'96 GTI compression

[Scott D]

I have 115 & 130 psi. Is it time for a top end or is this acceptable?

[Bill O'Neal]

Scott,
Definately time for a top end, or a new pressure guage.
When a Sea Doo motor loses 5-10 psi of compression, the rings are toast.
Being that a two stroke uses an air and fuel mixture from the crankcases, it is important not to have leaking rings that contaminate that mixture.
If you suspect that your guage may be reading inacurately, double check your compression with another guage that is known to be good, or use your guage on another Sea Doo that is in good condition.

[Scott D]

Thanks Bill. If I do the labor what would it cost for a fresh top end?

[Bill O'Neal]

Using oem parts and gaskets (recommended), you'll be out about $300.00

[smitty]

Bill :
could you outline the parts you would replace and gaskets fo a top end job?? My 96 GTI isdown about 8-10 lbs on one jug but runs fine. Very interested in what you would replace or re-use and the costs associated with this.

Would this simply be a re-ring and light hone job or would you look to replace both pistons and rings and inspect the cylinders for wear ??

Thanks

Smitty

Hey this is a good time for me to do this.. The water is down into the 60's here.. too :D cold for me!!

[Bill O'Neal]

Smitty,

720 top end rebuild.

2- ProX piston kits, $215.00
2-caged wrist pin bearings $37.90
1-base gasket $8.50
2-exhaust manifold to clyinder gaskets $10.00
1- exhaust manifold to headpipe gasket $10.00
2- Clyinder orings. $8.00
You can usually Re-use the other orings under the head cover, they rarely fail or leak, or you can purchase a whole top end gasket kit from SBT and replace all of the gaskets and orings for about $30.
One tube of 518 or equivilent gasket sealer.

Get both clyinders bored and honed to fit the new oversized pistons, $80.00 or less at most PWC machine shops.
Do not use WSM or Wiseco piston kits. Either use oem or ProX.
Better yet, spend an extra $300 or so, and buy a rebuilt motor from SBT. You will get all of the above plus the knowledge that your crankshaft, seals and RV components are new and warranted for 1 year. You will also cut your labor time drastically, and know that it is put together correctly. It is very easy to screw it up trying to rebuild the top end at home.

[Scott D]

Bill, Thanks for the info. I have my ski at a shop now. Can you tell me more about why you don't use wiseco pistons? I want to tell my shop to use pro-x or oem, but I also would like to know why.

[Bill O'Neal]

Because they are lightweight forged racing pistons and not the best for rec motors. Use a good quality cast piston and it will last much longer, which makes it alot cheaper in the long run.

[polaridoo]

I am no 'expert' at all, but can you explain how forged pistons (which are stronger) would not last longer? In all high performance applications they are ideal because of their strength. So, why would a stronger piston not be ideal?

[Bill O'Neal]

Polaridoo,
I did not say a forged piston could not be stronger and last longer than a cast piston. I said, "to use an oem type piston". The reason I said this is that I do not use Wiseco forged pistons in recreational watercraft, and that the cast oem type pistons will run quietier and last longer than a Wiseco piston in a watercraft motor.

From the head guy at Wiseco, in a conversation last summer, We were told that "Wiseco watercraft pistons were designed by the same engineers that designed their motorcross pistons". We were told that they are "too light for the watercraft applications". We were told that "Wiseco is re-designing their whole watercraft piston line to make their watercraft pistons more durable and stronger". We were told to "expect to see a completely differnt piston in the near future from Wiseco for watercraft applications".
I just got a call this week from a shop owner who had bought some replacement Wiseco pistons to replace his Yamaha 86mm Wiseco pistons in his big bore Rotax motor. He told me that the new pistons were much thicker and stronger than the Wisecos he was replacing. Maybe they are starting to replace their line of watercraft pistons allready? They did say it would happen soon. Obiviously, Wiseco knows their pistons are too light as they were previously designed, or they wouldn't be changing them.

This was well known by me and many other watercraft motor builders for a long time.
With that siad, I will tell you that I have built several racing motors that had Wiseco pistons in them. But, we use more piston to clyinder wall clearance and we change our pistons out for new pistons quite regularily in racing engines. The added clyinder wall clearance keeps the forged piston from siezing as easily, because the lightweight piston does not transfer heat as well as the cast piston. The cast piston can be run in a tighter bore and not rattle nearly as much.
Another problem of the lightweight piston is the skirts are very thin. Watercraft get abused with water injestion. A small gulp of water can easily cause the skirt to be compressed inward, causing excessive piston slap and broken skirts. The Forged piston takes longer to warm up when you first start the motor. When recreational riders get onto their skis to go play, I doubt many are thinking to themselves, I should take an extra 5 minutes to warm up my pistons.

The wrist pin bosses in a Wiseco watercraft piston is typically minimum in weight and I have seen failures due to the wrist pins pulling out of the piston pin bosses.

Having personally built several dozens of motors for all types of applications of watercraft, from rec riding to world championship winning engines, I recommend that if you are building a recreational motor, you stay with the type of pistons that the OEM's use, or an aftermarket high quality oem type piston. It will run quietier, and it will last longer, given proper lubrication and use.

A watercraft engine has alot more load on it than does a dirt bike motor. The pumps really load these motors down. For our 1050 big bore kits, Miller designed the pistons from scratch, He then had Wiseco build them. After the first batch, he re-designed them again. The new 1050 big bore pistons are quite a bit thicker in all areas. Knowing that the new forged pistons could be made stronger and still weigh approximately the same as the oem pistons ( for balancing concerns ), he made them stronger by thickening the crowns and the skirts and beefed up the pin bosses. This made his Wiseco made pistons very reliable. I believe Wiseco learned from Miller about how to build a watercraft piston that lasts longer in a watercraft, and the proof is why we only sold 6 pistons in a year that did not come with the orginal big bore kit. Of that 6 pistons, none failed structurally. All were failures caused by tuning. Our piston replacment rate for that year was under 5%, not a bad ratio considering that these were mostly racing engines and serious recreational riders engines.

So, to answer your question, I say Yes, forged pistons can be stronger than cast pistons, but only if they are beefy enough for the job at hand.
From my experence with watercraft motors, my opinion is that Pro-Lites have not proven to be the best way to go when building a recreational type watercraft motor. I will continue to build our motors using an OEM or similiar type piston, until I find a better way to do it.

[polaridoo]

Understood. Makes perfect sense. Originally I thought you had an issue with forged pistons. I now see its how the piston is made / designed that you take issue with. Did not know Wiseco's were that puny.

[Bill O'Neal]

Polaridoo,
In all fairness to Wiseco, "Puny" is not a good description of their pistons. Very light would better serve to describe their "Pro-Lite" series of watercraft pistons. They are strong enough to have won dozens of World and National Championships in watercraft racing.
But using racer orientated parts is not allways the best choice for recreational watercraft.