Sophomore Rider
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Post by ilovenxstage on May 27, 2013 21:53:42 GMT -5
Is it necessary to modify the crankcase to install a stroker crank in a 139qmb engine? If so, what has to be done?
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Post by larry001964 on May 27, 2013 22:05:27 GMT -5
Is it necessary to modify the crankcase to install a stroker crank in a 139qmb engine? If so, what has to be done? Nope no mods to the case, just and extra base gasket, and a big bore head must be used... |
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Post by onewheeldrive on May 27, 2013 22:10:21 GMT -5
How much more power/faster do you get out of those, in general?
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Post by millsc on May 27, 2013 22:10:27 GMT -5
I haven't done a stroker project , but it seems with the longer stroke and extra gaskets.you wouldn't gain much over the stock crank with one gasket or especially an hd crank with higher reliablity.
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Post by alleyoop on May 27, 2013 22:26:10 GMT -5
Longer cranks will give you more low end power, just think of a long breaker bar you have more leverage and power to turn a NUT that won't come off. You also may loose a little rpms due to the longer stroke. That is why the 2 strokes scream, little tiny half doller piston and short crank produce a lot of high rpms.
Alleyoop
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Post by larry001964 on May 27, 2013 22:28:12 GMT -5
I haven't done a stroker project , but it seems with the longer stroke and extra gaskets.you wouldn't gain much over the stock crank with one gasket or especially an hd crank with higher reliablity. Well what gives the stroker it's power is the connecting rod being out further on crankshaft... I know a 1 or 2 mm don't sound like much.. but it does make a difference..... Fawkes will be a stroker again, only this time 16/50 gearing, with a long case engine for a bigger rear tire... Trust me 50 to 55 mph speeds you want the bigger tire. |
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Post by millsc on May 27, 2013 22:32:13 GMT -5
My new scoot is a long case with 120/70/30 tires if all goes well with your project ill try it
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Post by larry001964 on May 27, 2013 22:34:25 GMT -5
My new scoot is a long case with 120/70/30 tires if all goes well with your project ill try it Already lining up the parts ;D will be ordering them probably next week.. Sooner depending on what scrappy does Tuesday.. |
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Sophomore Rider
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Post by psychedelicode on May 28, 2013 9:16:40 GMT -5
What happend to your stroker, larry?
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Post by onewheeldrive on May 28, 2013 10:11:59 GMT -5
Seems like it would throw the timing off?  |
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Sophomore Rider
Currently Offline
Posts: 137
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Joined: Mar 4, 2013 17:29:00 GMT -5
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Post by ilovenxstage on May 28, 2013 11:02:14 GMT -5
As soon as I can afford the parts, I think I'm gonna go all out with an 80cc big bore and stroker, plus all the other performance goodies  |
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Post by hank on May 28, 2013 12:30:09 GMT -5
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Post by larry001964 on May 28, 2013 18:46:04 GMT -5
What happend to your stroker, larry? First assembley I didn't use a second base gasket, and I didn't use matched parts, they were just unaviable at the time.. Any way Fawkes was hard to start... This winter i had to store him outside making him even harder to start, to compasate I used a oil that was too thin... Mobile1 0w-40 European blend.. Any way it trashed my crank bearing... Sooo I do it again, this time using matched parts, and that second gasket... That stroker when running was just wayyyyyyyyy to much fun.... |
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Post by larry001964 on Jun 2, 2013 3:12:36 GMT -5
Been so busy working I have not had time to update this, Scrappy got my engine and it's on it's way... A few little things to do when it arrives and Fawkes is running again,,,, Then Ill rebuild his original engine, and then Ill start the stroker.
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Post by RapidJim on Jun 2, 2013 11:27:23 GMT -5
Along with what Alley said about leverage, a stroker crank also increases displacement. The standard stroke of a 50cc motor is 41.4mm and the bore is 39mm. The formula for figuring displacement in mm is Bore(mm)*Bore(mm)*Stroke(mm)*0.0031416/4 So 39 * 39 * 41.4 * 0.0031416 / 4 = 49.4561 or 49.46cc Lets change just the crank to a 44mm stroker crank 39 * 39 * 44 * 0.0031416 / 4 = 51.612 or 51.61cc Since you most likely would not tear a motor completely down to just change the crank, lets compare using a 52mm big bore kit, the largest you can go without case mods. 52 * 52 * 41.4 * 0.0031416 / 4 = 87.92cc add the 44mm crank 52 * 52 * 44 * 0.0031416 / 4 =  .44cc Jim |
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..BAD TO THE BONE..
.44cc