Hi,

can anyone tell me if it is ok to use "C3" clearance bearings in place of the factory fitted larger "C4" clearance bearings on my Sherco? There is a significant price advantage to doing so and I have used them in the past but not on a trials bike.

Cheers

Hi Firecat

As you indicate the C3 / C4 is the internal clearance of the bearing. This is needed because of the potential rapid differential temperature that can occur across the opposing "races" of the bearing during warm-up or if the engine sees significant sudden heavy load from a cool start. When this occurs the bearings skid on the races causing further heat, wear and ultimately a seized or failed bearing.

While the dealer answer is clearly go with the intended C4 design, trials engines are low output and therefore dont generate large amounts of heat. Motocross or road race engines on the other hand can see rapid temperature rise due to the nature of the riding / track layout. They also have a specific output more than double that of a trials bike.

So my guess (and it is a guess) is that you could get away with it. Personally i would ask myself if the extra cost is worth the risk though. Do you really want to risk splitting the cases again?

This article explains it well..http://www.bikebearings.co.uk/crankshaft-gearbox-bearings/

Dom

Edited November 14, 201213 yr by dombush

Hi, thanks for taking the time to reply - much appreciated!

The difference between the C3 is half the cost of a C4, roughly £15 for a pair vs £30. You may be right re the risk but the C4's have nylon snap cages, part of the reason behind the rebuild is ethanol attacking the crank seals, it also attacks nylon. I guess £15 aainst the cost of all the gaskets and seals is not worth the risk.

All the best

The updated sealed bearings of the later models seem to be the ones of choice. SKF 6206 2RS1 TN9 C4/GJN

Available from Splatshop, along with the Viton(crank seals), this combo would be my choice.

If you are quibbling over a few quid, I am sure you can find some Chineese made bearings for less!

Having said that, fact is that standard open bearings of either C3 or C4 clearances do work, and have for years upon end. However one must agree that any open bearing is succeptable to whatever dirt that gets into the motor, and it will! Yet ones own riding and maintanence practices are just as important.

Be your own judge, but when I do this type stuff, I try to use the best I can find, and I only want to do it one time!

Copemech

i read your profile and it says Texas, yet your replies sound every inch a Brit.

Whats the truth?

Dom

Copemech

i read your profile and it says Texas, yet your replies sound every inch a Brit.

Whats the truth?

Dom

Truth is I have been around here too long! And I do not have a

£ on keyboard!

Well that was fun! Finally got the cases apart and can see what has caused the rough idle and noise/vibration. The seals are knackered and the main bearings are well and truly clapped out! They are actually rusty, and make a noise rather like a steam roller on gravel! I am glad I decided to pull it apart. I have had the bike from new and up until the time I put it away for 18 months or so it was running sweet. I have not seen anything like it in 35 years of working on bikes. Is it the ethanol and the water it attracts that has caused this damage to the seals, bearings and carb?

It turns out the originals were C3's. the recommended C4's are twice the price (same brand C3's) and are also unshielded.

The flywheel side was more rusty than the clutch side so maybe also down to leaking tingling cover?

The flywheel side was more rusty than the clutch side so maybe also down to leaking tingling cover?

Sitting is never good on things, then you combine other factors, it is not surprizing. Hopefully you got some Viton seals, reguardless of bearing choice!

Sitting is never good on things, then you combine other factors, it is not surprizing. Hopefully you got some Viton seals, reguardless of bearing choice!

Sure did!

Another thing to consider is the (lack of) corrosion protection provided by running weak premix ratios and synthetic premix oil. Semi synthetic premix oil is better for protection from corrosion.

Fogging oil as used in Marine and Aero engines looks to be ideal for trials bike storage. I by chance use Yamaha semi synthetic marine oil which has additives for damp storage, its also for reasons of lowering pollution in inland waterways designed to run in low ratio's so is ideals for trials where extreme temperatures and forces are not issues. MX oil is the norm and when you think about it, the requirements are quite different. I wonder why they are not in more widespread use in trials?