KT100: What caused my bearings to die this fast?

Well, I took the crank in and got it checked over, and it was OK. I ended up getting a new piston installed, and put in another set of brand new main bearings. Put it all back together and made sure there was about .010 crank end play. The crank was rock-solid up/down. Put it on the kart and idled it on the stand nice and rich for 15 minutes for the new piston. For the heck of it, I checked to see if there was any up/down slop in the crank after it ran. The result? .010. Again.

I think it's time to get a new hobby :(
 
Did you do the work yourself or have a shop do it?
What you are talking about is not rocket science. A $10 pair of calipers from Harbor Freight is all you need to measure the bearing journals on the crank and the ID of the new bearings to see if the slop is between the bearing and the crank.
If it isn't then the only other place it can is between the bearing OD and the pocket in the case ( a little harder to measure with a caliper :) ).
Did you or the builder check all four interfaces of the parts before assembling the engine?
You said the crank was checked out by someone... did you watch as the crank was inspected? How was it checked? Between centers or in V-blocks?
Steve O'Hara
 
Steve,

I had a reputable builder check over the rotating assembly just before I built it this time to make sure it was alright. I had recently torn down my engine a few weeks ago with the same exact problem I have now (about .010 of play)- this was after only two runs following a crank rebuild and new bearings. When he just checked it this time, he removed the piston and checked it in the blocks- He said there was only a couple tenths run out and a couple tenths twist, which he corrected.

See post #8-- Back when I tore down the engine I mic'ed the savers and the ID of the new bearings. There was only around .001 to .0015 clearance there. As for the OD of the bearings and the cases, there's no slop there (especially when the engine's cool) because I measured at least a thousandth or two of interference and had to heat the cases up really well to drop the bearings in.

I was also told that new bearings should only have around a thousandth or so of internal clearance. Thus I could understand a couple thousandths of crank play up and down, but .010? It has to be the same problem as before- the bearings themselves. The question is why are bearings with 15 mins of idling (or I guess I should ask, the last two brand new sets of bearings I've put in) doing this? Are they just supposed to be that way? Everything I'm reading says I should have less than .001 of radial play.

This is driving me absolutely nuts.

Shane
 
Did you have the case halves checked as well? If the bearing bores aren't perfectly in line with each other and all 4 sides machined square and flat you could be twisting the cases as the engine is bolted down. That causes the crank and bearings to no longer run true and could cause excessive wear and runout.
 
I didn't have the cases checked. Do you think even if the halves are slightly off it would ruin the bearings like this after 15 mins of idling?
 
Shanec80 said:
Steve,

I had a reputable builder check over the rotating assembly just before I built it this time to make sure it was alright. I had recently torn down my engine a few weeks ago with the same exact problem I have now (about .010 of play)- this was after only two runs following a crank rebuild and new bearings. When he just checked it this time, he removed the piston and checked it in the blocks- He said there was only a couple tenths run out and a couple tenths twist, which he corrected.




See post #8-- Back when I tore down the engine I mic'ed the savers and the ID of the new bearings. There was only around .001 to .0015 clearance there. As for the OD of the bearings and the cases, there's no slop there (especially when the engine's cool) because I measured at least a thousandth or two of interference and had to heat the cases up really well to drop the bearings in.

I was also told that new bearings should only have around a thousandth or so of internal clearance. Thus I could understand a couple thousandths of crank play up and down, but .010? It has to be the same problem as before- the bearings themselves. The question is why are bearings with 15 mins of idling (or I guess I should ask, the last two brand new sets of bearings I've put in) doing this? Are they just supposed to be that way? Everything I'm reading says I should have less than .001 of radial play.

This is driving me absolutely nuts.

Shane
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What tools are you using to measure the up and down play in the crank to come up with .010 play. I would be curious to know how you achieved these numbers and was it the same on both crank half's.
Collins
 
I learned early in the machine shop to check my gauges after repeatable suspect readings. :) That was going to be my suggestion. Make sure that the orientation of the gauge isn't magnifying the reading too...
 
I've been measuring with a dial indicator secured to the kart frame and measuring off the end of the starter nut. I was talking with my engine guy and he said measuring there could be magnifying the measurement. I'll pull the clutch today and measure the shaft right at the seal.

Thanks
 
A misalignment in the cases would side load the bearings and cause excessive wear, plus increase rolling resistance in the engine which greatly affects performance. With only 15 minutes run time its hard to believe it could wear that much, did you check the crank runout and play readings with it both hot , right after shutting it down, and again after the engine cools down? It would be curious to see if your readings are changing much as the engine cools. If your crank was checked and it was running true and your journals measure within specs, and your bearings fit properly in your cases and onto the crank journals the only place the play could be coming from is in the bearings, or your measuring instruments or procedure are faulty, as others have suggested.
 
If you're measuring at the end of the starter nut, you're probably tripling the actual play. If the cases are warm/hot when you measure, you're adding more yet (warm cases have less interference on the main bearing, which increases the internal clearance.

Also, if you're pushing up and down on the crank, you are essentially doubling the movement since the far side bearing moves in the opposite direction of the near side.

All of this should be checked on a surface plate after everything has been properly machined.

PM
 
Pete_Muller said:
If you're measuring at the end of the starter nut, you're probably tripling the actual play. If the cases are warm/hot when you measure, you're adding more yet (warm cases have less interference on the main bearing, which increases the internal clearance.

Also, if you're pushing up and down on the crank, you are essentially doubling the movement since the far side bearing moves in the opposite direction of the near side.

All of this should be checked on a surface plate after everything has been properly machined.

PM
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OK, good to know. I measured it right at the seal and was only getting a couple thou. Considering what you and others have said, I'm probably not measuring the e exact movement very accurately and it's probably being exaggerated by some of these factors a bit. Since they're brand new bearings I'm not too worried about it.

Thanks!
 
Shane,

Things to remember: If you can "slip" the crank into the bearings (i.e. it's not a press fit to install the crank into a main bearing), then you're probably looking at a minimum of .0003" clearance there. If the sleeves have been on the crank for a while, then it could be a few tenths more (OD of the sleeves polished down a bit from use).

So... push down on the crank on one side (say the PTO side since it's easy to get the indicator close to the seal), and you immediately remove all the play in the downward direction on "your" side (that's crank clearance IN the inner race of the bearing PLUS internal clearance OF the bearing), and you also remove that same amount on the far side in the opposite direction (crank goes up, and internal clearance of the bearing gets removed in the "up" direction).

Ideally... or let's say "theoretically", total clearance should be measured at the center of the bearing race (which of course is not practical, and actually impossible with the engine assembled). So now you have a "lever", that at the "down" side you are measuring probably 20mm away from the centerline of the bearing (half the bearing race width, plus seal width, plus the gap between the two). That magnifies your up/down comparison on the near side. While you're doing that, the far side is doing the opposite, which probably adds another... ohhhh... 20%-30% or more to the actual number. (if the far side was theoretically held rock solid, you would only be reading the near side bearing plus the percentage you are "outboard" of the centerline of the race).

Bottom line, I would not be concerned by seeing "a few thousandths" at the place you are measuring with a slip-fit crank (which is what we all run).

Pete
 
flattop1,

What are you looking for in particular?

Conceptually.... cases, crank, bearings, and clearances are all similar across engines.

PM
 
Vintagequest said:
Shane, I've had that happen on a TKM, rebuilt it afterwards, ran the balance of the season with no problems. I just guess what my father always said has merit..... "Just because its new it doesn't mean it was blessed by the man upstairs to make it perfect." may have just been the bearing itself.
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Over 50 years in karting and I've never heard this story before. "Reputable builder" I've heard that before! Not saying he's not, but I know of "reputable builders" that miss, occasionally. It happens!
One thing, not knowing what condition everything was in before you had it rebuilt, is a disadvantage. Not knowing if it's the bearings or the crank causing the sloppiness makes me wonder. Running a KT100 for a year without maintenance makes me wonder. I could never get away with that!
Do you know what brand of bearings were used? I'm going to assume you didn't changed brands of fuel or oil?
Maybe I'm being a little harsh, but I have a solid knowledge of what these engines can do, and how fast they wear out, and I'm just at a loss to understand what went wrong.
 
A long story made short as possible; met a young man at the track one day and he was having trouble. The motor just wasn't as fast as it was previously and it was just recently rebuilt by a "reputable dealer". Let me check I said. I pulled the head, the piston was hitting the head. This "reputable dealer" was at that race. When questioned about the KT100, and the piston hitting the head, his response was, "you have to set them real close if you want to go fast". I told the owner "I'll look into it". I took the engine back to the shop. Dial indicator, crankshaft up and down, .008". Totally unacceptable.
If the crankshaft has any up-and-down movement, it doesn't get better, it gets worse, and very quickly. Not saying that this is what happened, but it's been known to, even after being rebuilt by "reputable" dealers. Not saying there was any willful intention, but sometimes things get busy and you don't check everything.
 
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