KT100 Rebuild. What parts to use?

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b40

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Looking to go through my KT100. I have all the tools needed, and have rebuilt a couple of them. What I don't have is the knowledge on the parts that should be used. Our local builder, who is unfortunately no longer with us, taught and guided me on the parts to use. The other issue is that it has been over 10 years since I've touched one, and maybe things have changed. I can certainly tear it down and find out what was used, but would also like to see if things have changed.

This will be for an oval kart.

Parts list, and what I recall...
Pistons - not sure
Rings - O&H Gold
Con Rod - believe I used light weight rod.
Upper rod bearing - Cannot recall, but believe it was a wider bearing, and aligned at the bottom with shims.
Lower rod bearing - Don't recall any options here.
Crank bearings. - I think I used the self aligning FAG bearings. One of mine has sleeves, the other does not.
Yellow crank seals - Can't recall brand.

Anything else I'm missing? I'm open to any input, thank you very much!
 
I'm no expert .
The shims on the crank are to protect the shafts .
Also was told the. o-h gold ring needs a perfect bore .
 
Above message by "b40" quoted, with answers in bold.


Parts list, and what I recall...

Pistons - not sure -- I personally prefer OEM Yamaha pistons.

Rings - O&H Gold -- That's fine, though make sure to "light check" the ring after the cylinder has been honed. And make sure it has *enough* end gap. Fit and tension are far more important than end gap.

Con Rod - believe I used light weight rod -- If you can find NOS (but very old) con-rod, those were the best (IMO)

Upper rod bearing - Cannot recall, but believe it was a wider bearing, and aligned at the bottom with shims -- OEM

Lower rod bearing - Don't recall any options here. -- OEM with OEM thrust washers. Crank-pin might need replacement too.

Crank bearings. - I think I used the self aligning FAG bearings. One of mine has sleeves, the other does not. -- I would not waste money on double-row self aligning bearings. If there is a significant alignment issue with the case, then you could use self-aligning bearings on both sides, but I'd be more inclined to fix the case if there's a significant problem.

Yellow crank seals - Can't recall brand. -- I've personally always used OEM, and trimmed the "outer" lip out so it doesn't touch the crank.


PM
 
Make sure its sealed up good.as in gasket mating surfaces.thats the battle im going thru now on a engine.cylinder to head.
 
Pete_Muller said:
Con Rod - believe I used light weight rod -- If you can find NOS (but very old) con-rod, those were the best (IMO)


Yellow crank seals - Can't recall brand. -- I've personally always used OEM, and trimmed the "outer" lip out so it doesn't touch the crank.


PM
Click to expand...

Thanks for the reply Pete! I do have a some questions of the above replies.
The NOS rod, is that the heavier one, or light weight? What would be the reason to use the light vs heavy? I thought both the light and heavy were Yamaha brand, but can't recall.

And are you saying you remove the outer lip completely on the seal? Wouldn't that give a bit of a "one way" valve and suck in outside air as the pressures inside the crankcase drop, or maybe that was your goal to get a little extra air in there?
 
b40,

For many years, the *only* available con-rod is what is now called the "light rod".

The con-rods made back from the beginning of the KT100 (late 70s) for probably 10+ were just a better rods, in my opinion. I believe the raw material used in the original forgings was better then the current rods. They were more difficult to straighten if they need to be adjusted, and just "felt" tougher. They are difficult to find now though.

BTW: Target clearance on the big-end should be .0013"-.0015" in my opinion. Use a micrometer to check the crank pin, add two roller diameters, and then add .0013"-.0015". That should be the range of the bore size on the big end.

No, the seal will not suck in air when the outer lip is trimmed. The function of the outer lip is simply to keep dirt from getting to the actual lip that does the sealing.

PM
 
Good advice has been given but I’d like to advise you that O&H gold rings are no longer available from O&H, you may be able to find nos somewhere. KSI rings are the replacement for the O&H gold. Yellow seals haven’t been available for years, the go to seals are black with a goldish green ring near the center.
For a sleeved crank I like 6205 C4 SKF but FAG and ORS are good too. I’m not a fan of double row bearings either.
Unsleeved cranks call for 6304 C4 SKF or Fag
 
I finally got to tearing my engine down. Looking through my parts bin I have an OEM Yamaha light weight rod P/N 50W-11651-00, I also see two different piston pin options. One is aftermarket, and about 24grams, the other is OEM Yamaha, P/N 131-11633-00 is around 29grams. Do you match a light weight rod with the heavier pin, and the heavier rod with the light weight pin for balance reasons? My engine had the lighter pin in it and what I'd guess was the medium rod.
 
Always run the light rod, it's more than strong enough for a KT. If you going thru tec then run an oem piston pin, if not you could shed a few grams with the other one. A Burris big bore piston is 17 grams heavier than a std Yam and yet the balance factor change wasn't enough to warrant adding tungsten to the crank wheel, so 4 or 5 Grams you'll never notice. When the piston was 28 grams heavier than a std. KT. like the big bore Australian piston made by Strike , then it was a problem at high rpm without rebalancing.
 
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Use the thicker oem piston pin. Light weight pins can flex and break your piston. I like the IAME wrist pin clips with no tang or you can just cut most of the tang off of the OEM clips. If there's no real wear on your wrist pin, you can polish the bearing area it and reuse. Also, if the crank pin looks good you can polish it, rotate it 90 degrees and put it back in but most of the time it's best to use new parts.

There are different rod bearings depending if you have lower side plates or the older top plate configuration. For lower side plates you want the wider wrist pin bearing. Getting the axial clearances on the lower rod and overall crankshaft end play can be challenging. Set the clearance on the lower rod at .025 first then use bearing shims or split line gaskets to set crank end play .012 - .015. Don't compromise the lower rod clearance to set crank end play.
 
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Thanks for the replies, I'm double checking if running the light weight rod P/N 50W-11651-00 with heavier OEM Pin, Yamaha, P/N 131-11633-00 is recommended? Again, this is running oval kart on a track with just a little lifting getting in the corners.
 
One thing I might suggest checking is the "fit" of the wristpin into the piston. There are certain combos that are, in my opinion, too loose. As you know, aluminum grows double the amount per degree that steel does, so an "already too loose" wristpin starts to have a pretty decent amount of clearance at running temp. If you're planning on turning really high revs, this "play" in the wristpin can be really hard on the wristpin bosses in the piston.

So, IMO, check the wristpins you have available and run the one that has the least clearance in the wristpin bore.

PM
 
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