Oversized Piston?

[Matthew Sean]

I openned-up my 95 XP 720 engine. The piston is oversize and is inscribed with a 1.00mm marking. What size of piston do I order? Seems like the available sizes in online stores for the Type 717 motor are 78mm, 78,25 and 78.5? Logically, wouldn't I need a 79mm? I'm confused?

 

[CReynoldsMIZ]

I believe that 1.0 mm is the maximum overbore on any seadoo engine. Can you give us some more details, like what happened to the engine. You are probably looking at needing to resleeve the cylinders or just get a new engine, cuz you are already maxxed out

 

[Matthew Sean]

I bought it broken. The sellor told me he had a Factory Pipe installed and the motor stopped working in the first 5 minutes after the installation because of compression loss on the magneto side cylinder. The piston has rings melted into the piston on the manifold side with a chip above the melted rings. The cylinder looks good minor dark spots.
Now I'm worried that if I rebuild the motor and the problem is linked to the FP, I'm screwed. Not an easy decision to make as a full stock manifold, exhaust, and waterbox will be costly. What should I do. What caused this meltdown in the piston?

 

[Dr Honda]

If you want to return it to stock.... a manifold, and pipe are normally cheap. Check ebay.

 

[Spimothy Leary]

ya 720 pipes will be cheap, and honestly, there is still demand for the FP so i'm pretty confident you'd get somebody to offer you their stock parts & cash in exchange for your FP, it wouldn't be costly for you, but for the buyer it will.

if you put an add out i'm guessing a couple of guys would make that offer right now

 

[Dr Honda]

A few things to keep in mind......

1) Rotax engines already put out A LOT of power... and trying to add more isn't easy. (and that pipe gives 20 hp to the 720)

2) The jetting the FP says to use, is just a guide-line. Final tuning has to be done by the installer.

3) Trying to get more power, will bring out every issue you have now, but don't know about. So... if your squash band is tight, you have a slight case leak... or your timing is a little high... those things could be the "Nails" in waiting, for the "Coffin" that the pipe will bring.

4) Trying to get more HP out of a 17 year old or un-known engine is just silly. If you don't know the history... a full rebuild is in order, if you want to build a hot engine.

Just my 2 cents... take it for what it's worth.


Finally... sorry for the loss.

 

[Matthew Sean]

Thanks for the honesty everyone. Too bad I didn't know all of this befor estarting the rebuild project.

 

[Matthew Sean]

Are the any electronics to consider when removing a factory pipe? I noticed the rev control solenoid wire was hooked-up in the electronics box. Is the a special computer to reinstall for a switch from FP to stock exhaust system?

 

[Dr Honda]

Not really. Just clean up the splices, and remove the Micro-touch.

 

[Spunbearing]

I think you can get a ProX set of 1.5mm oversize pistons for that engine. I did a quick web search and they have some listed here:

http://www.atlanticjetsport.com/index.php?l=product_detail&p=2942

I am having my cylinders bored for a set of 1.5mm over ProX pistons now. After this set of pistons they need to be resleeved.

 

[Matthew Sean]

Now that's the kind of info I wanted to hear. Thanks guys. Matt