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Old 02-13-2013, 08:16 AM   #1
moto_psycho OP
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Best type of big end bearing?

Ok, if you look in the thumpers section you will notice me crying about my ktm 640 big end being dead. Now, the only kits for this are needle roller bearings, which for some people only seem to have a 15000 mile life span on them.

I was considering machining my own bearing out of leaded bearing bronze. But I need to weigh up the pro's and con's and of course draw from other people's EXPERIENCE (not opinion).

So, is there any reason I shouldn't use bronze, bearing in mind it is used in 90% of vehicles on the road for the crank?

Also, what sort of tolerances do you recommend. Interference with with the rod? 0.001" 0.003"? And what sort of clearance on the pin? Bearing in mind linear temperature coefficient of expansion?

Cheers guys!!!
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Old 02-13-2013, 08:22 AM   #2
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I'm guessing there is a very good reason not to but I don't what it is. Probably due to oiling
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Old 02-13-2013, 08:33 AM   #3
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If you are thinking of switching, what are you going to do about the huge difference in size between the roller bearings (larger spaces for bigger bearings) vs. the smaller relative size of plain or shell bearings.

If you're in it for the fun of it (engineering geek) I say go for it. If you like to ride, I would just replace the rollers.

Why is the lifespan so short on these? Other engines use them and don't seem to have such a short lifespan. I'm thinking of high mileage KLR's in particular.

If the roller bearings weren't bathed in oil, I could see them having a short life, but aren't these fully oiled?
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Old 02-13-2013, 08:35 AM   #4
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I would surmise that since the lc4 has roller, needle and ball bearings for every bearing, that pressurized bearings like you are talking about won't work because the oil system doesn't provided enough pressure.

BTW: have you determined the problem with your bike or are you guessing which bearing has failed?
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Old 02-13-2013, 09:04 AM   #5
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Rebuild it as needed, don't attempt to redesign the engine, it will be way more $ and time/effort. The bike is sold as "race ready", as such, longevity in design was traded for lightweight/output. Ride it, love it, rebuild it... Or get a tamer bike, and ride longer/wrench less.
There is SO much more at play to change to a plain bearing than may be appearant.
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Old 02-13-2013, 09:07 AM   #6
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Quote:
Originally Posted by Boon Booni View Post
I would surmise that since the lc4 has roller, needle and ball bearings for every bearing, that pressurized bearings like you are talking about won't work because the oil system doesn't provided enough pressure.

BTW: have you determined the problem with your bike or are you guessing which bearing has failed?
I'm pretty certain it is the problem, stripped both sides down, no play in anything, removed counter balance, noise still exists. The oil pressure point is valid, but I'm pretty certain the little end uses a plain bearing.


Also, to the other questions, I can make the bearing for free at work using the same material that goes in aircraft landing gears.
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Old 02-13-2013, 09:15 AM   #7
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Quote:
Originally Posted by Boon Booni View Post
I would surmise that since the lc4 has roller, needle and ball bearings for every bearing, that pressurized bearings like you are talking about won't work because the oil system doesn't provided enough pressure.
This. You'd have to change all the bearings to plain and raise the oil pressure. No telling what other things that would affect, since the KTM engineers didn't anticipate 40+psi of oil pressure when designing the engine. Now if you own a machine shop and time and money are endless, then sure, go for it, if you enjoy that kind of challenge. If you want to do it because you think changing bearings every 15k is too expensive, find somewhere else to save money, because re-engineering the lubrication system and rotating assemblies won't do that.
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Old 02-13-2013, 09:26 AM   #8
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Anything you do will make it worse.
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Old 02-13-2013, 10:08 AM   #9
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*IF* the bearing is a standard size, see if someone like Timken or SKF makes a "clean steel" version. Rolling bearing life is usually limited by the number and size of non-metallic inclusions in the steel.

While you're at it, look up hydrodynamic lubrication.
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Old 02-13-2013, 10:32 AM   #10
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To play on an old saying...if you have to ask, you're probably not capable of it. That said, I don't know you from Adam, and I have no idea what you are actually capable of. I am a mechanical engineer, if that makes my advice any more meaningful to you. I don't doubt your ability to machine a part or parts that will fit into the space originally occupied by the needle roller bearing. However, it's just not that easy.

You need the engineering knowledge to be able to analyze the entire rotating assembly and lubrication system, and re-engineer any part as necessary. I do not have a 640, nor am I intimately familiar with the engine, so I quickly googled a schematic of the 640 oiling circuit, and it's pretty clear that a change to the big end bearing would affect multiple other items. At the bare minimum, you'd be looking at potentialy redesigning and replacing the crankpin, the main oil pump, the oil jet, and the conrod itself. In the case of the oil pump...if it needs to be changed, would you have the wherewithal to source custom gears, etc? Would it even fit in the space available for the existing pump? So on and so on... Then, even if you think you've got it figured out and have your investment made in any/all changes, you'll have no proof of the reliability of the new parts prior to starting the engine for the first time. If the big end seizes the first time you fire up the motor, what do you do then?

You commented that the small end is a plain bearing, and you're right. However it's not pressurized, it's splash lubricated by the oil jet. The two ends of the rod have very different loads and lubrication requirements...that's why one is big and one is small. For the big end of the rod, splash lubrication and a plain bearing simply would not work; you'll need a pressurized oil supply.

Anyway, my advice is the same as the others: stick to the simple job of replacing the OEM bearing as often as needed. It's going to be quicker, cheaper, and most likely more reliable than trying to do anything different.
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Old 02-13-2013, 11:40 AM   #11
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In my direct experience that bearing is good for 60K miles. In my indirect experience I have not heard of too many 640's lunching that bearing before 15K miles. For it to fail at 15K it has probably been run out of oil, was put together wrong, or had a rare manufacturing defect.

There are other things I'd have a go at redesigning inside the 640 mill before that if I was looking for ultimate reliability - eg the cam follower rollers that generally fail much sooner.

Cheers
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Old 02-13-2013, 05:14 PM   #12
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If the engine has rollers stock there most likely is not enough oil pressure in the motor to use a plain bearing.
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Old 02-14-2013, 05:07 AM   #13
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Quote:
Originally Posted by hellfire76 View Post
If the engine has rollers stock there most likely is not enough oil pressure in the motor to use a plain bearing.
This is a summary of what was said previously.
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Old 02-14-2013, 05:33 AM   #14
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If you're asking this question you do not posses the ability to perform the work. And yes that is my experience fixing other peoples fuck ups not an opinion.
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Old 02-14-2013, 07:58 AM   #15
moto_psycho OP
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Quote:
Originally Posted by Lutz View Post
To play on an old saying...if you have to ask, you're probably not capable of it. That said, I don't know you from Adam, and I have no idea what you are actually capable of. I am a mechanical engineer, if that makes my advice any more meaningful to you. I don't doubt your ability to machine a part or parts that will fit into the space originally occupied by the needle roller bearing. However, it's just not that easy.

You need the engineering knowledge to be able to analyze the entire rotating assembly and lubrication system, and re-engineer any part as necessary. I do not have a 640, nor am I intimately familiar with the engine, so I quickly googled a schematic of the 640 oiling circuit, and it's pretty clear that a change to the big end bearing would affect multiple other items. At the bare minimum, you'd be looking at potentialy redesigning and replacing the crankpin, the main oil pump, the oil jet, and the conrod itself. In the case of the oil pump...if it needs to be changed, would you have the wherewithal to source custom gears, etc? Would it even fit in the space available for the existing pump? So on and so on... Then, even if you think you've got it figured out and have your investment made in any/all changes, you'll have no proof of the reliability of the new parts prior to starting the engine for the first time. If the big end seizes the first time you fire up the motor, what do you do then?

You commented that the small end is a plain bearing, and you're right. However it's not pressurized, it's splash lubricated by the oil jet. The two ends of the rod have very different loads and lubrication requirements...that's why one is big and one is small. For the big end of the rod, splash lubrication and a plain bearing simply would not work; you'll need a pressurized oil supply.

Anyway, my advice is the same as the others: stick to the simple job of replacing the OEM bearing as often as needed. It's going to be quicker, cheaper, and most likely more reliable than trying to do anything different.
Yay! A decent reply!
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