Noob taking Rotax Max apart, need advice

Hi guys, winter is coming so my kart is hibernating.
In this time i want to refresh a few bits. I wanted to check my engine out as i have no clue the hours on it(except my hours). Soo, here i am asking advice. It’s a rotax max snr 125 (pre evo). I bought it with my first kart.
1)The plastic gears look ok, i will purchase new oil for these. Any recommendations?

  1. Looks like oil got into the barrel, i had a spark plug come loose one time. Bit of beige slime like water oil mixing. Would this likely be the cause?

3)Will my piston go again? I was thinking of buy a piston ring.

This is my first tear down so go easy. Thanks guys

Hey Peter, I think there’s probably a guide for the various things that should be checked. I think some of the stuff you are looking at would get addressed with the usual maintenance which is top and bottom end rebuilds. I personally don’t wrench but basically a top end is piston and gaskets, stuff like that and I think it’s bought as a kit.

I’m guessing that since you don’t know hours someone here might have a thought where to start.

I couldn’t find the pre evo manual but it’s out there. Maybe this guys video will be helpful:

Normally, you should measure the piston to determine wear. But in this case, visually it looks tired, some blowby and more importantly you said you don’t know the hours…that means you need a new piston kit.

Over the years, I’ve learned that the cheapest way to go about engine maintenance when you buy used is:
-IF engine hours are known and comes from trusted person: keep going with the original schedule
-IF engine hours unknown or known but bought online or from somebody you don’t know: full rebuild (top, bottom, every bearing and seal etc)


I second this.

I’ve had several engines that was supposed to have only 1h since rebuild. Yeah straight BS, I’ve tore every engine apart and you could clearly see marks from a lot of hours. And this has been from random people

So always check engines before use if not bought from a reputable guy.

This crank was supposed to have 1h in it, same for bearings. But bearing don’t rust within 1h of use :joy:

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Piston looks really tired and needs to be changed…
If you want to do it yourself do it, be carefull when putting in the piston ruing in cylinder and on piston, falsely matched will ruin you engine

I dont reammy know if you hone a rotax (rotax folks help me on this one) if you do i would recomend to leave it to a builder with a proper hone machine,

For oil go with the rotax gear oil if you want to have a normal oil,
I would never change piston ring without changing the piston.


Rotax is Nikasil plated

Check the crankshaft for correct tolerance. The cylinder is nicasil plated so it cannot be honned but you have to check it . Piston seems tired so need to be changed with the correct size . I will find the engine manual from pre evo and put it here . You sould work as the manual said .

This is only the operation manual, its strange coz i cant find the fr pre evo manual on line now . But i have it printed so its must be somewhere on line …

The other guys here can tell you if the service manual from the rotax max evo is compatible with you older version engine coz is a manual easy to find on line today .

1 hour

1 hr sure, that bottom end has fought more battles than Napoleon :man_facepalming: :joy:

Wierd right? They deliberately took it down it seems from where it was. It’s all Evo stuff.

Maybe one of us can ask them to make it available.

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Iv check and found my physical copy of the service manual , so if you need specific help in something , ask me here and i will photo and sent the info you may need

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thanks for your replies, most helpful. So really i need a full rebuild considering i have no knowledge of the history of the engine. So here are my thoughts on this.
i was tempted to upgrade to an evo engine next year if money allows it. This is making me unsure if i want to spend a load of cash getting this one good. My other idea was to replace my chassis as its an old evk otk. That might mean keeping the engine for another year in which case sorting the engine could be worth it. i dont race, just hobby karting.
i think everyone would agree it needs a new piston and ring, and other seals to get it back together. could i get away with reusing the bottom end and bearings? there was no known issue whilst using the kart on track, i just wanted to get the kart sorted over winter so i can get in and go when the spring rolls round. i cant feel any real play in anything and i run a little oily hoping it would help with longevity.
thanks guys

EDIT- a quick thought after i wrote the reply. is it worth buying evo stuff and making a franekstein? pre evo bottom end and evo barrel and piston? i dont know if it all works together?

This should be fine. You can buy a kit for doing this. It’s more than a cylinder and piston. You’ll need to source the new electronics and power valve. Also I believe the clutch is different.

If you are not running official Rotax races I would just do the rebuild as-is and not worry about doing the update.

Make the rebuild and do some seat time practice ,
Dont spend the money on going evo upgrade if you dont need an angine that is gonna be race competitive. Spend it on consumables and practice on the racetrack . Enjoy karting . Thats my opinion


Yea im not racing so thats a valid point. I will rebuild as long as its all in spec and get a chassis instead. Thanks guys for your help so far, i appreciate it. I’ll be back if i encounter any issues. Cheers again.

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Good luck! Here is another good video resource, helped me as a quick reference when I pulled mine apart this year.

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