KT100: What caused my bearings to die this fast?

Crank

First you need to determine is the crank out of align or the bearings bad.

If the crank is checked in V blocks and straight and there is runout with the crank in the case ....THEN the bearings are bad.

If the crank is out of alignment in the V blocks...it's a crank out of alignment..

Yamaha cranks move out of alignment fairly easily...If a chain gets wadded up in a clutch, or if the crank takes a hit..it will move...

This could be a crank that got knocked out of alignment, not a bad bearing issue..yes the quality of bearings does vary, but the cranks do move out of alignment easily...it's built to Yamaha's quality level, not IAME.
 
The pic of the "new" FAG bearing you show has no markings on it. I am guessing they are on the other side? Look good at the markings and see if they are stamped in it or lightly acid etched. They look just like the junk bearings I just bought. They may be genuine FAG's that you have, or maybe not. Not saying the kart shop screwed you, but they could have bought them from a supplier thinking they were getting the real deal, and ended up getting knock-offs. The genuine FAG's I have here are solid and smooth with no obvious play up and down. The knock-offs are sloppy and cheap feeling.
 
Correct, the markings are on the other side and they are stamped. I'm pretty darn sure these are genuine- I guess I'm just surprised with the difference between the old vs new ones. They must be fine to use as most kart shops stock them as their only Kt100 bearing. I'm going to bring them in when I take my crank in to double check. I may take a look at some other brands as well- has any one tried ORS?
 
The discussion points in this thread are good, but may still be missing a few issues.

They key values and dimensions in a ball bearing (particularly the ones that matter to us in karting) are:

1 - The accuracy of the OD of the outer race and ID of the inner race (are they round, and are they on-size?)

2 - What is the *precision* of the bearing (this has no relation to its internal clearance, rather it's how accurately the grooves are ground in the races, how accurately the balls are ground and "matched" in size, and the surface finish of all these items.

3 - The internal radial clearance of the bearing

4 - Thickness of the inner and outer races (which of course directly effects the size of the balls since the OD and ID of the bearing are defined).

5 - ... and to a lesser extent: things like contact angle inside the bearing (largely determined by the size of the ball and the radius of the ground groove in the race).


Very generally speaking, higher *precision* bearings have less internal clearance. This is due to a number of reasons:
1) greater accuracy of the components allows this
2) high precision bearings are typically used in more precise/controlled applications where bearing fit is precisely controlled
3) lubrication is typically much higher quality and more accurately controlled

The environment where "we" use bearings is actually quite "crude" (for lack of a better word): A very wide temperature range, inconsistent bearing bore size and accuracy, supporting an "out of balance" load, and quite often poor setup/installation. Add in the inconsistent and generally poor lubrication we have (compared to a controlled environment), and it's actually amazing how long a main bearing can last!

A bearing that "feels" tight in the hand is not necessarily a good thing, as internal clearance is required in the free state in order for the bearing to STILL HAVE internal clearance when sitting at room temperature and installed in the engine. The specs for internal clearance are clearly defined (can be found online at most any bearing manufacturer's web site, or I can supply if someone needs them). Also be aware that there is a LOT of overlap in the internal clearance spec -- a "loose" C3 actually has more internal clearance than a "tight" C4. All this can (and should) be measured if the goal is to set the lower end of an engine up very accurately.

The 3 main variables that effect where you "end up" when the bearing is installed are:
1) Thickness of the outer race
2) Bearing interference in the case (and is the bore actually round?)
3) Internal clearance of the bearing.

Also... don't forget that "running clearance" is very different from what it feels like on the bench, as the bearing bore grows twice as much as the bearing does with temperature (CTE of aluminum vs. steel)

... and then there is bearing precision. How do you measure it? A number of ways but the way I do it is I slide a super-precise test bar through the main bearings with the case assembled and torqued (no crankshaft). I then place a .000050" (50 millionths) test indicator on the shaft right next to the bearing and spin the shaft. Note that your hand on the shaft influences the reading, so I just lightly spin the shaft and watch the indicator for movement as the shaft "spins down". I do this on each side. A bad bearing stands out like a sore thumb -- I've seen bearings with more than .0010" internal runout. A good bearing should be under .0002", and a *really* good one should be under .0001".

While the case is sitting there on your surface plate with the shaft in the bearings, it's also very easy to see what the *actual* internal clearance is at room temperature (note that your test shaft must fit very accurately in order to get a real value). Simply "lift" the shaft on the end where the indicator is and let it settle back down. Also note that most bearings have an odd number of balls, so ball orientation will slightly change what you see for internal clearance. The bearing MUST have internal clearance at room temperature, or it will quickly damage itself when the engine is started cold.

Enough rambling for now... I can provide more information if anyone is interested.

Pete
 
1 more important thing, while assembled case is on surface plate, I'd check to see if all 4 mounting bosses are the same, No feeler gauge will fit under them.
 
Ah... thank you Jack! Excellent catch. Every engine that ever went through my shop was machined flat and square on the bottom, so missed pointing that out.

It's important! Thanks!
 
Not a problem, Pete, after all you've generously shared with us on here, it's the least I could do, I have learned a ton from your postings over the yrs.
 
Shanec80 said:
The old bearings have "Germany" stamped on them. They are super rigid and have absolutely no vertical slop in them that I can feel. They still feel very tight and precise when I spin them. They even look a lot "beefier."
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Why did you change out the old bearings?
Quality aside, there are really only "good" bearings and "failed" bearings. Like electronics, either they work or they don't.
 
I figured it would be good measure after getting the crank rebuilt. I have no idea how many hours are on the old ones. I'm tempted to run them again though.

Pete- can you explain the bearing interference in the case you mentioned?

Thanks!
 
Well... the main bearing should have "the correct amount" of interference. Too little, and the main bearing can spin in the case (this can sometimes first happen when an engine is shut down for only a couple of minutes and restarted, as the cases will actually get *hotter* for the first couple of minutes that airflow over them is stopped). At that point, the case (i.e. bearing bore) is at its largest point.

Aluminum grows right at 12 millionths of an inch per inch per degree... and steel grows almost exactly half that. Many bearings in karting are 52mm OD (2.0471" in diameter). So for every degree over ambient, the bearing bore grows about 25 millionths and the main bearing grows about 12 millionths (let's call the bearing just growing half of the aluminum to make the math easy... 12 1/2 millionths). So for every 8°F over ambient, you lose .0001" (one ten thousandths) of an inch of bearing interference.

I'll cut to the chase: on a KT100 case, if it's the newer die-cast style case... anything under about .0012" interference at 70°F and you will eventually spin a bearing (may take certain weather conditions, or someone doing the aforementioned RE-starting of the engine when it's still hot, but there's a decent chance it will spin a bearing within a few races or so). On older permanent mold cases (early production), you can safely run maybe .0002" less bearing interference.

So... here is the flip side: very early Yamaha cases had as much as .0022" bearing interference. This "squeezes" the outside race of the bearing to a smaller diameter (which reduces the amount of internal clearance the bearing has... over its entire operating temperature).

Not only does bearing interference in the case need to be considered, but strength ("beefy-ness") of the case, as well as cooling capacity all need to be considered when setting up the correct bearing interference.

Most people do not have the means/equipment to re-machine bearing bores in cases, and this is why it became common to run C4 main bearings instead of C3. Manufacturers didn't want a bearing spinning in a case, so they just bumped up the interference a bit (or lets say: set it a bit higher than required). The solution was for karters to "adopt" C4 bearings as being de rigueur (fashionable). 99.9% of the karters never check bearing interference, and probably only the better or best engine builders check it.

The combination of interference, correct internal clearance in the bearing, measuring the quality of the bearing, running the correct lube, and a myriad of other things is what gives an engine not only performance, but longevity as well.

fwiw...
PM


PS: I'll have to politely disagree with MikeFlipp -- there is a lot more to bearings than simply "good" and "failed". A WHOLE lot more, as a matter of fact. :)
 
30 years ago, my engine builder, Jim Barkley from Joplin Mo, was considered to be one of the absolute best in the country on Yamaha's. I put him in an unspoken genius class, as did many others. One day, he says he sent my cases to a Pete Mueller. When I ask, who this Mueller guy was, Jim's response was- "A Friggin Genius!" Yea, well, he was right! Listen closely !
 
Pete_Muller said:
PS: I'll have to politely disagree with MikeFlipp -- there is a lot more to bearings than simply "good" and "failed". A WHOLE lot more, as a matter of fact. :)
Click to expand...
So enlighten us.
What lies between good and failed?

Quality aside, there are really only "good" bearings and "failed" bearings. Like electronics, either they work or they don't.
 
There is a way to listen to the noise a bearing makes while running, and it will do a graphic print out. Used in Industrial applications, to predict conditions to anticipate failure. I can't remember who makes it, but the Candy Factory used it a lot.
 
MikeFlipp said:
So enlighten us.
What lies between good and failed?

Quality aside, there are really only "good" bearings and "failed" bearings. Like electronics, either they work or they don't.
Click to expand...


Ah... maybe what you're saying is "functional" and "non-functional" ?

Yes... a bearing can spin somewhat/relatively fine, or it's destroyed and no longer functions as a bearing. Agree on that.

When it comes to quality of bearings though (and it *does* effect performance), there is a significant difference between a good (high-quality, accurate, precision) bearing, and an average bearing. I'm focusing more on describing the differences between bearings that might all be considered "good", but in reality cover a wide spectrum of quality.

My mistake.

Pete
 
Pete_Muller said:
Ah... maybe what you're saying is "functional" and "non-functional" ?

Yes... a bearing can spin somewhat/relatively fine, or it's destroyed and no longer functions as a bearing. Agree on that.

When it comes to quality of bearings though (and it *does* effect performance), there is a significant difference between a good (high-quality, accurate, precision) bearing, and an average bearing. I'm focusing more on describing the differences between bearings that might all be considered "good", but in reality cover a wide spectrum of quality.

My mistake.

Pete
Click to expand...

Oh yeah quality is all over the place...used to not be as big a problem.
Some of the really sketchy ones are only good for a sinker or a paperweight.

Maybe I should have said, "With good quality bearings, there are really only "good/functional" bearings and "failed/non-functional" bearings."
 
Anyone that has worked on Yamaha's know the cranks move easily out of alignment. Check the crank for alignment.

If you don't have those resources...install the best mains you can get...install the crank and dial indicate it...if it has run-out...its the crank that is out of alignment. Each piece needs checked before building an engine...assume NOTHING!
 
on the subject of bearings, anyone know if the small journal 6205 (20x52) are available anywhere. I've seen 2 versions, 6205 B20-TN9 and NTN 6205 T2x2 (maybe Tzxz)
 
Sean Cook said:
Anyone that has worked on Yamaha's know the cranks move easily out of alignment. Check the crank for alignment.

If you don't have those resources...install the best mains you can get...install the crank and dial indicate it...if it has run-out...its the crank that is out of alignment. Each piece needs checked before building an engine...assume NOTHING!
Click to expand...

Sean,

A KT100 crank will actually stay in place quite well if it's assembled correctly. You may have noticed before that when you disassemble a Yamaha crankshaft, there are often "scratches" in the crank pin bore? Those are caused by simply taking new parts out of the packaging and assembling a crank. The ends of the crankpin should be polished to a mirror finish (right where the radius meets the OD of the crankpin), and the crank halves should have a slight radius polished onto the entry point (where the chamfer meets the pin bore). Ditto on this... needs to be like a mirror. I check them under a microscope before assembling.

Once that's done, there will not be any scratches in the pin bores, and the crank will not only true easier, but will also stay put once it's trued.

One major trick to truing crankshafts (not just Yamaha) is to assemble them, and then just bump them in close (say within .001" or .002" or so), then let the crankshaft sit for about 24 hours, and come back and finish the truing. There is a lot of stress on the parts (due to the amount of interference), so giving the crank a bit of time to "relax" makes all the difference in the world when it comes to that final bit of work to make it dead true. True a crank the night before a race and assemble the engine, and it's just about guaranteed that the crank will not be true any more when you check it next time.

Of course it's always a good idea to measure the radius of the crank half centerline to the crankpin bore centerline, and try to use halves that match up really well (within .0001" or .0002" if at all possible). Don't trust the centers in the crank half when checking this, as they are typically not accurate at all from the factory -- need to be lapped into position if you plan to support between centers to check crankpin bore radius. And of course always use high-pressure lubricant when pressing the pin into the bore.

PM
 
Pete,

I've build hundreds of winning engines, so yes I know every trick there is in the book on cranks...and then a few.

I'm talking about when a chain gets wadded in the clutch..or any impact to the crank...the crank will move out of alignment on a Yamaha, this is common knowledge.

Do you know how many people try to pull a clutch and in won't come off...so they hit the puller with a hammer....that just pushes the halves together opposite the crank pin and it's out of align. Most people don't know how careful you need to be when working on a Yamaha.

Was just trying to point the guy in the correct direction and checking the crank is needed.

The quality is not that of an IAME crank.
 
A KT 100 crank suffers from a primary problem that is hard to fix. Metallurgy and tempering. Not that the characteristics are all negative. They are acting in the way that the materials are designed. It's a lower modulus material. There are some advantages. Absolute rigidity is not one of them
 
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