KT100: What caused my bearings to die this fast?

Ron Tuttle, the machinist at Nelson manufacturing, (Invader karts) once showed me a device he built to measure the distance, on Yamaha cranks, from the crank centerline to the pin centerline. Nelson's retail store, California Kart Sales, always had several crank halves in stock. Ron would measure them all and he could pick and choose halves that were within .0001". He found them to be all over the place. Now being all over the place is not bad in itself, because if you get 2 that measure the same, when you put them together, you get a straight crank with minimal run out. Minimal run out is good.

If you measure a batch of KT100 cranks, crank center to pin center, you'll find that measurement can vary from .0000" to .0009", maybe more. Both ways, plus/minus. Watching Ron measuring crank halves was a real eye-opener.
 
Yes, Al that is so true. The strokes vary and perfect alignment is not possible in some of the Yamaha cranks. It is just what it is. A great engine that has done karting well, the crank being the weak link. Years ago I spoke with Lynn and he wanted to have a 'good' crank built and the organizations did not want it. What people didn't realize is a more expensive, better crank would have cost the karter less money in the long run, since it would have stayed true and lasted a much longer time.
 
Sean Cook said:
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.
Click to expand...
I have no idea how "tender" Yamaha cranks are in comparison to other cranks, however you should NEVER "pound" on anything that is supported by bearings or bushings especially if you are not going to replace the bearings/bushings before use. Never.
 
I personally believe that KT100 has been a success for karting beyond measure -- 40 years so far, and they are still being used everywhere.

Looking back over the history of the engine (and yes, I raced one when they first appeared) -- I feel the main reason for 90% of the issues with the engine are that we (in this country, primarily) have tried to turn it into something it isn't. The KT100 was designed and manufactured to be a "moderate performance" engine for the masses, run direct-drive, and with a low peak-power (wide range) exhaust pipe.

Unfortunately in this country, we had what I like to refer to as "rule creep", where not only was the intake port allowed to go lower and the exhaust port higher, but combustion chamber shape was altered dramatically, and of course we shoved exhaust pipes on them that created a gigantic peak torque spike (aka: heat) in the 10,000-12,000 rpm range.

As mentioned, the idea of a better crankshaft sounds great on the surface, however what's next? Oh wait... now I need a better con-rod too since with the better crank I can turn the thing another 1000 revs. Oops, the piston probably needs a redesign too since the wristpin boss is a bit weak on the OEM design. While we're at it, it would be really nice to update the head with maybe 50% more surface area and better material so that there would be less chance of detonation (which of course leads to an even more radical exhaust pipe, which leads to more issues).

Many years ago, I put together a direct-drive kart just to see how well the engine would run and hold up in an "as intended" setup. Pure direct-drive (no dry clutch or anything), an exhaust I sourced from Australia (thank you St. George Kart Centre). The engine still met our U.S. rules, though the intake was higher and the exhaust port was lower than allowed (considerably closer to original intent). 11cc's, stock everything, and even ran OEM steel-cage Koyo main bearings. At my local sprint track (Santa Maria), I ran 10/96 gear, with hard/spec tires. The engine would run 7200 in the tightest corner and just touch 17,000 at the end of the straight.

I ran that engine for over 17 hours, and finally pulled it apart because I felt guilty not checking it over (still turning the lap times it did at the beginning). The piston, crank, and bearings looked like they had a couple hours of "typical" running on them. I could have easily just cleaned all the parts and put it back together.

Conclusion? The KT100 is a fantastic engine if run as intended. We have managed to turn it into something it was never supposed to do, and then we complain that the internal parts aren't good enough. Of course this is typical racer mentality in just about every form of motorsports there is... :)

Just sayin...

PM
 
Pete_Muller said:
I like to refer to as "rule creep", .........combustion chamber shape was altered dramatically,
Click to expand...

I have put about every known shape into the KT head except the old K77 style trench chamber, and the newest toroidal shape.
Every year after seeing how rules were relaxed, another few heads would get cut up.

Yup, endless rule creep is right !
 
My 1st KT100 (1976) came with a dry clutch. The exhaust came out the front, as the engine was mounted to the frame, and curved up and back and around. It looked kind of like a veevee pipe. It had 12 1/2 CC's in the head. It was quickly learned that turning the barrel around only increased the performance, it did not hurt it. With the stock ports, and my pipe, the engine reached peak torque at right around 9200 RPM. I'm not positive on that, (peak torque) but if I slipped the clutch, (Burco) any lower, the engine would bog. This was before Haddock came up with the rules we have now.

Right after I bought this engine, one of the 1st 100 imported into this country, Yamaha gave us a new head. It was definitely different from the original head, and at the track, after a few laps with the old head, I put the new head on and immediately had to lean down the carburetor. CC's, what's that, I had never heard of CC'ing an engine. I don't remember any improvement in performance.

If you want to see that original head, see Terry Ives, he has it. I gave it to him before I moved to Texas. Only God knows why I saved that thing, but I did. I got it in 1976 and gave it to Terry in 2008. Why somebody would save such a worthless piece of junk is beyond me. lol
 
donrokon said:
Are the use of ceramic bearings appropriate in these applications?
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never tried it, but if there's any place that they would be a help, I would think the engine would be one place they could help. I don't think putting them on the axle helps much, if any
 
Al,
Did your old head look like the one on the right? I have a 79 straight shaft head on the left that supposedly only has the head gasket surface cut for comparison. not sure if the images work as the preview shows a picture icon, not the image.
SDC14417.JPG

and look how thick the fins are:
SDC14420.JPG

and look at the spark plug seat:
SDC14422.JPG


There is no radius on the transition from the squish to the bowl. The spark plug is about a turn from being flush with the bottom of the hemisphere. It is noticeably heavier. It has never been run. I was going to cut it as all my heads are cracked or at 8.9 cc - 9 cc.
 
GoFast said:
I have put about every known shape into the KT head except the old K77 style trench chamber, and the newest toroidal shape.
Every year after seeing how rules were relaxed, another few heads would get cut up.

Yup, endless rule creep is right !
Click to expand...

I found that straying too far from the stock combustion chamber shape might equal more power but it also equaled more CHT but not EGT. CHT would build so fast it would turn into a dog by the end of the race and no amount of richening it would really help. I gravitated back to a slightly wider squish band at ~ .030" squish at just over 11 cc as the best compromise. Now that I'm not racing and "retired" I have gravitated to about .024" -.025" squish, 9ish ccs, and I just back off and head for the pits when it gets up to 400 CHT. I've tried putting around for a lap then running hard but within one lap I'm back to 400.
 
moebiusgold said:
Al,
Did your old head look like the one on the right? I have a 79 straight shaft head on the left that supposedly only has the head gasket surface cut for comparison. not sure if the images work as the preview shows a picture icon, not the image.
SDC14417.JPG

and look how thick the fins are:
SDC14420.JPG

and look at the spark plug seat:
SDC14422.JPG


There is no radius on the transition from the squish to the bowl. The spark plug is about a turn from being flush with the bottom of the hemisphere. It is noticeably heavier. It has never been run. I was going to cut it as all my heads are cracked or at 8.9 cc - 9 cc.
Click to expand...
none of your links work.
 
Thanks for the hint Al. The problem is both sets of links work on my computer but that is probably because I made them. So the question remains is the combustion chamber shape on the right head worthless? I'll have to cut it to get mid 20s squish for a valid comparison at the track. I kind of like it because of the wide squish band but I don't like how deep it is. I'd go cc it on one of my known engines but its 106 outside and my shop is almost 140. Real shop next year....
 
Something strange going on. Every time one of those links show up, this morning, if I click on it, it works. Now why do you suppose something that would happen. They didn't work before, but now they do.
 
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