Vintage Dirt Bike Q & A

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PostPosted: Sun Feb 12, 2006 9:19 am 
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Joined: Thu Jul 22, 2004 3:54 pm
Posts: 150
Location: Surrey, BC Canada
I plan to put a 76 MR175 engine in my 75 CR125. Will the 75 CR125 crankshaft fit into the 76 MR175 engine? I want to be able to bolt on the CR's ignition rotor so I can keep the CR's CDI ignition. The crank "wheels" ("cheeks") are a different shape. Maybe they need to have a different balance to match the greater weight of the MR175 piston so the engine doesn't vibrate. Has anyone done this? Advice appreciated, thanks.
Mark


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PostPosted: Thu Feb 16, 2006 10:14 pm 
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Joined: Fri Apr 15, 2005 9:49 pm
Posts: 86
The CR125 crank has different size rod bearings on both ends so it will not go in the MR175 motor. I think you can space the ignition on the MR175 crank so you can use the CR125 type ignition. I want to try the opposite mod by using two MR175 crank halfs with a 1981 CR125 rod kit which should mount to the MR175 crank without mods. Don't know how much vibration the MR175 crank will cause though.


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PostPosted: Sat Feb 18, 2006 6:12 pm 
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Joined: Mon Feb 02, 2004 3:05 pm
Posts: 14
Hi! I have done both mods, and it is not to complicated at all. The CR125 CDI ignition will work just fine on the MR175 crank, (stroke is the same as a 125) and will give you a little quicker throttle response. If you are going to put the MR175 carnk into a CR125 bottom end, you need to machine the cases ot accomodate the slightly larger crank weights. It is not to tough do this, it is only a millimeter or two. They all use the same main bearings, so no worries there. If want a real screamer with a bit more power, the MR175/CR125 transmission is a great setup. Good luck!


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PostPosted: Tue Mar 14, 2006 5:19 pm 
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Joined: Thu Jul 22, 2004 3:54 pm
Posts: 150
Location: Surrey, BC Canada
Thanks. The CR125 rotor and stator bolted right on. Easy!!
The CR125 ignition cover fit on too.


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