657 vs 657x cylinders

[wabash]

Trouble shooting an issue with one of my two 94 GTXs I have found they have different casting numbers between them on the cylinders. I have done complete rebuilds on them but one was about 8 yrs ago and the other last year. My issue is with the " newer " one. It has casting number 6913382 on the cylinders and they are not original. I had to do an exchange for them my old ones for new machined ones matched to new pistons and rings. The "older" one I had the original cylinders machined and I re used them. They have casting number 6913380 on them. I am pretty sure the 6913380 is the correct cylinder for a non X 657. I read somewhere the 6913382 might be for a 657x which is 1mm different in height. Can someone confirm for me if this is true and what the height difference is? So the x jug taller or shorter?

 

[wabash]

Ok I think it's confirmed after browsing thru some listings on eBay that the 6913382 is off a 657x. So it appears I was hosed by JJ Machining who sent me 657x cylinders in place of my 657 cylinders. Nice!! And I'm guessing this is where my problem is. Either by the different compression and/or porting of the x jugs on the non x motor it is lacking some low end grunt. I am seeing 140 psi compression on this motor vs my other one which is all correct parts and shows 160 psi.

 

[Strizzo]

I think the 657 and 657x jugs are close enough, only real difference is the carb tune and the x has oval instead of round ports on the rotary valve that gives it the extra hp.

 

[wfo speedracer]

You can run either cylinders on either engine as long as you have the head that matches the cylinders, the 657x cylinders are 1mm shorter than the standard 657 cylinders and the pistons on the 657x protrude slightly from the top of the cylinders, the heads are of course cut to accommodate this difference.

So in short just get a 657x head, bolt it on and call it a day.

 

[wabash]

Ok, the info I've been getting off the web is completely confusing and contradictory. I did some searches last night and found about every different explanation of the differences between the 657 and the 657x that you could imagine. Some say the cylinders(jugs) are 1mm different (the height measured from the case sealing surface to the top of the cylinder, within that some say the x was shorter some say the x was taller (ie piston didn't protrude above the cylinder in the x). Some say the two cylinders were the same height and the deck of the case was where the extra 1mm is and its the x case that has a taller deck thus making the piston in the x not protrude above the cylinder. The heads were then different to compensate and provide proper squish clearances for the piston the common theme being the x head has less squish clearance. This was also supported by several posters warning not to run an x head on a non x motor as there will not be enough squish clearance. I know for a fact that I have non x cases and non x heads on my machines as they are original, the only thing in question is the cylinders on the one machine that have different casting numbers than the known originals that I have. Also it was noted the porting in the x cylinders is different than non x which stands to reason if the rotary valve timing is different and intake ports are larger to allow better air flow.

So here's what I know. I have non x cases and non x heads. the cylinders on the problem machine appear to be x cylinders based on casting numbers I can see on eBay associated with cylinders off of 657x engines. The problem machine has approx. 20hrs of use on it and runs well except for being a little weak in the low and mid range under load. It shows 140psi compression on both holes where the other machine shows 160psi on both holes.

It stands to reason that either 1. The x cylinders are 1mm taller and combined with the non x head (more squish clearance) provides a lower compression ratio thus reducing the cylinder compression psi and also affecting low - mid range power Or 2. the cylinders are the same and its the x case deck that's 1mm higher which means my motor with the x cylinders and the non x head presents the piston to the head the same as a normal non x motor and its all because the porting in the x cylinders that is giving me the loss in low-mid range power and compression psi.

My vote would be #1. But man I tell you there is a ton of misleading, confusing info on the forums about the 657 657x motors.

 

[wfo speedracer]

This is the best info I have ever come across regarding the differences in the two engines, cut and pasted from PWCT from the man himself Bill Oneil , he has forgotten more about these engines than I will ever know.

If there is a 1mm difference in the two clyinders hieghts, 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 sieze. 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.

And here is a pic from the same thread which absolutely verify s the cylinder length difference. If you need the link to the thread PM me I don't think it will let me post it here.

 

[wabash]

So based on Bill's information (x motor pistons were lowered in the bore by 1mm at TDC) and your picture, the x jugs are taller by 1mm. Combined with my non x head that makes my motor have less compression. So technically I should be able to bolt up an x head and gain back what I need? If it were the other way around (x jugs shorter and non x head less squish) I would have more compression and would have melted the pistons by now.

 

[wfo speedracer]

So based on Bill's information (x motor pistons were lowered in the bore by 1mm at TDC) and your picture, the x jugs are taller by 1mm. Combined with my non x head that makes my motor have less compression. So technically I should be able to bolt up an x head and gain back what I need? If it were the other way around (x jugs shorter and non x head less squish) I would have more compression and would have melted the pistons by now.

I say you are correct

 

[wabash]

Confirmed this morning. I checked the squish and its .079 to .085."Or about exactly .04" (1mm) from middle of spec. I show spec as .035 to .051". Strangely tho my SeaDoo shop manual does not have a squish test in it. I pulled this info from Seadoosource

657x head on its way.

 

[wabash]

Installed the 657x head and all looks good. The squish came into range, towards the upper end and compression is 160-165. Hitting the water today for a couple weeks.

 

[wabash]

So to straighten up bad information that I found all over the Internet. The 657x cylinders ARE 1mm taller from the base gasket surface to the top of the cylinder, than a 657 cylinder. There is a 1mm relief cut into the domes (squish band) of the 657 head. Thus moving the cylinder up in relation to the piston at TDC on the 657x but keeping the volume in the combustion chamber above the piston the same. The deck of the cases between the 657 and 657x is the same. The extra 1mm is not in the cases it's in the cylinders. So the cylinders and heads go as sets and yes you can swap them as a set between motors. DO NOT use a 657x head on 657 cylinders as Bill O has said. Not enough clearance. You can tho run 657 head on 657x cylinders but you will sacrifice some compression and a minor amount of low end torque. The non aggressive rider would never notice it and you would have a motor with much lower stress on it. Now as for porting I don't know the details. The 657x cylinders are ported differently but it doesn't seem to have much effect vs the 657 cylinders when using a 657 case and rotary valve.

 

[wabash]

Ran both my GTXs today. They run essentially identical. One has 657x cylinders and head the other just 657. Both have 657 cases and rotary valves, exhaust etc. there is not one noticeable difference in power delivery or top speed between the two. Both run very clean and strong.

 

[dalef62]

What is the horse power difference between the 657 and 657X? How do you route the pulse line on the 657X to carbs that were on a 657?
Thanks

 

[wabash]

I think it's a 10 hp difference on paper. 70 vs 80. But you'd be hard pressed to tell the difference on the water. Save yourself some grief and don't put a 657 manifold and carbs on an x. Use the x set up. Wish mine were. Waaaaaay easier to work on. My cousin has a pair of 95 GTXs. Much better set up but x motors are higher compression thus more hp and if you ain't careful you can burn a piston quick. Absolutely no ethanol gas and only 93 octane. I remind him all the time. I've run his and compared to mine I can't tell a difference.