Ski- Doo Rotax Engine Pistons and Rings

 

The REAL reasons behind the Ski Doo 600/700/800 Twin Engine failures!

If you have read the article on pistons HERE, and a little more history HERE, then you have a good understanding of the various functions (duties) of the piston ring.

It is the opinion of RK Tek that the OEM pistons found in the 700/800 Rotax Series III Engines and 800R/ETEC Engines do not have adequate heat transfer from the ring to the cylinder wall. This equates to excessive heat on the piston crown and ring and leads to premature ring failure.

DOO has had ring flaking issues ever since waaaay back in the 583 and 700 triple engine with their moly impregnated rings. This is not a new problem .. what it is... is a problem that is much more apparent with the larger bores and lower revving engines. If you read an article on the function of the ring you will see how one of the primary purposes of the ring is to transfer heat from the piston to the cylinder walls where it can be dissipated by the cooling system. SO.. it stands to reason that if the piston is larger.. it will have a larger surface area to retain heat. So... the heat transfer path is longer due to this larger piston.. it will take longer for the heat to travel from the crown to the ring to the cooling system..(..since Time = Distance/Rate ). Smaller piston.. will transfer the heat in a shorter time because the distance is shorter.

OK...The 800 twin is NOT the only Rotax twin having problems.. the 600 HO and the 700 twin are also plagued and, Yes, their bores are smaller. So, what is happening???? IMHO there are several reasons and some of them elevate the effect of the other. Keep in mind.. these are only MY opinions...

1) The biggest reason is the ring design/material content. The ring is comprised of a cast alloy with another alloy impregnated in/on it. You have 2 dissimilar metals with different chemical properties joined as one. Keep in mind.. I am no Chemist.. so I do not have all the chemical compositions of each alloy and how they are bonded. In short... I believe that these 2 alloys work well together under most conditions but under high heat, high stress and oil submersed conditions.. I THINK they have some issues with maintaining the molecular bond. NOTE: Replacing this ring with a chrome ring WILL stop the flaking. BUT... it will NOT stop the power-fade problems that are apparent with this engine. Read more about this below!

2) The ring material is insufficient to handle the amount of heat it is subjected to and actually dis-tempers during heat transfer. This causes an actual de-lamination of the ring facing and leads to pitting of the ring face. Goes with what was stated above.

3) Single ring.... Single ring set-ups are great for making power in high rpm engines. In fact, in low rpm engines also, for that matter. What a single ring is NOT good for is transferring excessive amounts of heat. The ring is overloaded with heat and can not adequately transfer the heat from the crown. DOO has made the ring very thick in hopes of alleviating this problem but it is not doing the job. The ring is simply overloaded with heat!

4) Loose piston to cylinder wall clearances. The excessive piston to cylinder wall clearances found in ALL Series III engines allow for the piston to "rock" slightly in the bore. This rocking places the ring at different angles on the cylinder wall and can act as a "scraper" and scrape the oil barrier from the cylinder wall. Besides this scraping effect.. it also places undesired loads on the ring and the piston (thrust points). If one were to exaggerate this "tipping" of the ring .. one could envision the ring only making contact (transferring heat) through partial contact (not full surface area) of the ring face on the cylinder wall. This would also equate to much less heat transfer to the cylinder wall and the heat being imprisoned in the ring, itself, because it was not mating completely with the cylinder wall. NOTE: This is only a worst case scenario... I do not know, or have any way of knowing, if this is actually the case. Just a theory.

5) Poor port chamfering... MOST of the cylinders I have seen have FAIR port chamfers (bevels) I have seen some that have very POOR chamfering and others that have EXCELLENT chamfers. Poor chamfering could , and most likely ,would, cause stress on the ring.

People are all to quick to point the finger at detonation for this ring failure. While detonation may be to blame for some of these ring failures, RKT feels that it is not the "BIG CHEESE"

Having said all this...it all boils down to having an insufficient medium to rid the piston of its heat. This is VERY apparent because ALL of the ring flaking I have seen, and , believe me... I have seen PLENTY!, starts at the ring pin. The ring pin is the hottest spot of the ring. WHY.. because there is less surface area at the pin. People like to think that the exhaust port side of the piston is subject to the most heat, and they would be correct.. But, heat will always find the path of least resistance and the ring pin is that path. So if you think of the ring as a heat sink... there is less of a heat sink at the ring pin and that is why the flaking will ALWAYS begin there. So, inspecting your ring via peering through the RAVE slot is always a good idea.. not seeing ring pitting at the exhaust port will NOT assure you that you do not have pitting at the pin.. where it all begins. Chrome Rings are not the answer.. Are they better than the OEM Moly ring??.. YES.. Will they cure the inconsistent power and fade that is associated with your engine?? NO!!

NOW.. Why is it so apparent in the Mountain sleds vs. the Lake runners??? ANY Mountain sled engine will be subject to MUCH more engine and frame load/stress then its twin Lake runner. Mountain sleds are put through much more loading because of several reasons

a) The engine is subject to longer intervals of engine loading. During this loading. the entire drive train is also load much more heavily.. which, in turn, puts even more load on the engine.

b) Mountain sleds are heavier and have heavier components to move.

c) Mountain sled engines, due to elevation, are not putting out the power of a Lake runner engine. So, any load placed on it is amplified because it is NOT making the same power as it would at lower elevations. For example.. say you could place the Rocky Mountains in Michigan. And take 2 identical sleds and put them in the same condition. The Michigan sled would see less of a load on the engine doing the exact same task because it would have more power.

These failures are more apparent in the engine of the Mountain Riders. The reason for this is simple. Mountain riding places a much greater strain on an engine. It is not uncommon for an engine to be under a FULL LOAD condition for over 3 minutes in everyday deep powder riding. This pro-longed "loading" of the engine is very hard on the piston and ring due to the fact that the piston is having a very difficult time ridding itself of heat via the ring.

Eventually the ring will fail and cause a VERY LIGHT seizure or flaking of the ring face. This will usually not stop the engine or leave you stranded BUT will cause a power loss and eventually total ring failure and engine seizure.

The 800 twin (and the 700 twin for that matter) power-plants are not capable of SUSTAINING power on long pulls!! WHY??? because the engine is constantly heat soaked!! meaning... your engine is ALWAYS overheated, internally, and this will cause power loss! BIG TIME!

The reason for this heat soaked engine is the pistons/rings combo. The OEM pistons can NOT adequately rid the piston of the heat. The OEM rings are always in a state of failure during extended loading of the engine (meaning long pulls). The ring's function is to transfer heat from the piston crown to the cylinder walls where it can be disappated. This does not happen. The rings dis-temper and can not sufficiently transfer the heat from the crown. This results in elevated piston crown temps and severe power loss!

The OEM equipped engines will never be able to continually charge a hill and make progress. They will keep going lower and lower on the hill with each pass after the 2nd try (or so). You may think that it is your clutching getting hot but it is your engine getting hot causing the problems.

These Rotax Twin Engines are an awesome power-plant when "repaired". In the repaired state.. they can and will out perform most of the other modified engines in their respective cc class.

Great engine.. just needs a little fixing IMHO

It is very possible, and likely, that better than 75% of all Series III engines have had a ring failure (flaking) and the owner is not aware of it.

Another problem with these pistons is that they seem to have a short life span, about 1200-1500 miles, before stress fractures occur in the skirts. A fractured piston skirt WILL eventually break off and will ,most likely ,lead to severe engine damage!

Now.. What is the solution?????

Well.... The solution for the 700 engine is the 738cc kit which uses a much better piston whilst adding power. Please see details below in this article.

RKT has a solution for the 800 and 800R Engine. This solution comes in the form of a replacement set of pistons and a custom head to match the pistons.

RKT will be offering a Piston and Head kit for the 800 Series III and Series IV (R) Engine. This kit will include 2 new pistons with wrist pins and a custom head. These new pistons are a dual ring piston with a slighlty higher piston crown. The combination of the cooler crown temps and the better heat transfer to the cylinder walls will lead to an increase in the power output of the engine on the long pulls when the OEM pistons are failing. These pistons will have more longevity associated with them and last 1 or 3 seasons faithfully.

RKT feels that the stock head can not be properly re-shaped to accommodate these new pistons and will not be offering that service. RKT feels that the only way to acheive the proper combustion is to have a head made for these new style pistons.

RKT feels that these new pistons are a "must have" for the serious deep powder mountain riders in order to avoid ring failure.

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