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Rusty Main Bearings


portman
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Has anyone got any experience on how to prevent main bearings going rusty?

I purchased a used Sherco 290 (2005 model) a while ago and I recognised from the piston slap that it needed a new piston and relined barrel. When I stripped the bike I saw that the main bearings were very rusty, but thought nothing of it as I did not know how well the previous owner had looked after the bike.

I fitted all the necessary new parts, including good quality mains and once the bike was back together I started the engine about every fortnight to ensure fresh oil was taken into the motor. After 3 or 4 months I detected the mains were rumbling, getting progressively worse. In fact on some occasions before starting the bike it was obvious from the noise coming from it as it turned over that the mains were once again a couple of rusty ball bearings.

Once more I stripped and replaced the mains, using top quality SKF parts and this time I ran it on freshly bought premix oil of a different make, again starting it about once a fortnight.

Now, again after a few months the mains are emitting the tell tale noise of rusty ball bearings!

It is not losing coolant and I do not believe water is getting in from washing as it doesn

Edited by Portman
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Not s...ure what you are doing, but two things are for sure.

You do not store them after a muddy trial without cleaning them and inspecting/protecting the airbox.

You do not wash them without protecting/cleaning the airbox and filter. :rotfl:

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Hi Portman,

I haven't worked on these motors before so can only generalise. If you're having both main bearings go rusty then there is water getting in the crankcase, which may suggest that there is water getting into the gearbox/clutch area usually from a water pump shaft seal. As it has happend often, it is pretty odd. If you are sure it is not from the carb/airbox area and there is no water leaking past the pump seal, then water is entering the crankcase from the cylinder. Have a look to see if the water passages have not corroded through the base of the barrel and past the base gasket into the crankcase. What work was done to the barrel? Was it bored out a little and re-plated, or was a sleeve made up and fitted? Was it a reputable engineering company that has done this type of work before?

Hope this helps, PeterB.

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I bet its from water getting into the airbox while riding it or washing it, when washing it you should tale off the rear guard and tape up the airbox opening, then proceed with the washing

hope this helps

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After-ride easy-to-do sequence to minimise risk of rusty mains:

Wash bike

Fit a clean and freshly oiled air filter element

Run motor enough to warm it up and to blow any free water out the exhaust

Pull the starting knob to get it running very rich just before you turn the motor off

Plug the exhaust

For extreme protection:

Before storing, run the bike on 20:1 premix using mineral based two stroke oil and petrol with no ethanol.

Store it somewhere with a low relative humidity and minimal daily temperature swings.

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Thanks for the information so far guys, and just some comments.

I've checked and there's no loss of coolant. (PeterB - on the Sherco the mains are lubricated by the petroil and on the clutch side are isolated from the clutch/primary drive/gearbox oil).

I'm satisfied that the problem is not washing or other water getting in. I rarely wash the bike as I generally do not ride in muddy/dirty/wet conditions. Also I have frequently detected the "rusty bearing noise" when the bike has not been cleaned since last use.

As I mentioned in my original post I have been starting the bike about every two weeks with the idea of keeping fresh oil going through the motor. Obviously I do not wash it under such circumstances and yet I still get the "rusty bearing noise".

Feetupfun -you may have identified one way to help, namely to run the bike with the "choke" on for a while before stopping the motor. You also mention running on a mineral oil premix before storage. Are you aware whether mineral oils are better than semi-synthetic at preventing corrosion?

I did wonder about fitting stainless mains next time around but (i) I cannot source them in C3 clearance and (ii) I believe stainless bearings' load ratings are less than chrome steel.

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Thanks for the information so far guys, and just some comments.

I've checked and there's no loss of coolant. (PeterB - on the Sherco the mains are lubricated by the petroil and on the clutch side are isolated from the clutch/primary drive/gearbox oil).

I'm satisfied that the problem is not washing or other water getting in. I rarely wash the bike as I generally do not ride in muddy/dirty/wet conditions. Also I have frequently detected the "rusty bearing noise" when the bike has not been cleaned since last use.

As I mentioned in my original post I have been starting the bike about every two weeks with the idea of keeping fresh oil going through the motor. Obviously I do not wash it under such circumstances and yet I still get the "rusty bearing noise".

Feetupfun -you may have identified one way to help, namely to run the bike with the "choke" on for a while before stopping the motor. You also mention running on a mineral oil premix before storage. Are you aware whether mineral oils are better than semi-synthetic at preventing corrosion?

I did wonder about fitting stainless mains next time around but (i) I cannot source them in C3 clearance and (ii) I believe stainless bearings' load ratings are less than chrome steel.

My 07 Sherco came with sealed bearings I initially thought this must be an error but it wasnt. Wont fitting sealed bearings next time cure your problem ? The bearings are c4

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Sherco's are prone to main bearing problems, I have replaced three sets of them last month in the shop. I do not think it is necessary to run with the choke on prior to shutting the engine off, this will actually cause more problems in the long run.

I would definitely switch to a fully synthetic two-stroke oil though. Another thing to watch is to make sure that when you periodically start the engine, make certain that it gets completely up to operating temperature before you shut it off; if possible ride the bike around for a little while, otherwise condensation will form in the engine.

Our Beta's all come with sealed main bearings from the factory and I have never seen water related problems with them, so that may be something to try as well.

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Where are you getting your bearings? Are you going to a local bearing supplier or are you getting them from your local Sherco dealer. If I remember correctly Sherco updated the bearings a few years back, I do not know if they are anything special other than clearance but you may want to check with the local dealer or importer to see if there are improved factory bearings available. They may be more expensive but for the number of times you have had to replace them you may want to try the factory parts, especially the latest 08 parts. I just looked at the 05 and the 08 Sherco Spare Parts book and there is a difference in the bearings, the 05 is a 6206 C3 and the 08 uses a 6206 TN9 C3C3, I don't know the specific differences but a good bearing shop should be able to tell from the numbers.

Wayne

Edited by Wayne Thais
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Many thanks for your time folks and keep those suggestions coming please guys.

Baldilocks - I cannot understand about the bike coming with sealed main bearings. There are main bearing seals outboard of the bearings, as separate parts in order to keep the crakcase charge where it should be, but if the bearings are sealed with either metal or rubber seals, how can the oil from the premix ever reach the balls? I am sure the grease packed within a sealed bearing wouldn't adequately lubricate the bearing over time at engine speeds and heat. The Sherco parts lists only indicate a "normal" (see later) bearing, so I'd be interested if anyone else has come across sealed ones before.

JMcK - as a matter of interest did you have to replace them due to wear or rust? You say that running with the "choke" on before shutting off will lead to more problems. I realise this could lead to plug fouling, but did you have anything else in mind? You also mention that it's necessary to make sure the bike is fully up to temperature before shutting off or condensation may form. This may be the clue. I sometime run it until it's warm, but not always so hot that, say, the fun cuts in.

Wayne - The first set of bearings I obtained from a well established Sherco dealer and parts supplier (I won't name names) who supplied a reputable make of bearing - nothing cheap and nasty. The second set I obtained from a reputable bearing supplier - this time I specified SKF type, C3 clearance. The original ones which came out of the bike were SKF and had a TN9 suffix. Looking on the SKF web site this means an injection moulded cage of glass fibre reinforced polyamide. Most bearings have a metal bearing cage, and both replacement sets I fitted had metal cages. Clearly replacing with the TN9 type will be as per manufacturers spec, but I cannot see why the cage material should make any difference to rust resistance, and after all the original TN9 type were rusty when they came out. I wonder if Sherco specify the plastic cage as I expect this will cause less damage to the motor should it break up. Incidentally I think the C3C3 is an English typo (see the French and Spanish descriptions in the parts books) and should be just a single C3 (the internal clearance in the bearing).

Portman

Edited by Portman
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About how the water gets in there-

There are many possible routes for water to get inside the crankcase of a Sherco two stroke.

You have checked for water leaking from the coolant circuit so scratch that.

You don't often wash your bike (me neither) so scratch that.

Water can condense out of the air if the inside of the engine is cooler than the dewpoint of the air. Depending on daily temperature swings and the amount of water in the air, it is possible (and in some places very common) for liquid water to build up inside things like fuel tanks and engines. It is why steel fuel tanks rust out so quickly.

Water is also a significant component of the products of combustion in petrol engines so no matter how warm your motor gets, when you turn it off, some of the water which is in the form of a gas in the combustion chamber will turn to liquid water as the motor cools down. Being a two stroke, the combustion chamber is usually open to the crankcase with the motor stopped so it gets water in there too.

About the oil-

Yes mineral based two stroke oil will protect the bearings from rust better than synthetic oil when stored but if you are riding your bike every two weeks it is probably not necessary nor advisible. It would be the sort of thing to do if you were storing the bike for months in a wet climate. Using semi-synthetic oil can be a good compromise for preventing corrosion and minimising engine wear. Thats what I use too.

Something else to think about-

I hope you are totally sure that the bearings are failing due to corrosion. There is another possible mode of failure that is common in 1970s Bultaco motors that sounds like corrosion failure with the motor running, but when you have a look at the bearings, there are thousands of impressions in the bearing running surfaces caused by tiny particles of hard carbon being squashed between the rolling and fixed elements. Bultaco motors are more susceptible to that mode of failure than many others due to a couple of factors. One is that due to the design, there is no through-flow of fuel-air mixture to keep them cool and clean. Most other bikes have a hole ventilating the space between the bearings and the seal which provides a cooling flow. The other relevant Bultaco design feature is that there can be significant side loading on the main bearing races due to all four bearing seatings having an interference fit. Some people fit a roller bearing on one side to avoid this problem.

Its a hunch, but maybe Shercos also suffer from this carbon particle induced damage.

If the reason for the short life of your mains is this, then you should use the highest quality full synthetic oil you can get, at the mixing ratio recommended by Sherco, to minimise carbon formation.

Using sealed bearings would also provide protection of the surfaces from carbon particles, but you would want to be sure that the grease inside was suitable for the duty.

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Many thanks for your time folks and keep those suggestions coming please guys.

Baldilocks - I cannot understand about the bike coming with sealed main bearings. There are main bearing seals outboard of the bearings, as separate parts in order to keep the crakcase charge where it should be, but if the bearings are sealed with either metal or rubber seals, how can the oil from the premix ever reach the balls? I am sure the grease packed within a sealed bearing wouldn't adequately lubricate the bearing over time at engine speeds and heat. The Sherco parts lists only indicate a "normal" (see later) bearing, so I'd be interested if anyone else has come across sealed ones before.

JMcK - as a matter of interest did you have to replace them due to wear or rust? You say that running with the "choke" on before shutting off will lead to more problems. I realise this could lead to plug fouling, but did you have anything else in mind? You also mention that it's necessary to make sure the bike is fully up to temperature before shutting off or condensation may form. This may be the clue. I sometime run it until it's warm, but not always so hot that, say, the fun cuts in.

Wayne - The first set of bearings I obtained from a well established Sherco dealer and parts supplier (I won't name names) who supplied a reputable make of bearing - nothing cheap and nasty. The second set I obtained from a reputable bearing supplier - this time I specified SKF type, C3 clearance. The original ones which came out of the bike were SKF and had a TN9 suffix. Looking on the SKF web site this means an injection moulded cage of glass fibre reinforced polyamide. Most bearings have a metal bearing cage, and both replacement sets I fitted had metal cages. Clearly replacing with the TN9 type will be as per manufacturers spec, but I cannot see why the cage material should make any difference to rust resistance, and after all the original TN9 type were rusty when they came out. I wonder if Sherco specify the plastic cage as I expect this will cause less damage to the motor should it break up. Incidentally I think the C3C3 is an English typo (see the French and Spanish descriptions in the parts books) and should be just a single C3 (the internal clearance in the bearing).

Portman

The three sets of main bearings I replaced last month were very noisy when the bike was running, (you could even hear the noise when spinning the engine over by hand with the spark plug removed).

All three were rusty. One engine, (a 2005 2.5) the nylon bearing cages were in multiple pieces. Due to the amount of silicon gasket material in the engine and sloppy workmanship I believe that this was at least the second time that this particular engine had been apart.

The other two engines were both 2006 2.9

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A few comments regarding Sherco main bearings:

I replaced the main bearings in my '03 290 in April, 2007. My problem was a worn left-side bearing. The right-side looked OK, but I replaced the bearings and crank seals on both sides. There was no rust on either bearing. Actually, my issue was wear and heat-related damage - pretty much the opposite end of the spectrum from the rust issue. I feel certain that the bike may have experienced a rough childhood before I got it. Most likely a diet that contained a lot of dust and lack of proper air filter maintenance.

I ordered my bearings from Ryan Young Products using the part number from the '03 Sherco parts list. When I received the bearings I noticed that they were sealed on both sides. I immediately called Ryan to make sure I had not received the incorrect bearings. He assured me that I had received the correct parts and that the revised bearings were supposed to be sealed. This seemed rather odd to me even after our conversation, so I sent him an email to get written clarification. He replied with the same answer. "The new bearings are sealed. Leave the seals in place when you install them." I also discussed the sealed bearing issue with Brad Baumert at TTC this spring. Apparently, some engineer at the Sherco factory had explained the sealed bearing reasoning to the RYP guys and was confident that this was the way to go.

I am happy to report that these sealed bearings finished out the '07 season and have performed flawlessly through the '08 season as well. They still seem just as fresh as they did when I installed them. The sealed bearing concept seems odd, but I am now a believer.

For what it is worth, I run premium unleaded with Maxima K2 @ 80:1

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Portman

I know what you mean but my bike did the SSDT with sealed bearings and it was fine. I only changed them as a precaution, my dealer told me of the carbon issue mentioned above and given 07 and 08 Shercos have given a lot less trouble I'm assuming this theory is correct.

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