Compression Test Rotax Max

Hi I’m new to karting ,
iv just bought a crg framed Rotax 125 pre evo.
I’m just doing all the standard things before I go out on it , changed gear oil , cleaned carb , repaired reeds , checked fuel pump, new fuel lines etc…
Now we have gone into lock down I might have a go at the piston and rings. (What’s the worst that can happen :thinking:) Iv got a Compression tester for the barrel , my pressure is 125psi

Iv been looking on the web and am I right in thinking that a really good barrel pressure is between 145 to 155 psi . If so what psi is considered bad ?
I’m not a black belt mechanic just know enough to get by so any help and input tips or tricks will be appreciated

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Are you troubleshooting a problem, or just building this engine up?

125psi is on the low side alright… but…

Gauges can vary a lot. Have you had this one a while?
If you don’t have enough oil on the cylinder walls you’ll see a lower number as well.
Most of the cylinder compression is made by the ring, so for a quick check you could measure the ring gap. I don’t have a number for that unfortunately.

Thanks for the reply :ok_hand:
The pressure gauge is new and Iv has the kart around a year but not had much use . all the tracks are shut around here because of COVID , so I thought I’d just make sure when they do open the kart will be the best it can be .
I’ll have a go at changing the ring , see if that will build the cylinder pressure.

Make sure of three things for an accurate reading on a compression test:

  1. There’s adequate oil and fuel getting to the piston ring (run the engine first or crank it over with the ignition off before testing)
  2. The plug is grounded so that you don’t kill the ignition coil
  3. The throttle is opened the whole way after the engine has been spun up by the starter with the throttle closed.

If you’re still getting 125 psi after this, the engine needs a rebuild - maybe as little as a new piston

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One thing we should throw out there before you take it apart… If you plan racing the engine you’ll have to take it to an authorized dealer to do the work as it’s a sealed engine class.

If you’re not racing it, servicing it yourself may impact the resale value of the engine as it would have to be resealed by the next owner, assuming they plan to race it of course.

Cheers for your input guys .
My engine isn’t sealed , so I’m going to have a go at changing the piston .

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Let us know how it goes.

Would you guys say this is a bad piston

Looking at the carbon on the crown and blowby past the rings (carbon on the skirt) I’d say it’s due a fresh one.

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Ring’s shot, which you can see by the blow-by past it, and the coating’s pretty well worn. You could save it for an emergency but I’d say your engine deserves a new one.

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Iv honed the cylinder and ordered a piston and ring and a few gasket. Hopefully they come in the next few days so I can put it all back together and get her running great. Again thanks for your input guys :+1:
Hopefully the tracks around here open soon , I’m missing them and can’t wait to test the kart

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Isn’t the cylinder Nikasil-lined aluminum? I didn’t know those cylinders could be honed.

Nikasil lined cylinders “can” be honed, special diamond hone stones are needed.
I used “normal” stones to clean off fused on, piston aluminium once. Which to my surprise worked, and did nothing measurable to the bore.
I was pretty careful and out of other options at the time though…

Kudos to you for giving it a crack but you have clearly given zero thought to skirt clearance by honing prior to measuring new piston and most likely using wrong hone type and stone.
A wiser choice may have been taking cylinder to a builder and getting it honed and sized correctly with a piston supplied as well.

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I would be surprised if honing the cylinder was necessary in the first place - the Nikasil is a lot harder than the aluminum piston rubbing against it, and more wear-resistant than an iron liner. It’s one of the things I miss about running a Rotax or KZ…

Honing Nikasil properly is not easy and requires the right equipment, meaning diamond stones and a machine that can apply the proper cutting pressure. The harder the honing stone, the greater the pressure needed, and diamond is as hard as it gets. Without enough cutting pressure, the diamond will just plough rather than cut, and you’ll end up with a polished, and likely distorted, bore. When guys talk about honing Nikasil, a lot of time what they really mean is just running a brush hone through the cylinder to “deglaze” the bore.

I have no experience with that engine so hard to evaluate if your compression pressure is correct or not.

Seems low to me! Still, I only have 100 mL rotary valve engines to go by. A good TT 75, with a good snap-on gauge, measures 215 psi. A KT100 will pump 160 psi.

Going by what you said, I would say your engine has low compression. Maybe needs rings.

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Piston coming Monday so hopefully Tuesday I’ll be doing it . Iv got a 53,97 piston . It’s has cost me a lot of money buying the stuff needed, Material and some of the tools. It would probably have been the same price for a professional mechanic to have done it . But I am enjoying learning and gaining knowledge. If I get it right and it runs and don’t blow up :joy: I’ll have the tools and knowledge to do it again and again and maybe help someone eles. From what I know and have read I’m confident it’s going to be a success, I even understand the squish gap . Do you guys with the rotax have yours around 1.05mm ??

1.05 mm seems really loose for squish gap, on my ICA I ran 0.7 and that turned 20300 RPM at Point Pelee without the piston hitting the head

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