Opinion needed on Rotax Max

Senior Evo is nearing 20 hours. Do you think it’s possible for someone with no prior mechanical knowledge to undertake a top end rebuild. I was thinking of buying Power Republics instructional video.

Do I need any special tools? Also when I was looking at the kits online, they seem to come in different sizes?? Like 53.95, 53.96 etc… what actually needs to be measured?

I can get help of an automotive mechanic, but just trying to make sure there isn’t some extremely difficult aspect to it.

thank you

Is your engine sealed, and do you race in a series that seals the engines? If you open it up yourself, a service center will need to do a complete rebuild and inspection ($1300 these days) before resealing it.

If not, then the Rotax manual will tell you what piston-to-wall clearance you need, and you’ll measure your cylinder to find what piston is needed.

I haven’t raced yet, just practice. May start next year. The engine has not been touched since new. I’m not aware of any seals on it.

The seals are on the wires connecting a case bolt to a head bolt.

I’ve read the rule book, they seem to attach a new seal every race meet. If something seems fishy they will inspect the engine.

Sounds like there’s two seals at play then possibly? One at the track, another than would happen at an authorized Rotax dealer. Unless the track is just doing it’s own sealing by itself?

Changing a piston is doable for a DIY’er, I’d recommend that you at least check the bore for ovality and send it out for a professional hone if needed. Lastly, do not overlook checking squish clearance during assembly which has caused a problem for more than one DIY’er.

Where is this other seal that you speak of?

The “offical” rotax seal that performed by authorized Rotax dealers.

I second the above. In my opinion, if you want to go DIY route, you need at least:

External micrometer (to measure piston),
Bore gauge (to measure cylinder bore),
Dial gauge with spark plug adapter (to set advance/find TDC),
Wrist pin compression tool & wrist pin extractor tool (piston tools),
Magnifying lense (to check wear),
Head insert & glass burette (to set chamber volume).

This will equip you to do top ends properly, farming out bottom end which is never an option to DIY (cost of tools and expertise required are both high).

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James, remember that a Rotax has a Gilnisil coating on an aluminum cylinder - it won’t need to be honed unless the engine has seized


Where abouts are you from? Some countries require rotax seals and others don’t. Here in NZ we only use engine seals handed out at race meets, no rotax seals.

The problem with the rotax engines is the tolerances are still crap with regard to bore diameter and piston sizes. Importers here have had brand new engines require honing to get the bloody things circular instead of oval! Also the piston size tolerances out of the factory are whack. My builder has to go through 3-4 pistons in stock just to get one that matches the correct bore size.

If you were doing something with better tolerances like KZ then I’d say yeah do it yourself, but rotax… I’d suggest builder. Especially since top ends are really not frequent

I’m surprised at this given the cylinders are supposed to be CNC machined for some time.

How large are these variances?

I’d have to check with my builder for cylinder tolerance but my guess is it would be unevenly applied lining since as you say they are CNC now.

As for the pistons I recall him saying he’s seen them as far as 0.06 away from their stated figure which when were talking pistons is substantial. Far more inaccurate than a Vertex TM piston.