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106cc Kt100 torque monster build for super cheap..
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Tim Brown



Joined: 12 Nov 2009
Posts: 71

PostPosted: Thu Feb 14, 2013 9:30 pm    Post subject: 106cc Kt100 torque monster build for super cheap.. Reply with quote

Just Poking around in the garage and tearing down a PRD RK100 that I got for free, to have a 52mm Yamaha piston installed, instead of the 49mm that would make it a 106c torque monster. The PRD has a 50mm stroke and looking closer at the crank journal it looked like the same size as the KT100 with crank savers, so for kicks I shoved it into KT cases and it fits!!! and spins freely... and uses the same ignition rotor

So with the piston pin being the same except a little lower and a very small amount of fitment issues, I'm building a 50mm x 52.50 KT100 or should I say KT106 practice engine

I will keep posting the results of the build if anybody is interested... I think it should make 22-24hp on gas, with huuuge bottom end.


Last edited by Tim Brown on Sun Feb 17, 2013 11:05 pm, edited 4 times in total
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Bob Beasley



Joined: 07 Feb 2012
Posts: 2

PostPosted: Thu Feb 14, 2013 11:49 pm    Post subject: Reply with quote

This one looks like fun.
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John Mulvihill



Joined: 14 Oct 2001
Posts: 1151
Location: United States, New York,

PostPosted: Fri Feb 15, 2013 7:49 am    Post subject: Reply with quote

Neat. Please keep us posted.

Is the PRD crank a pretty tight fit in the case?

John
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Tim Brown



Joined: 12 Nov 2009
Posts: 71

PostPosted: Fri Feb 15, 2013 7:57 pm    Post subject: Reply with quote

Tight in terms of side to side or rod clearance in the crankcase? Just to let you guys know the crank has the journals that are the size off a kt with crank savers installed, but its cast in and induction hardened and has a knife edged rod, like an IAME, or Rotax, its really a nice looking crank.

Don @ CMC here in Oregon will assist in all the machining, very small amount if any, and getting the port timing correct to have the best Torque and Hp over the RPM range, this engine would be brutal in a stock appearing class, even in stock form

I wonder where peak torque will end up at, I hope lower than 10300, cuz you could use a very low stall clutch and you could use more of the rev range, just an idea...
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Walt Gifford



Joined: 19 Jul 2002
Posts: 4390
Location: United States, South Jerrrsey,

PostPosted: Sun Feb 17, 2013 6:31 am    Post subject: Reply with quote

http://karting.4cycle.com/showthread.php?t=472430&page=2&pp=35
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Tim Brown



Joined: 12 Nov 2009
Posts: 71

PostPosted: Sun Feb 17, 2013 10:26 am    Post subject: Reply with quote

Hey Walt.... was there a specific post you wanted me to look at, or just more helpful info to help me along...?
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Tim Brown



Joined: 12 Nov 2009
Posts: 71

PostPosted: Sun Feb 17, 2013 11:15 pm    Post subject: Reply with quote

Just found out the ignition armature is the same also and no TCI needed, hmmm, I wonder if its the same ignition curve too?? anyone?? I'm just posting my findings to let racers know there's another ignition system that would work or out perform the stock kt stuff and it might be waaay cheaper too...

When I'm done I will dyno them both to see the difference.
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Chris Parks



Joined: 09 Sep 2001
Posts: 1510
Location: Australia, not USA state,

PostPosted: Mon Feb 18, 2013 9:37 pm    Post subject: Reply with quote

You might find the crank case volume is the limiting factor when the Yamaha capacity is increased. It is most probably marginal in the first place.
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Chris
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Walt Gifford



Joined: 19 Jul 2002
Posts: 4390
Location: United States, South Jerrrsey,

PostPosted: Mon Feb 18, 2013 9:49 pm    Post subject: Reply with quote

Just another interesting KT100 build with some info on boring for a 55mm piston.

Is the RK100 shaft heavier than the KT100?


Gif Cool
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Tim Brown



Joined: 12 Nov 2009
Posts: 71

PostPosted: Tue Feb 19, 2013 8:06 pm    Post subject: Reply with quote

Okay Walt here ya go...

Yamaha 3lbs 12.0ozs
PRD 3lbs 13.5ozs

And about Chris's comment, I have heard very good things about stroking a kt100, can anybody help out here and further comments about the Crank Case volume issue, as I do enjoy the info about this build...
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Gordon Duax



Joined: 22 Dec 2009
Posts: 139
Location: United States, Texas, San Antonio

PostPosted: Sat Feb 23, 2013 12:30 pm    Post subject: Reply with quote

I wouldn't worry about it.
Read the book by Gordon Jennings called 'Two Stroke Tunner's Handbook'.
One of the most detailed books on modifying 2 strokes, and what Mr. Jennings said on case volume is "You will need some".
In other words, not a big factor. I thiunk his rule of thumb was about a 1.5 compression ratio.

Damn good book, but you need to be up on your math skills.
If you can't find a hard copy (and they go for big bucks, as he's left this world, and the book is not in print any longer),
then several sights have it on line, like here:
http://www.bridgestonemotorcycle.com/documents/2stroketunershandbook.pdf

Post pictures, I am very interested.
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Tim Brown



Joined: 12 Nov 2009
Posts: 71

PostPosted: Sat Feb 23, 2013 4:25 pm    Post subject: Reply with quote

Thanks Gordon!!!!!! I will print it off and start to educate myself on this stuff. I'm not trying to build a cheater engine, just a practice engine that gets it done!! and lasts for a while, and to let people know about the fun options that are out there too.
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Walt Gifford



Joined: 19 Jul 2002
Posts: 4390
Location: United States, South Jerrrsey,

PostPosted: Sat Feb 23, 2013 8:04 pm    Post subject: Reply with quote

Looks like an extra .040" of stroke to deal with at both ends.

If you're stacking that many base gaskets I would go with the aftermarket aluminum type.

I don't know what kind of exhaust system you plan on running but I would suggest first thing to do would be to shim the cylinder so you get a maximum of piston pop out at TDC, (say 1/2 the distance from the ring to the crown above the head gasket seat). Then check the exhaust port opening to see if the numbers look reasonable. Strokers always open too early.

You can get a .040" head gasket and cut the head to take the extra pop up just be aware of the spark plug to piston clearance.

Two Stroke Power Units by P.E. Irvine is also a good read.

Hope that helps,
Gif Cool
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FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience
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Tim Brown



Joined: 12 Nov 2009
Posts: 71

PostPosted: Sun Feb 24, 2013 11:21 am    Post subject: Reply with quote

Thanks Walt..
Well on the pipe I don't really want a bottom end pipe, so I wont use be using a L-2 pipe, might go with a a L-3 or L-4, depending on the track... if you can suggest a pipe that would be great. Remember this is a practice engine only and will never be in competition. Don @ CMC in Tigard OR. will be the man setting the ports on this little beast, I will mention everything you said about the piston pop out.

Hey Walt you never commented on the crank weights I posted for you??
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Walt Gifford



Joined: 19 Jul 2002
Posts: 4390
Location: United States, South Jerrrsey,

PostPosted: Sun Feb 24, 2013 4:38 pm    Post subject: Reply with quote

Looks like no significant difference in weight but, I'm wondering if the PRD crank is as hard as other high end 100cc cranks like Parilla ect. I suppose if those crank journals get worn they could be machined off and sleeved.

I'd go with the L2 unless you're on a mile long track otherwise, you're going to work your clutch to get any power off the tight turns.

Gif Cool
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FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience
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