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Pete88

Crankshaft Bearing Replacement

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Hi All,

Just after some advice & top tips.

I have a 2004 Sherco 250, it is a great bike & rides well but it has developed a bit of a tapping engine noise,  I dont think it is piston slap but I could be wrong! I think it is crankshaft bearing play.

Having read up on this I have learnt they are quite susceptible to wearing out.

I am going order the parts I need and have a go at replacing them whilst I have all this time at home during lockdown!

Do I need a clutch basket tool?

Any top tips or useful knowledge would be greatly appreciated,

Thanks in advance 

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If the crank bearings are worn you should be able to feel play by pushing and pulling on the flywheel.

If the piston rings are gummed up or worn they can make a tapping noise, be worth checking before splitting an engine.

While you've got the barrel off you can feel for any play in the big and little end bearings as well.

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Thanks Kurtas,

What is normal/acceptable play wen checking the crankshaft bearings by push/pull on the flywheel?

Are there any specific indicators I am looking for when checking for gummed or worn piston rings?

What is normal/acceptable play in the big & little end bearings?

Thanks again for your help

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13 minutes ago, Pete88 said:

Thanks Kurtas,

What is normal/acceptable play wen checking the crankshaft bearings by push/pull on the flywheel?

Are there any specific indicators I am looking for when checking for gummed or worn piston rings?

What is normal/acceptable play in the big & little end bearings?

Thanks again for your help

Pete88 you didn't ask me but I'll put in my 2c worth

1 for main bearings any discernable movement axially or radially of the flywheel means they need to be replaced.

2 If a ring stays in its groove in any section of the groove rather than moving freely in and out  the groove, it is stuck. If there is gum, you will see it. Sticking rings is very commonly seen on two stroke trials bikes. Worn rings are measured by fitting them into the bore squarely between the top of the exhaust port and the head surface and measuring the gap between the ring ends. The pattern of carbon deposits on the walls of the cylinder also is a guide to there being ring sealing problems or not.

3 There should be zero discernable movement up and down of the conrod relative to the big end pin. Slight left-right rocking movement of the top end of the rod is normal. The little end can be inspected with a magnifying glass after cleaning the rollers and pin and the eye of the rod. If there are any visual defects in any of the bearing surfaces the parts should be replaced. It should also have zero discernable movement in the direction of the rod axis but it is normal for the pin/piston to be able to rock slightly in the eye of the rod.

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Thanks Feetupfun,

This is awesome advice, exactly what I need.

I am a confident hobby engineer but i need the specific details for this type of bike, being relatively new to trials still, getting used to these machines after years of working on boat engines is interesting.

I will start by checking for play at the flywheel & then get the head/cylinder off to look at the piston & rings this afternoon.

I assume I can get the cylinder off with the engine still in the frame?

 

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Thanks Miss Crabstick (great name btw)

The noise tends to be most noticeable once I let off the accelerator, on the run out as the engine comes back to idle.

I am guna check piston and play at the flywheel before ordering the parts but guna do a bit on an overhaul whilst I have the time! 

Any other hints or tips would be great,

Thanks

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There are a good how to from RYP USA. 

Clutch holder aren't needed if you have a impact wrench (good for flywhhel nut too) or if you jam the gears with a rag or something like.

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10 hours ago, feetupfun said:

Pete88 you didn't ask me but I'll put in my 2c worth

1 for main bearings any discernable movement axially or radially of the flywheel means they need to be replaced.

2 If a ring stays in its groove in any section of the groove rather than moving freely in and out  the groove, it is stuck. If there is gum, you will see it. Sticking rings is very commonly seen on two stroke trials bikes. Worn rings are measured by fitting them into the bore squarely between the top of the exhaust port and the head surface and measuring the gap between the ring ends. The pattern of carbon deposits on the walls of the cylinder also is a guide to there being ring sealing problems or not.

3 There should be zero discernable movement up and down of the conrod relative to the big end pin. Slight left-right rocking movement of the top end of the rod is normal. The little end can be inspected with a magnifying glass after cleaning the rollers and pin and the eye of the rod. If there are any visual defects in any of the bearing surfaces the parts should be replaced. It should also have zero discernable movement in the direction of the rod axis but it is normal for the pin/piston to be able to rock slightly in the eye of the rod.

Hi Feetupfun, 

So with the top end off and all apart:

I cant feel any play at all at the flywheel.

There is some rocking play in the big end but none up and down (certainly none that I can feel with my hands/finger tips)

The little end has quite a lot of rocking play and a small amount of up/down play. The gudgeon pin has slight wear on it, I think I will replace the bearing, gudgeon pin and clips whilst I have this apart.

The piston rings both seem free in their slots, I cant see any scoring on the rings, there is very light scoring on the cylinder wall, is this something to be concerned about? 

From what I can work out it has had a newer piston fitted as the width of the little end bearing is the same as splat shops description of the past 06 virtex piston 

I also discovered the transmission oil looking quite milky so I am going the do the water pump seals & check the shaft.

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4 hours ago, Pete88 said:

Hi Feetupfun, 

So with the top end off and all apart:

I cant feel any play at all at the flywheel.

There is some rocking play in the big end but none up and down (certainly none that I can feel with my hands/finger tips)

The little end has quite a lot of rocking play and a small amount of up/down play. The gudgeon pin has slight wear on it, I think I will replace the bearing, gudgeon pin and clips whilst I have this apart.

The piston rings both seem free in their slots, I cant see any scoring on the rings, there is very light scoring on the cylinder wall, is this something to be concerned about? 

From what I can work out it has had a newer piston fitted as the width of the little end bearing is the same as splat shops description of the past 06 virtex piston 

I also discovered the transmission oil looking quite milky so I am going the do the water pump seals & check the shaft.

Yes that sounds like a good plan. Sounds like you found the source of the noise.

Very light scoring is normal to find. If you can feel lines then there is a problem.

If it was me I would also clean the rings and ring grooves (using non-caustic paint stripper and a tooth brush) and clean any carbon off the piston crown and exhaust port with either a scotchbrite pad or bead blasting.

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3 hours ago, feetupfun said:

Yes that sounds like a good plan. Sounds like you found the source of the noise.

Very light scoring is normal to find. If you can feel lines then there is a problem.

If it was me I would also clean the rings and ring grooves (using non-caustic paint stripper and a tooth brush) and clean any carbon off the piston crown and exhaust port with either a scotchbrite pad or bead blasting.

Thanks again Feetupfun,

I can only just feel a very light score line above one of the ports, but I really have to use a fingernail to detect it (by no means the worst I have ever seen)

Would acetone be any good for cleaning the piston rings & groves? 

On another note how much movement/rotational play should there be between the primary gear?

When I hold the clutch basket still and rotate the flywheel there is a bit of movement, enough to make a small noise, is this normal of square gears?

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57 minutes ago, Pete88 said:

Thanks again Feetupfun,

I can only just feel a very light score line above one of the ports, but I really have to use a fingernail to detect it (by no means the worst I have ever seen)

Would acetone be any good for cleaning the piston rings & groves? 

On another note how much movement/rotational play should there be between the primary gear?

When I hold the clutch basket still and rotate the flywheel there is a bit of movement, enough to make a small noise, is this normal of square gears?

I haven't played with a Sherco primary drive but it would be very unusual for those gears to be worn. Some clutches have a cushion drive between the clutch gear and the basket and they can get a bit of inconsequential play.

Go ahead and try acetone if you want. It didn't work for me and before you ask, neither did throttle body cleaner, methylated spirits, kero, turps, eucalyptus oil, melaleuca oil, methanol or WD40. I didn't try xylene on its own.

An alternative to paint stripper that would also work but risk taking off some metal would be to degrease then bead blast.

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43 minutes ago, feetupfun said:

I haven't played with a Sherco primary drive but it would be very unusual for those gears to be worn. Some clutches have a cushion drive between the clutch gear and the basket and they can get a bit of inconsequential play.

Go ahead and try acetone if you want. It didn't work for me and before you ask, neither did throttle body cleaner, methylated spirits, kero, turps, eucalyptus oil, melaleuca oil, methanol or WD40. I didn't try xylene on its own.

An alternative to paint stripper that would also work but risk taking off some metal would be to degrease then bead blast.

Is it worth cleaning the inside of the cylinder with a scotch pad too?

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Nice. no worn out bearings.

Check your clutch. (Water can unglue discs friction material).

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2 hours ago, Pete88 said:

Is it worth cleaning the inside of the cylinder with a scotch pad too?

I don't know enough about your cylinder or rings to say

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