787 RFI crank tolerance

[benaspx]

Hi All..

My son wants to be a marine engineer, we bought a dead Jetski and have been trying to revitalise it in time for Christmas.

We got stuck with removal of fly wheel and took crankshaft to an engine rebuilder, they managed to remove it with a lot of heat and rattle gun, they also changed some bearings and seals for us.

When they gave us back the crankshaft we were told that the pistons were out by 2000th and that i needed to check manual to ensure that was within tolarance in the shop manual.. I can't find any reference to it in the manual, the engine technician said that it was minimal error well within tolerance of most engines.

What do you think guys? should I rebuild the engine, or try to correct?

787 RFI 1999 GTX
Many thanks Ben

 

[mikidymac]

Not sure what he meant by pistons are out on a crankshaft, that isn't normal terminology.
You can check crankshaft deflection to see if it is twisted. .002 in on MAG side and 0.001 in on PTO side.

It is in the shop manual...
https://seadoomanuals.net/download/pdf/service/manuals/1999/1999-seadoo-shop-manual.pdf

 

[benaspx]

Hi Thanks for getting back to me,

I'm very new to engines, I rebuilt a 50cc engine with my dad.. that was a looooong time ago, so please forgive me if I don't use the correct terminology.

So from my understanding, one of the cranks was held in a vice to remove the Fly wheel, he used a big rattle gun to get the fly wheel off, with all the vibration and heat either one of the cranks have moved fractionally or it's always been slightly off within tolerance?

I read in a 787 manual cranks supposed to be within tolerance of180 degrees to each other, however I couldn't find in that manual what the tolerance was.

I'm pretty sure the link you sent relates to the deflection of the piston rods.

So I think the 2000th measurement he was giving was the offset of the pistons?

does that make more sense?

 

[etemplet]

More likely he meant (.002") on the crankshaft bearing journals. If you are that close I dang sure would rebuild anything. I'm a machinist by trade so I deal with those measurements. Put it together.

 

[benaspx]

More likely he meant (.002") on the crankshaft bearing journals. If you are that close I dang sure would rebuild anything. I'm a machinist by trade so I deal with those measurements. Put it together.

Thanks we'll give it a go tomorrow, I double-checked with them it was the run off? a senior guy in the workshop also said if it was down to him he'd rebuild.

I think they were just covering themselves. In my head the last thing in the world I'd want to do was rebuild and reassemble the ski only to find it seized after it ran for 10 minutes.

Thanks for your help

 

[etemplet]

I would touch base with them again because there are people that rebuild their own crankshafts. They don't have a lot of tools for this but they get it done. If that shop rebuilds cranks they know what they are doing. As I said, I'd ask questions. Just get them to feel comfortable that you are not gonna be an a-hole if something happens. I don't like wasting money and I see a lot of that going on. These skis are not new and you don't need a brand new engine. You just need it to work reliably. Believe me this stuff will run "half tore up" and you won't even know it till it finally gives up the ghost. LOL The engine will tell you if it doesn't like something.

 

[benaspx]

I would touch base with them again because there are people that rebuild their own crankshafts. They don't have a lot of tools for this but they get it done. If that shop rebuilds cranks they know what they are doing. As I said, I'd ask questions. Just get them to feel comfortable that you are not gonna be an a-hole if something happens. I don't like wasting money and I see a lot of that going on. These skis are not new and you don't need a brand new engine. You just need it to work reliably. Believe me this stuff will run "half tore up" and you won't even know it till it finally gives up the ghost. LOL The engine will tell you if it doesn't like something.

Thanks good advice, after getting new pistons and top end honed it would be silly to go off half cocked. they suggested that they assemble the bottom end for me to ensure everything was right, I'll just go with that to be safe.

thanks Ben

 

[mikidymac]

Ask if they measured it with V-Blocks and a dial indicator. If they say no then they don't have any business touching the crankshaft. Basically they should be checking to see if it is twisted from removing the flywheel and that is what the specs are that I have you above.

Also you need to have the cylinder bores measured to see if they are within spec. Typically on a used ski you don't just "hone" and install new pistons because the cylinders are already worn out of spec. You need to bore to the next size and install oversized pistons to get back within spec.

 

[etemplet]

Thanks good advice, after getting new pistons and top end honed it would be silly to go off half cocked. they suggested that they assemble the bottom end for me to ensure everything was right, I'll just go with that to be safe.

thanks Ben

As @mikidymac stated, the bore needs to be measured to make sure it is in tolerance. Good Luck !!