Where to get a crank stroked?

KawieKX125

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#1
I am back after a long break from motocross while at college and a long break while building the Shelby Cobra replica but I have a new project in the works for the 144 cylinder I have and the extra engine I got off ebay for 100 dollars for my bike. I am going to build a stroker motor. I have the equipment at school to weld and redrill the crank pin hole, as well as a CNC mill to clearance the cases for the longer stroke, but I do not think that I am capable of stroking the crank accuratly and I also don't think I can balancce a crank, so does anyone know of where I can get a crankshaft stroked for less than an arm and a leg? I am not sure if Eric Gorr does crank stroking and all I know of so far is Falicon ($$$$$$$$) and RPM ($$$$$$$$). I don't want it assembled as I will do that myself (unles it needs to be assedbled to balance it).

Thanks a bunch I need to start coming back here more often!
 

bclapham

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#3
welcome back, kawi- its been a while!

Eric wrote a nice article on this a while back if you want to do a search- dont forget there is other stuff to consider to such as PV linkage and spacer plates and rods, etc. etc. i am thinking about doing this to my rm144 in the summer when it comes to rebuild time, i will be bumping that thread at some point with some more Q's for Eric. last time i spoke to him, he wasnt doing the cranks himself, but was doing all the other stuff including building the engines etc etc.
 

KawieKX125

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#4
I read the article (the only one that came up when I did a search for crank stroking) and it helped clear some stuff up. From what I understand, with some creative head mods you don't need a cylinder spacer. Maybe I should post in Eric's forum too?

Thinking about this bike has me salivating on the keyboard. Right now I have a fully built TMR motor that came with the bike and it is faster than stock new 125's and unbelievably easy to ride, I can't imagine the spare engine I have with ported cases and cylinder plus an extra 40 some odd cc's. It would probably be over 40hp.
 

bclapham

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#5
RPMs does the head mod route but i am not sure this is the best way forward since it messes up the port timing quite a bit, either a shorter rod or a spacer plate seems a good way forward IMO.

as for HP, i dont think 38HP will be exceeded very easily, but i am sure these motors will have loads more power before peak.
 

EricGorr

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#6
Although I offer the service through my business, I only assemble the cranks and rely on TNR Machine in Dowling Michigan to do the machine work. I've tried and evaluated all the companies offering the service of boring, bushing, welding, and finishing the big end bore and TNR's work is consistent and the service is relatively fast. TNR doesn't offer all of the other services as Crank Works or Falicon, but quite frankly on a 125 you don't need it. Most Japanese 125 cranks are over-weighted anyways and most of the balancing services are just a guess at the proper change in balance factors.

I've built about 25 engine with bore and stroke combinations including high revving small bores and they're all running fine. So the whole balancing thing is a bit of a black art. Some companies add weight to the bottom of the flyweights, some remove it from the top. I don't do either and I haven't had anything fly apart or produce excessive vibration.

The KX125 can be stroked a safe maximum of 2mm off-set for a 4mm total stroke change. The cases must be bored to accomodate the connecting rod swing. Don't be tempted to grind the rod for clearance like some west coast companies. That will weaken the rod and cause it to break prematurely.

One last thing about Crank Works, I've recommended them in the past and I think that the owner Phil knows more than anybody in the business of crankshafts, but those guys have some serious organizational and order-fulfillment issues to focus on. I never got a crank returned in less that 8 weeks from them, whereas TNR is about 2 weeks turnaround.

TNR Machine, 2050 W. Dowling Rd, Dowling, MI. 49050  Tel(269) 623-2827(Tom Watson)
 

KawieKX125

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#7
Thanks for the info Eric. We have a CNC mill at school that I think would be able to clearance the cases 2 mm, but I am not sure I feel comfortable doing it. How much would you charge to do all the required mods and porting to a bare set of cases, a crank (maybe I will just send that out on my own?), and a cylinder that has been bored and plated with no porting work done? I have a friend sending in his RM cases and cylinder and another CR cylinder so maybe I will throw it in the same box as his stuff.

Thanks
 

bclapham

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#8
Originally posted by EricGorr
Don't be tempted to grind the rod for clearance like some west coast companies. That will weaken the rod and cause it to break prematurely.
it seems to me that around here (and probably most places) that the emphasis is to do the most amounts of mods with the miminum of altering of any of the regular parts. i have to wonder if using an expensive offset crank pin, grinding the rod and not the cases, and just milling out the head so a spacer isnt needed doenst make the best engine, or worse, provides an unreliable engine which becomes essentially disposable as soon as it goes wrong- all for a massive number on the reciept!
 
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#9
I work for a company called Vertucci Performance Equipment. We do a lot of Stroker cranks for mostly all two-strokes from 65's to 500's. If you have no previous experience in building strokers I would not attempt it. Some of the best machinists can screw up a crank in no time. Every aspect of flatness, truness, and squareness must be checked and rechecked. You also need to use different tooling to cut a crank. Plus you must know the required press fits because they are different than stock. As for machining the cases, you must know your new rod spec because too little will require rod modification and too much will open the crank case volume too much which will result in loss of power. Depending on the added stroke the rod might have to be modified which should not be ground! We machine the rods precisley to what clearance we need. I've never had a rod break in one of our motors.