the accepted increase in power due to coatings, that most builders claim.

But you tell me. Prop pitch had been set for a long time on this latest plane so it would pull about 6600 rpm, straight and level. After break in, this latest engine pulled over 7200rpm, static, on the ground. We didn't touch the prop during the rebuild process. How much more power is that? It is well over a 12% increase. This is a single carb 503 with dual ignition. This is the only engine with which we can compare before and after power outputs.

The earlier 503 was originally built with a fan and a single carb. The fan was removed and a free air cooling shroud was added, along with dual port fuel injection from a Hirth. These two things probably add at least 10-12 hp, so a comparison was not possible with that one. That engine makes a lot of power though.

Now, this is not a "How To Rebuild Your Engine" article. This is for folks that already know how to rebuild an aviation two stroke. No basics, just the added steps we do that make a difference. We think you can rebuild a 503 and have it last for many more hours than it did before, probably safely running to something like 750-1000 hours. Consider using a synthetic oil, or at least a blend. That will keep the carbon formation down and the rings free. If your engine sits through the winter, you can do like I do, and run some Pennzoil through it for the last run of the season.

Here's what to do with a 503

Send your used crank to a reputable crankshaft shop. We have been using The Crank Shop in Essex Jct., Vermont, as in my opinion they are the best in the country. They do snowmobiles, and are generally considered the best. Here's what you want done.

Inspect crankshaft, change out bearings as necessary to bring it up to new spec. If your crank is low hours it will probably not need bearings.

True the crank. In case it has already shifted.

Have the crankshaft welded. A tiny bead is welded onto where the pins join the throws. This locks the crank and prevents it from shifting in the future, a major problem with the 503. It can easily be ground off later for another rebuild.

All this will probably cost a lot less than a new crank. It will be renewed, but much stronger than stock, absolutely true, and it will hold that trueness.

Once you have pistons/cylinders/heads and exhaust header in hand, and are confident that they are ready for a rebuild, send these parts to a reputable coatings house. We use Finish Line Coatings in Portland, Oregon. They are really good at this. Permanently mark the parts so you can get them in the right place when you get the pieces back. The coating may cover up the arrow on top of the piston, so mark that somewhere too. What you want done is--

Ceramic coat the piston domes, cylinder head combustion chambers, and the exhaust ports.

Put Thermal Coat, which looks like shiny black paint, on the exteriors of the heads and cylinders. This paint contains silver, and is a miracle for getting heat out of the cylinder fins. Put ceramic coat inside and out on the exhaust header. Do the whole exhaust if you want, but it is just the header that is going to help hold temps down. With no coatings, the header runs very hot and transfers a lot of heat back into the cylinders.

A note of caution here. Every coatings shop you call will try to talk you into getting some of their other coatings installed, the main one being a low friction coating on the piston/skirts. DO NOT DO THIS! I let the first shop I used talk me into it the first time I did this, and my Hirth seized 30 seconds into the first flight. Let my mistake be your gain.

Airplane engines are different. We run at higher constant average power outputs than even racing engines do, and there is no extra clearance for a coating between the piston and the cylinder. Besides, that piston coating wears completely off in 50 hours or so anyway, at least the portions that contact the cylinder walls.

When you get the parts back from the coatings shop, there is one more thing to do. Have the pistons zero balanced. It's normal for Rotax's to have pistons way off from each other, 6-8 grams difference is not unusual. Any good machine shop can do this for you. You're going to have a very smooth running engine, in addition to cooler operating tempatures.

Order a set of after market crankshaft seals from Tri State Kite in Ohio. These are non-factory seals made of Viton, and will not need the usual replacing in three years. Do not use the factory seals, they are junk. Tri State has a whole gasket kit that works great and is considerably cheaper than the factory item.

Don't shirk on the balancing. Any experienced high performance builder will tell you that a zero balanced engine can make up to 10% more power than a stock one with no other changes. Our most recent example above, illustrates this point.


You've already built two strokes before, right? Make this the most careful assembly you have ever done. Clean the bolts and holes before assembly. Bring the crankcase bolts and cylinder head bolts up to torque very slowly, going round and round just a few pounds at a time. Use anti-seize all around. Set the ignition timing to perfection. Do your best work.

None of these mods are hop up mods. They are more like reliability and cooling mods. No porting, no tricks. Unless we are totally mistaken, this will be the best running 503 you have ever seen.

One more thing. Before first startup and break-in, increase the main jet size by 3 steps. This will probably be too much, but that's better than too little. You'll have to tune the carbs again (and the prop), like a new engine setup. Set the EGTs at cruise and WOT the same as you would with a normal engine. If you have been using a wood prop, you may need a new one with more pitch.


An internal combustion engine is a heat engine. Expanding hot gases push the pistons around, making usable power in the process. A certain amount of this heat is absorbed by the engine parts, and of course, that heat is not available to make power. The coatings change this relationship all around.

With the coatings, the engine parts absorb less heat, and then, there's no place for the heat to go except to stay in the burning mixture and make more power. And of course, if the engine parts absorb less heat, the engine runs cooler in the process. And the Thermal Coat on the outside acts to remove the remaining heat more efficiently. That is why you must increase the size of the main jets. EGTs will go way up if you don't. It's all about efficiency.

This is spin off technology from NASA. Remember the Space Shuttle and its ceramic tiles? This technology has been available and being used by top engine builders for almost 20 years.

I know, it all sounds too good to be true. It will cost you an extra $500-700 to do this. The Thermal Coat on the fins is the most expensive part. The crank will probably be a savings. If you have them do everything, the cost will probably still be less than a new crank.

Why does the motor run so much cooler? We don't really know, and it would probably take some really sophisticated test equipment to find out. We have also done this to a couple of 4 strokes, an HKS and a VW. Good results, maybe 15-20F cooler CHTs, but nothing like on one of these two strokes.

My opinion-- it is the coating that is on the exhaust port that makes the most difference. The exhaust port sees the hot exhaust gases during the exhaust part of the cycle, and then it sees them twice again, as they reflect back from the exhaust system. These two cylinder, two stroke aviation engines have an unusual exhaust setup for a two stroke, with a single muffler for both cylinders, where each exhaust pulse reflects back to both cylinders, and so the exhaust port of each cylinder sees two hot exhaust reflections for each revolution. So, the exhaust port is almost constantly exposed to hot exhaust gases. And, the exhaust port probably has more area for heat absorption than the combustion chamber in the head does.

Heat is the major enemy of engines. Very seldom does some engine part fail that is not overheated. By extension, cooler running should also translate to a longer useful life. Let's hope so.

Here are some pictures.

This was the first engine to get these mods. A Hirth 2706. I was leery of putting the Thermal Coat all the way to the bottom of the cylinders, but I should have.

Hirth mounted in Challenger. This picture is before the coatings were added. The only picture remaining from several computer crashes.

This is the first 503 we did. Here you can see all the coatings. Oddly the coating in the exhaust ports is black, but very slippery. Supposedly carbon has a hard time adhering to it.

Same engine as last picture, all assembled. You can see how shiny the Thermal Coat is. This engine also got fuel injection, adapted from a Hirth. The EFI addition was not an easy one, and took almost a year to get sorted out and running right.

And here is this engine mounted in the plane. Front cowl is off. No cooling fan, Dual port EFI where there was a single carb before. Probably at least a 10-12 hp increase over stock. Very smooth running. 325F CHT is the highest seen so far, after about 50 hours.

This is the latest rebuild, in an LMA 85% Cub. You can see the ceramic coated exhaust header (leading to the old rusty exhaust). This engine makes well over 12% more power than it did before the rebuild and does it with 80-100F cooler CHTs. All the work described above was done to this motor.