What’s difference Between rotax 657 & 657xmotors regarding pistons, head clearance and compression

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I’m rebuilding two 657x motors from a 1995 speedster boat. I got to the point of doing a squish test for piston to head clearance, following the Seadoo repair manual and the gap is too large to measure. The base gasket below the jugs has 5 reference dots. Would a thinner gasket fill that much gap? New pistons and gaskets came from WSM Performance Parts. I called and asked about the large gap. I was asked if I was racing the machine, if not would not worry will run just fine. I’m wondering if I may have wrong pistons, I compared new with old and seemed to be same from wrist pin to piston top, only visually. I completed motor assembly, all 4 cylinders are at 130psi. Any opinions, advice would be appreciated.
Re: 657 vs 657x

First off all 1994 XP's come stock with 657X motors. The ID tag is on top of the magneto housing.

The differences,
The cases are different, 657X cases are thicker walled and have a 1mm higher deck height.
The crankshafts are easily ID'ed by the holes drilled through the counterbalancer webs on the 657 crank, and the Labrinyth seal on the back of the 657X crank, plus no holes in the counterbalance webs on the 657X crank.
The clyinders are the same on both motors, but the heads differ. The 657 head has a step cut squish band and the 657x head does not have a step cut squish band.
When you remove a 657 head, you will see the piston rises all the way to the top of the clyinder sleeve and the 657X piston will only rise up to within 1mm of the top of the sleeve when you set it at TDC.

In 1994 XP's, they used the SBN 38mm I series carbs. In the 1994 SPX and GTX ( dual carb models that are not XP's ) they used the SBN 38mm carbs ( you can easily tell the difference by looking at the way the carb bolts onto the manifold. The SBN's use nuts to hold them onto the intake manifold and the 657X I series carbs use allen bolts through the whole carb body to attach them to the manifold. There are three bolts holding the airbox mount on a 657 carbs and two bolts on the 657X carbs.
The RV covers and intake manifolds differ, as the 657X has larger oval ports in the RV cover and engine cases, and the intake manifold has a different bolt pattern on the top and where it bolts to the RV cover.

The 657X pto flywheel is shaped differently, and it has a dual coil stator and a different Magneto flywheel.
There are different ignition modules in both models, and different coils.
Is that what you wanted to know?
Last edited by Watercraft Magic; 08-08-2007, 12:13 PM.

If there is a 1mm difference in the two cylinders heights, you are mixed up about something. The pistons in the 1993 motor came to the top of the bores, thus the recessed squish bands in '93 heads. the '94 motor, the X, had pistons lowered by 1mm in the bores, and no cut into the chamber squish bands to even out the compression ratio of the two 657 motors.

Also, do not tell people that you can use a 657X head to raise compression on any non X 657. It will only last minutes in an engine with piston crowns at the very top of the bores before it melts the aluminum on the piston crown and if run lng enough, either blow a hole in the center of the piston, or detonate the rings and edges of the piston crowns so badly that they break off. It will for sure overheat and seize. You cannot run a two stroke motor without minimum squish clearances for the quality and octane fuel that is being run through it.

And like Dan pointed out, you got your X crank mixed up with your 657 crank if both came from the correct model years of XP's, 1993 and 1994.
In an effort to make a lighter full circle crank, Rotax needed to put large holes in the counterbalancer full circle weights, and make them thin and light as possible while still maintaing some reasonable amount of harmonics in the engine so it would not crack the engine cases, rotary valve surfaces above the top case, and shatter the intake manifolds. In doing so, the 1993 657 crankshafts were weaked to the point when they got hot from hard running, and were put under the shocking stresses of an impellor losing and regaining bite as the hull was sometimes in the air and other times in the water, it would twist the crankshafts in a heartbeat. I never checked index on a used 1993 crank without finding it way out of spec, sometimes twisted a quarter of the way around. Because the counterblancer flywheels on the '93 crankshafts were made the way they made them, the pins in them would loosen in their holes as the crankshaft flywheels expanded with heat. One good yank on the impeller would twist a crankshaft out of index. Imagine the twist on a driveline and crankshaft when an XP came down from jumping a wave with the rider still pinned on the throttle to rev limiter hieghts. That was the cheezeist crank that Rotax ever put into a Sea Doo hull. Good idea, bad results.

If you closely inspect the two PTO's, they are quite different too. One is heavier than the other. And that effects the absorbtion of engine harmonics, a terrible problem for a two clyinder motor. Seems to me, the 1994 PTO was shaped much differently than the two in your pictures. It was shaped more like the 720 pto. with the majority of the weight further away from the centerline of the crankshaft which is a big factor in torque production.

The cases are different for other reasons, mainly to make X cases stronger. The thickness of the aluminum in most all areas is thicker. When we were cutting cases for 4mm stroker cranks to fit into them, we had to add alot of aluminum to the lower part of the cases right at the bottom of the cases, by welding layers of aluminum onto the bottom of the cases, before we flycut the cases to clear the longer stroked crankshafts in 1993 cases. But no extra aluminum is needed in 1994 cases, until we tried as much as 8mm strokers...........
The area right where the rotary valve surface is above the upper case half, if you look at the two closely, you will notice reinforcment webs supporting that thin aluminum rotary surface above the cases where 1993 cases allways crack but not on 1994 X cases.

The two intake manifolds have totally different stud alignment as they are inline with short studs on a 1993 inatake manifold, and non existant studs, and replaced with threaded holes that are both angled on the manifold and not inline with each other at all. I series carbs use long hex bolts to hold the carbs on. '93's use nuts on studs that are frustrating and difficult to remove or even access with conventional SBN carb flanges on inline shaft to shaft carb sets.

They probably eliminated the case drain plugs because nobody ever really used them. They are extremely difficut to access when that motor is in a Sea Doo hull. Kinda like the drain plug in a 787 case, it is accessable if you know it is there, and not in an XP model, but a screw-in cap above the counterbalance drive gear is so much easier to remove and then suck the water out.

The two motors share only a few parts that were not changed. the head cover, the rods are the same specs I believe. I am not even sure if the bearings on the crank were exactly the same spec, but they are interchangeable on either crank. I think the 1994 bedplate may even be stronger, not sure though. I believe they used the same oil pumps on both motors. And maybe the woodruff keys in the crank snouts. Pistons fit either motor.

I am sure I forgot more than I remember.
Thank you for the very good explanation of the differences btw these motors. I am working on a 95 Speedster with twin 657X motors. Over haul due to injection oil lines that were deteriorated. Jugs bored and new pistons, also replaced outer crank case seals, RTV seal. Just completed assembly and cylinder pressures on all 4 cylinders are at 130 - 132 psi. The new piston kits came with .5mm base gaskets (would thinner gaskets bring into range?) I was thinking the compression would have been closer to 150psi. I used 3 different compression testers, two matching and one read 25psi lower. Thought maybe possible wrong parts may be reason. Based on your explanation I don’t believe that to be the case. I measured ring gap making sure not too small, but didn’t confirm actual gap width making assumption cylinder bore was in spec for replacement pistons and ring sets were matched for over bore kit.
So with all that said, is 130psi ok to go with, or what could be a solution to raising pressure?
You need to measure the squish to determine the base gasket thickness.

Your compression should be 150 psi, not 130 on a fresh engine.
I attempted to do the squish test according to the manual. The piece of lead that was used was .8 mm diam and it didn’t crush. I’ll retest with larger diam solder. Not sure where I got that diameter reference from but I just now looked up the procedure and see an error in my method. That’s why I was questioning the motor type differences. Thank you, I’ll post results.
Attached are the results of compression test and squish test. Also the spec for new pistons. Hopefully you can advise how to increase the compression to achieve the 150psi value. Thank you.


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So the way you are supposed to test squish is with no base gasket.
You take the clearance measurement without gasket and subtract that from the desired spec and that will give you the gasket thickness.

I am not convinced that even with a thick gasket you could drop 20 psi though, that it quite a bit.
So what would be your guess as to the cause of this condition?
Work completed,
Jugs bored and honed .5mm over
New pistons and rings, wrist pins / bearings
Outer crankcase seals replaced along with two O-rings on each crank shaft.
Rotational valve shaft seal replaced.
All gaskets replaced.
Thanks again for your input