Compression Test Rotax Max

Max has a heavier slug and is a fairly different engine to an ICA though.

1mm is the min squish per regulations for senior, if it’s at 1.05mm I’d leave it there even though this is just for fun.

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Iv done the piston , compression is good now getting 155psi
It’s starts and runs
But when I get to a high rpm the gear oil spits a fair bit off gear oil out the breather cap , why is this does anybody have a idea ?
It was fine before
I have metal gears and have put 100mm of rotax gear oil in.
I have a 80 tooth back sprocket and a 12tooth front , Iv put a 102 chain on from a 108 could it be that ?
I’m puzzled :face_with_raised_eyebrow:

You say “compression is good now”? Compared to what? Using what brand of gauge?+++

I’ve never checked one of those engines. A good KT100 measures 160, an HPV 175 and a TT 75 215. I always used a snap-on gauge. Still, your procedure could change the readings.

My original psi was 120. Iv read up on the web that a rotax max should be around 150/160 .
So changed piston and ring . This gave me 155psi .
My gauge is only a £40 one so not snap on quality sadly .
It’s more the oil coming out of the fill cap that I’m worried about , it squirts out at least 2 to 3 inch at high rpm and I don’t know why .
Going to look at crank seals tomorrow but they was fine before the new piston and ring ?

Fine as in you did a vaccum or pressure test, or visually inspected?
Put the piston at BDC, make a block for the exhaust and intake and you can use the plug adapter from your compression tester along with some sort of pump to pressurize the engine. No more than 4-5psi. It should hold the pressure for at least a minute.

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I didn’t check visually or pressure test, just assumed as everything was fine before . Is the oil spitting out the filler cap a symptom of bad crank seals or am I wrong in thinking they are causing the problem?

Mine has the metal gears and Rotax recommends 100ml however, 25-30ml spills out the overflow for the first 3 runs. I compensate now so I just put in 70ml and haven’t had a problem but it’s up to you if you want to do that too.


50cc’s of ATF is all you need in a Rotax gear case. Any more than that and you need to run a long bend of fuel line off of the cap to capture the inevitable squirt.

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