657 engine parts swap

[jeannemark]

I have a 657 block with rebuilt crank, 657x cylinders with head and rotary valve and dual carbs. Are there any problems with different deck heights in the block or are they the same. The motor is already built and in the boat but not started. I'm not looking to make any more power just reliable. This is in a 1995 seadoo speedster.

 

[seadoosnipe]

Blocks...

I believe the blocks are of the same dimensions......:cheers:

 

[jeannemark]

thanks for the reply. Your info makes me feel alot better about the rebuild.

 

[seadoosnipe]

Blocks....

I have not done a super lot of research on the 657 verses the 657X. I do know there is a lot of small things that are different, like in the electrical system but have seen where a lot of the components are different.

It was just recently shown to me that the blocks are different in deck height. I know you've responded to this already but, I have got to check on a few more things with this.

But for now, assume that the deck height IS different..........sorry for the mistake.:cheers:

 

[timmyboy76]

Yeah,from what I gathered, the X case was 1mm higher than non X, used the same cylinders, but for non X head, they cut a "squish" groove in, thus with X motor, since raised 1mm, no squish cut to head.
So, with jeannemark situation, I'm assuming, using the cylinders is fine, but with "non" X case, they need a "non"X head, rather than an X head...right?, or stack 2-3 cylinder base gaskets and use the "non"X head....

 

[robin savell lloyd]

From what I have read from pros is that the cylinders port timing are a litle different but will work. The numbers on the 657 vs 657x cylinders are different. You can put 650 cylinders on 650X cases and vice versa but you need to keep the cylinders & heads as a matched set. This is not the best way of doeing things but should be ok. The rv holes [air fuel intake] on the 657 crank case are going to be smaller than your old 657x so you may need to adjust carbs. Just keep in mind that I can allways be wrong. Robin :cheers:

 

[seadoosnipe]

Just a bit different!

:agree: Robin.....but the question was about the deck height. Which is what timmyboy is pointing out. We have all looked at using the 657 and 657X in different applications and what parts are interchangable. But the question from the user is about the deck height. Like timmyboy said, it's about 1mm. Yeah, that's really so small it may not be a huge issue but from what I gather, changing the cylinders out with between the two could cause detonation. The 1mm difference is in the piston between the depth of the ring from the dome.:cheers:

 

[robin savell lloyd]

:agree: Robin.....but the question was about the deck height. Which is what timmyboy is pointing out. We have all looked at using the 657 and 657X in different applications and what parts are interchangable. But the question from the user is about the deck height. Like timmyboy said, it's about 1mm. Yeah, that's really so small it may not be a huge issue but from what I gather, changing the cylinders out with between the two could cause detonation. The 1mm difference is in the piston between the depth of the ring from the dome.:cheers:

:agree: My bad as I did give some bad info as I thought he would just rase his compression just a litle. There seems to be a litle more to it than that. From what I understand deck hiegth is the distance from the top edge of the piston crown to the top edge of the clyinder sleeve.
For some reason the early 657 motor had pistons with very little deck hieght. The head has a squish band recessed into it to provide deck hieght. The problem was that it let the top ring get too hot as it was too close to the combustion above the piston. The ring could overheat and either lose tension or glow, causing detonation.
rotax figured it out and made the engine case 1mm taller where the clyinders bolt down onto the cases ( in other words they increased the distance of the gasket surface on the engine cases 1mm further away from the centerline of the crankshaft). Using the same clyinders in both motors allowed the piston to sit 1mm lower in the bores. Then they used a head without the squish recessed into the head to bring the squish back into spec. This allows the top rings to run much cooler, last longer and not create detonation.
This is why you need to know the parts when mix matching parts on these motors so you do not run a head without a squish cut on a motor that needs a squish cut recessed into the head or vise versa..

 

[jeannemark]

thanks for all your info. As for now what it sounds like all it is going to do is raise the compression a little and minimally change some port timing. Seeing that the person that built me the motor gave me a year warrant, and he is confident there is no problem with this setup, I'll run it the way it is and go from there. As for the possible detonation I still have to reset the timing, so I'll just retard it a few degress. Thanks again and I'll post how it all goes.

 

[timmyboy76]

thanks for all your info. As for now what it sounds like all it is going to do is raise the compression a little and minimally change some port timing. Seeing that the person that built me the motor gave me a year warrant, and he is confident there is no problem with this setup, I'll run it the way it is and go from there. As for the possible detonation I still have to reset the timing, so I'll just retard it a few degress. Thanks again and I'll post how it all goes.

think'n, with runn'n the X head, like you said, the compression be up there, think'n, your gonna need to mix the gas w/ higher octane,...if doesnt run right...good luck

 

[robin savell lloyd]

think'n, with runn'n the X head, like you said, the compression be up there, think'n, your gonna need to mix the gas w/ higher octane,...if doesnt run right...good luck

Like I have said before when timmyboy is talking listen up. A higher octane gas does sound like it may work timmy but I can allways be wrong so good luck. Robin :cheers: