Compression Testing?

[RudiGreyling]

Hi Guys,

Related to the Sticky Rings subject on the forum: http://microlighters.co.za/viewtopic.php?t=1612
I have done a search on this site, and googled the internet, nothing...

1) How do one perform a Compression Test on a Rotax 503, without a started motor. i.e B Gearbox?
2) What is the range of readings allowed?
3) I guess it should be done on a warmed up motor?
4) Can one use a normal car engine compression tester?

Niren or any other Rotax engine gurrus input appreciated,

Thank you,Kind Regards
Rudi

[RudiGreyling]

Hi Guys, no reply yet??

Don't you guys do a compression test on the 503's??

I know the big boys on the lycomings do a regurly test, it show signs of things to come, if you start to loose compression.

I know the big boys also does a cut open and oil filter check, we don't use it on a 2 strike though..

I know a compression test wont show emminent crank failure, but at least it is something you should be easily done during every spark plug change..

Now how do I do it, and what are the parameters??

Thanks,
Rudi

[Duck Rogers]

Eish...patience, patience.

I know you can do a pressure test to check for leaking seals. I have the info somewhere on my other PC. Wag net 'n bietjie. I have to try and find it.

I have never done a compression test and I doubt if it will tell you much though. The 2 stroke doesn't have valves and is piston ported, so the results are probably gonna be meaningless

I'll see what I can dig up out my archives. Will post as soon as I get it.

Duck Rogers

[RudiGreyling]

Hi Duck,

Yes I'm aware of the pressure check when overhualing to check for gasket leaks etc, but you can't perform that test reguraly with spark plug changes, since you have to block of the intake and exhaust ports.

A two stroke does not have valves, but it does have a compression stroke so one should be able to check it, and measure it. But how and what is what I'm after.

My brain tells me if it is off spec you have sticky rings or worn sleeves and or pistons.

Thanks for checking, waiting in anticipation.

Dankie, Groete
Rudi

[nickjaxe]

Dont know if it helps but I did a comp check on my 503 last summer 120psi on both clyds but its electric start, I would be interested to know how you do a crank case presure test to check for leaking seals if anybody knows.

nick in the UK.

[Duck Rogers]

Here's the info that I can decipher from my scribblings...in no particular order: :D

Warm up the engine
Remove all plugs
Ground ALL the plugs...apparently you can damage the ignition coils units "black boxes" thingies if you don't...
Throttles wide open

Swing the prop BY HAND through 2 or 3 revolutions and take the reading.
The reason you're swinging by hand is as I've said before, the 2 stroke is "piston ported" and doesn't have valves to seal off the cylinder head, so you're not getting to read the cylinder head pressure, but rather the sealing effect of the rings only, so slow cranking will negate crankcase pressure

If you use the electric starter to spin the crank, and don't open the throttles, you will probably get a higher reading because of the pressure in the crankcase.

I can probably give you loads of THEORETICAL mathematic equations with regard to static pressure, compression ratio, volumetric pressure etc. etc. so you can THEORETICALLY work out what the compression ratio should be.....but that's all pie in the sky because we don't really know how fast or for how long to spin the prop, do we?

I'm gonna guess here, and it should give you a good starting point for a reference:
Compression by briskly hand-propping about 2or 3 swings will probably be about 100 - 105 psi.
Compression by electric starter will probably be about 120 - 125 psi.

I would rather suggest the following: Do a de-coke as per all the specs and make sure everything is ok...then do a compression test and use the numbers obtained as a reference value only. Then you'll always have the right base value to compare against :D

Actually, the best way to check is through the exhaust port to see if you have scuffed cylinder walls and/sticking rings....that's why Rotax suggests it in the first place

I hope this will help you some...but use this info at your own discretion :D

...I would be interested to know how you do a crank case presure test to check for leaking seals if anybody knows.

I will post the procedure for this later tonight ony, as I will be out all day....patience, patience :D

Duck Rogers

[The Agent]

Get hold of Glen at Microflight Aviation.
083 662 6565
He is a pro on those engines.

[RudiGreyling]

Hi Guys,

I have found the answer, and thought of posting it here, for future reference.

Last night I watched a DVD of a complete strip down, inspection, measure of components and rebuild of a 503 engine. Let me start by saying these little motors are not very complicated. Just a little trick here and there, a special tool here and there and they are equivalent to a lawn mower.

Being mechanical minded, having some good tools in the garage, I think I will be able to do my own overhaul!

Now back to the compression test:
1) They tested it with 3 plugs in the motor, one removed and connected a screw in type compression tester.
2) They tested it cold, just overhauled motor with a bit of lubrication. I guess it is better that you test both cold and warmed up, since they did not make a distinction.
3) They use the rope pull start as per normal start to achieve+/- 300rpm. (If you do it on the plane disconnect all your plug wires & make sure the mags is off, you do not want the plane to start and run away)
4) Reading range (110 - 120) PSI +/- 10PSI. With the difference between cylinders also +/- 10 PSI.

So I guess the bottom line is if you get close to the 100PSI or lower start thinking about opening up and inspecting the motor??

Wonder if you need a sign-off of some kind from a AP if one overhauls his own 503??

Your thoughts welcome,

Kind Regards
Rudi