bhambrook

Hello Bill:

I have a few questions for you:

1) Could you please tell me how to do a Squish Clearance test in a Rotex 717?

2) do you happen to know what cylinder base gasket thickness was used on a 1996 SeaDoo HX? There appears to be many different sizes available as OEM gaskets.

3) does shaving the cylinder head a specific amount (ex. 0.1mm) have the exact same effect as using a cylinder base gasket (or cylinder head gasket) that is thinner by the same amount (ex. 0.1mm thinner)?

Thank you very much for all your help.

Brent

Mr. Bill

To find the proper oem squish clearances, look it up in the technical pages at the back of a service manual.

To check squish,
Remove head. Place a .065 piece of soft core solder in the center of the piston directly inlne with the wrist pin in the piston and full length from the back of the bore to the front of the bore ( full width of piston from side to side when piston is below the top of the sleeve ). Replace head with a few bolts tight so the head stays put. Rotate the pto until you feel it smash the solder. Do not rotate it the wrong direction, you want the piston to come up, not down. Remove head, measure the squished part at each end of the solder, but not at the very tips that hang over the edges of the piston crown. Right behind that point, will be the thinnest of the measurents as the tapered squish band in the chamber gets thicker as you move away from the outer edges of the solder. This thin measurement is your squish thickness. Choose the right base gasket to put you within the minimum and maximum squish clearance. Most stock motors come with a 4, 5, or 6, base gasket. Your best choice is usually a 5 if no machining has been done to the head or the top or bottom of the clyinders. The 5 will work for 99% of recreational motors.

A quickey way of checking squish when the head is allready on it, is by bending a piece of .065 solder into an L shape and feed it into the sparkplug hole until it hits the sleeve. It must be directly over the wrist pin, centered on top of the piston to eliminate piston rocking side to side. If you do it that way, do it three times, using a new piece of solder each time. Measure at the thinnest part out by the very end before the end takes the drop into the gap at the edges of the piston as it will be thicker there because of the gap for piston clearance in the sleeve. Average the three measurements that you get. That will be very close to the actual squish clearance. Use soft core solder.

Do not cut your head surface if possible. But if you do, take an equal amount off of the squish band, or maybe just a little less to get the squish clearance closer to what you want it to be without swapping the base gasket out.

Ths is why we mock up a piston onto a con rod, bolt down the clyinder, measure squish gap with out the base gasket under the clyinder, then calculate the proper thickness gasket needed. Then we finish assembling the motor when we know what gasket it needs.

bhambrook

Thank you very much Bill for your informative answer. I really appreciate it. Another question, in order to remove the PTO an HX (in order to install a 580 PTO), is the easiest way to remove the exhaust (and shove a rag in the exhaust port) and slide the engine forward? And, what should I torque it to when I reinstall it?

Brent

Mr. Bill

A 1996 HX PTO has a coupler on it. No 587 PTO uses a coupler on the PTO, they have splines like the 1995 HX PTO and most other 720 PTO's that are not on an 1996 or later HX . So, I assume you have a 1995 HX driveshaft tube ? Or maybe your 1996HX is really a 1995HX ? Or, you are using the HX motor in another model other than the HX ?

I never liked stuffing a rag into the exhaust port as the cloth will get cut up and there usually will be some of it stuck into the rings or something. Use a wood stick or a piece of teflon, something softer than the metal parts but ridgid enough to not compress when you put the leverage to the PTO to remove it. Always use the PTO clyinder so you do not put the twisting power onto the front of the crankshaft when removing a pto. That eliminates the possibility of twisting the center pins or front crankshaft counterbalancer flywheel pins in the crankshaft.

You can easily twist a crankshaft out of index if a PTO is really stuck on the threads of the crankshaft badly and you have no heat applied to the pto to loosen it up.

bhambrook

My HX is a 1996 model. The Sport Spec rules now allow you replace the PTO with a 580PTO. Does this mean that I cannot install a 580PTO in my HX?

Brent

Mr. Bill

No, but finding a 1995 HX center driveshaft tube that has a splined front and a coupler rear end on it may be difficult to locate.
Then, if your motor makes good power, you may spin the splined insert inside the driveshaft tube as it gets very hot when riding or racing from heat transfered to it from the pto. It can get super hot. I my own raceboats this became a problem for us and we lost a race we were leading at the time because once it spins the splined insert in the driveshaft, you go nowhere. It is like putting your car in neutrual. We had to scramble to get the shaft pinned and welded into the driveshaft tube so it could not spin again before the next race. I was in Virgina racing at a National event and my shop is in California.

If it were mine, I would not put the 587 PTO on it, as the 1996 PTO is made to dissipate heat away from the aluminum and rubber coupler and has a better wieght to it for reducing harmonics in the engine. You are much better served by reducing the weight on the magneto flywheel. We used RAD aluminum flywheels. If you notice, a 720 crankshaft has a very long front tapered snout to make room for and to mount a magneto flywheel onto. This places a very heavey wieght out beyond the two from crankshaft roller bearings in the cases. Those two bearings take a real beating because of that weight. Add to that, the steel magneto flywheels are no where near properly balanced. In fact, if you run the engine at high rpm with the magneto cover removed and at the same time, shine a strobe light at the flywheel, you will run for cover or wish you had a block wall between you and that gyrating flywheel. I was amazed at out far out of round that thing was as it rotated out on the end of the crankshaft. No wonder 720's always have really bad front bearings on the cranks when you tear down a used motor. All this really drains power from the engine and because we run these motors much harder that a normal rec 720, and at much higher rpm, it becomes even worse of a problem for a racing motor.

Long before we had 657 and 720's or aluminum flywheels, we raced the 587 engines. I used to take my steel flywheel to the balancing shop after removing the ring gear and drilling large holes spaced equally around the ring gear to reduce weight and do as much machining to the steel flywheel to reduce weight as possible, then tig weld the ring gear back onto the flywheel in three equally spaced spots around the circumference of the flywheel. You will find on a stock 720 flywheel that so much heat is built-up into the flywheel that sometimes the ring gear will walk back on the flywheel until it jambs up against the front of the engine cases about 1/4 inch away from the ring gear when the ring gear is in the oem position on the flywheel. In my years as a Sea Doo mechanic, I have seen many of the "upright engines" ( 587,657,720 ) come into the shop locked up. Usually this is a rusty crankshaft condition, or broken piston or whatever causing the lock up. But quite a few times it was the ring gear moving back on the flywheel. The ring gear is pressed onto the flywheel from the backside and the only thing holding it is friction and red Loc-Tite unless you stitch weld it on there.

There is absoutely nothing I like about oem steel flywheels. They are grossly out of balance. They are steel and they retain heat. They are very heavy and placed too far out away from the two front cranksfaft bearings. They just beat those bearings to death and all that friction creates alot of unnecessay heat and heat and friction robs power from an engine. Eventually, the cases become wornout in the bores for the two front main bearings.

If you want to build a winning engine, you should think everything about that engine that can make your engine better than the one sitting next to you on the starting line, as it is easier to lead races than follow and pass other racers.

bhambrook

Hello Bill:

Thank you very much for that awesome explanation!!! We all learn so much for you.

Brent

greenwhite1

I know this is an old post but hoping to get a question answered. When you say soft core solder is that a 60/40 rosin solder, or do I want a 50/50 tin lead? I know that you do not want acid core because if its corrosiveness.

Mr. Bill

Brent,
1. I told you how to check squish in my last post. Do you have a specific question that I did not cover in that post ?

2. Most had a 4, 5, or 6 with the majority having a 5, which means 1/2 mm. or approx, .019 -.020 thousandth's of an inch. #8 .8mm is the thickest, #3 .3mm is the thinist. A millimeter is almost .040 thousanths of an inch. You need a caliper to measure squished solder, or a base gasket if you do not own a 0-1 in. Micrometer. Use .065 soft or flux core solder. Most top end rebuild kits are shipped with a .5mm base gasket as it is the most likely size to come closest to oem squish clearance thickness, without exceeding the minimum requirement for regular pump 87 octane gasoline. I recommend using 89 octane minimum if the head has not been milled.

#3. Yes, pretty much if they also cut .1mm off of the full width of the 11 degree angled squish band in each chamber of the head too when they cut .1mm off of the head gasket surface.

Bill