951 Piston Question

[ParamedicCurt]

I have a 2003 Seadoo GTX-DI (Rotax 951) that I thought only needed a fuel pump. A couple thousand dollars later and it still doesn't run. SO, I've been back tracking my work. It needed a new top end. I've included screenshots of the compression before and after. The picture of the piston is the original. My question is, I installed a WSM STD size piston. The machine only had 86 hours on it, so I couldn't imagine it had anything other than the original parts (and being my first project, I didn't have the tools to measure). IF I'm reading the piston right, the cylinder has been bored out to .045 over right? I didn't know at the time what those markings meant. And a STD size piston would be outside the tolerance? Could that be the reason it won't run?

At this point, I'm just about to buy a new core with a warranty so I can sell it and recoup some of my money. Please help! I'm just a guy with a carport and a few tools trying to fix something to ride. Now, I have no choice but to sell and start over. BUT, I DO NOT want to screw the next guy like I got screwed. The guy who sold to me SWORE the engine compression was 120+.

Another question: IF I were to buy STD bore cylinder sleeves and have a machine shop put them in, the STD pistons I installed would still be usable, correct?

99.6 and 116.6 are the MAG piston before and after
107.4 and 108.4 are the PTO piston before and after

I've done SO much work to this machine! And learned a lot along the way!

I've stripped all the old decals. Recovered the seats. New grips. New fuel pump. New reed valves. New fuel pump. New fuel injectors. New fuel and air regulator. New air compressor. New hoses. All the electrical connections have been cleaned and gotten new dielectric grease. The trailer has new lights, new fenders, new tires, hubs and bearings. IF I can get this running, someone will have a SWEET machine.

 

[etemplet]

If you did the top end "no way" are those the correct readings. Don't waste any money... stick with it. Borrow a good compression test gauge. Find the leak. The engine is just a pump not an all in one fix.

What is the reason you want to go standard bore with the cylinders? That is way more money than just a rebore. Sure wish you lived around me. We'd have that thing running.

 

[burtshaver2021]

I’m not sure if this has already been answered but we’re the cylinders bored or only new pistons and rings? Could those readings not be correct if cylinders are egged shaped?

 

[Frige]

Not sure Im reading that right. If the cylinder is bored 045 over, a standard piton would be way to small. The piston would be flopping around in the bore and ruin the engine.

 

[POPPS]

Would,,, remove the rings,,,put them in the cylinder,,,using the piston,,,look up the ring width clearance,,,if it is within limits,,,leave it alone,,,then measure the piston clearance,,,f they both fall within limits,,,you should be okay,

 

[etemplet]

Never heard of 45 lol.

 

[DI_Hoss]

I'm pretty sure that's the part number. at the exhaust side of the piston there should be a number that reads like 87.88 (which is OE piston size, the bore being a whole number, or 88.0mm in this case)


What does your bore look like? Any scuffs, pitting, rust? Anything that might make the rings not seal correctly?

 

[DI_Hoss]

Also, for what it's worth - if the sleeves are scored, just get it bored and new oversized pistons - it would be a huge disservice to get new sleeves installed (which rumor has still need to be bored true and deck milled) and install old worn pistons. The only real reason to install new sleeves is if the ones in the engine are beyond boring (+1.0mm over); and even then, I'd still look at a SeaDoo junk yard for a salvageable jug.

On the cheap, if the bores and pistons both look good, no scoring, you could hone the cylinders and just install new rings. Life expectancy - eh - not as long as a fresh bore and new pistons/rings (as the pistons already have some wear).

One last thing - watch out for and keep track of the OE needle bearings at the wrist pin (at least they are on my 2001) - they are uncaged and could go everywhere, like into the crank.