Decoking a 503 motor

[Morph]

Yesterday I decoked my 503 motor in my Challenger and although it seemed like a daunting task at first turned out to be quite simple. I got Aviation Engines to supply me with all the gaskets up front (about R150-200)

Since my plane has an inverted engine I decided to do the job with the motor installed, this saved a considerable amount of time. I wouldn't try this with an upright motor since you have the risk of dropping things into the motor.

First I stripped off the exhaust completely and had one of our local experts Donald look at the pistons through the ports. Both had a lot of carbon on the heads and there was presence of "blow by", dark burn marks along the side of the pistons indicating stuck rings.

So off came the carbs and engine cowlings. Then I removed the headbolts. The head came off easy and showed a lot of carbon buildup. A gentle tap with a rubber mallet on the side of the cylinder released it and it slipped easily off. (1 hour)



MUCHO Impotante: Do NOT get the pistons, rings, heads etc mixed up. Mark everything and keep them separate.




This is the carb side. The other side was much worse. It's amazing how much these motors can take and still run. You can see the burn marks past the rings. The ring you see is stuck solid.





At first I thought of cleaning the pistons while still attached to the motor but this is not practical. So, off with the cir-clip on the PTO side. We used a thin long socket to press the wrist pin out. Some guys use a g-clamp for this but left hand pressure on the pin while tapping the right hand of the piston with the back of a screwdriver was all that was needed. Be very careful to catch all 31 pins from the bearings. (30minutes)

To clean the pistons required a lot of elbow-grease. Be very careful not to damage the piston, especially the sides where the rippling effect on the piston is necessary to hold the oil so in other words, no sand paper. First we removed the rings. Heavy job, that you must be very careful unless you plan on spending R1000 on a new set of rings.

We used plastic, and then a soft aluminium plate to start tapping the ring to release the grip. They were solid. Eventually with one person holding the piston on a piece of wood and another gently lifting the ring while tapping around the piston they finally came loose. Don't overstretch the rings. Open them only as wide a necessary to clear the piston.

We reinserted the rings in to the cylinders and used the piston to push them all the way to the top. I then measured the gap in the rings with a feeler guage. They must be less than 1mm otherwise replace. Mine were .55mm

I used a Stanley Knife blade to remove the major carbon off the dome of the piston. Don't go mad, just lay the blade almost flat and gently shave the carbon off. For the sides I used the soft Aluminium plate as a scraper as it it much softer than the piston aluminium and has no chance of scratching it. The ring grooves were cleaned using the back of a broken hacksaw blade. The rounded edge of the hacksaw blade was used to clean the carbon off the heads. I then finished it off with a Brillo pad the get the last remaining carbon off. (2 to 3 hours)

Everything was then cleaned and ready to go back. Get a nice thick grease and coat the needle bearings with it. Use the wrist PIN and stick the pins around the wrist pin. Make sure there are 31. Then insert the whole lot into the con-rod. Once installed, push the wrist-pin out, leaving the bearings in the con rod, held in place by the grease. The pistons can only go one way. On the dome you should see an arrow that should point to the exhaust side. Alternatively the gap on the rings is on the carb side.

Now insert the wrist pin into the side of the piston(the same side you took it out) Make sure you put the needle bearing ring on the wrist pin as it protrudes though the piston. Carefully align the piston and wrist-pin before pushing it through the needle bearings. What vary careful that you don't push any of the needle bearings out and double, triple check that they are all there. Put the other needle bearing ring on the wrist-pin as it protrudes from the con-rod and push the wrist-pin all the way through. Install a new Cir-clip. This is the hardest part of the entire job and you need 2 sets of hands, light and patience to accomplish.

Add the new gaskets oil the cylinders lightly, and reinstall the cylinders and heads. Install the exhaust manifold and tighten this up (don't torque yet) to align the cylinders. Now torque the head bolts according to the specs in the Rotax Maintenance Manual. http://www.ultralightnews.ca/rotax503/index.html

Replace everything else in reverse order. Total Time 8 hours

I must say that I had the help of a lot of friends at Morning Star which is what I love about aviation. Griffin helped cleaning pistons and heads, Donald Williamson, Stephan and CLU-less helped on the engine. Stephan added good coffee and CLU-less, biscuits. But in particular it was Donald whose advice and experience and help made this job easy


Here is Donald closest and Stephan reinstalling the pistons

Griffin took better pics. As soon as I get them from him i'll publish them

[Duck Rogers]

You guys did good.
This write-up will help a lot of guys that are too scared too ask. Well done Morpheus.

Now, the million dollar question............DOES IT WORK AGAIN

Duck Rogers

[Morph]

Duck,

I took her for a number of ground runs this morning, The WX was closed in and threatening to rain . I must say that the motor has more oompf. From idle to WOT (wide open throttle) surges faster than before. I did 6 ground runs up and down the runway to get the temps up and ensure that the engine is working fine. I noticed now that I had to readjust the idling on the carbs a bit but otherwise she is running fine. :D I can't wait to fly her

[RudiGreyling]

Hi Morph,

excellent post with pictures etc. Did you manage to take a picture of the exhaust side of the piston to show the blow by??

Thanks
Rudi

[Morph]

Yeah, just waiting for Griffin to send them to me then I'll post them

[Leprachaun]

Well done guys , a question though? did ya replace gudgeon pins , circlips and small end bearing - its open , so close so do it . Well done anyway, send me a mail and I will send you a complete DVD on the 503 engine . Fly safely

[Morph]

Hey Leprachaun

we checked the tolerances and all was fine so there was no requirement to replace the wrist pin, needle bearings. We did however, replace the circlips since they get bent when you remove them.

I phoned Niren at Aviation Engines and said I was going to do a decoke. He sent me all the gaskets, and the circlips.

[kb]

Morph, when you next coming to durban..... Will GLADLY show you the inside of my 503

[Morph]

Any time,

If you can come fetch me in the R44

BTW I have added some pics of the pistons from the exhaust side. Thanks Griff

[RudiGreyling]

HI Morph,

Thanks for the reference on the exhaust port side pictures of the pistion, Yes one can clearly see the blow past caused by sticky rings.

When I tried to check for sticky rings, through the exhaust port without taking the motor appart, I could not move them, very little space in there to work with. My piston skirt was clean, no signs of blow by, which means my rings is still working fine.

Thanks again.

I'm considering doing a tear down of my motor soon to inspect the tolerances and crank. Maybe when I do it I will take some good photos just like you did for other pilots & DYI mechanics as a reference.

Kind Regards
Rudi

PS: Do you know how many hours your motor has done, since the previous clean up and inspection? It would be good to use as an isolated reference with the pictures.

[Morph]

I have only had the plane for 105 hours. It now has 545 hours on it. The last major, according to the logbook, was at 385 hours. all was checked but no decoking was done at that point. When I bought it the AP that did the MPI checked the rings and said it was still fine, although I personally did not verify his findings, besides at that stage I had no idea of what I was looking for.

It is difficult to see or feel if there is movement on the rings. The best indicators, in my still very limited knowledge, is to press on the rings from the exhaust port and see if you get oil pushed out from behind them as you press them. Use the back of a screw driver to press the rings. If you are getting "blow by" then they are definately stuck or worn. Other wise they should be fine.

It has not had a crank replacement nor bearing replacements. I was considering doing the bearings but we checked tolerances on the crank
it is within spec. I am getting no lateral movement from the bearings and it still feels smooth as silk. I think in another 100 hours (i.e. 640hours) I might pull the whole lot apart and check everything.

BTW there are a number of 503's here in the Cape that have done 1200 hours on them with only regular 25 hourly services, decoking and without crank and bearing jobs. Especially those being used in the training field. The more you fly them the better they seem to run.

From what I have seen and experienced it is a very simple engine to work on yourself. But then again I do have a strong technical background and I have worked on cars and bikes before.

[Aerosan]

Morph
you mention th crank, some guy here sugested that it isnt needed to change the crank at 300 hours or so, it could be sent for a check and then a decition made. Now personally, Im not comfy with this as Ive had an engine out before and it isnt the nice rollercoaster ride that I expexted but there might be credit in it. Isnt the problem with crancks that they get small cracks? and need to ex-rayed? Just a question :D

San

[Morph]

I did the Rotax course with Niren from Aviation engines and he recommends you check the play between the conrod and the crank. If that is within specification and the bearings turn fine without any play or roughness you should be OK.

I haven't heard of the cracks issue at all.

Give Niren a call at Aviation Engines on 011 455 4204

BTW if you are uncomfortable then replace the crank and bearings, It will cost you about R10K to do.

An alternative would be to replace bearings at 300 (about R1K) and crank at 600.

Our AP's here check the movement of the crank etc before doing anything. Whenever I talk to them (3 different guys at different times) and ask them about when I should replace my crank they give me the same blank stare and say, check it for movement and if it runs smooth without any roughness and movement, leave it alone for another 150 hours.

I have only ever heard of one crank failure here in CT. One of the bearings seized solid. It turns out the engine had previously had a seizure due to incorrect mixing of oil (100 to 1 instead of 50 to 1) and the bearings were not replaced at that time.

It's your choice, speak to your local AP. The most important thing is you must have confidence in your engine, if you are unsure err on the side of safety.