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Another 230cc CRF conversion question

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I'm obviously new to the forum so hello. :)

Like those that have also done this conversion - Before you ask why I'll say because I can . :)

I need some opinions on frame mounts. I'm using a Zongshen 250 (232) air cooled motor and a 05 Honda 450X frame for a trail bike build. I was wondering if it would be strong enough to have just the head stay mounted, the two front (red) engine mounts, and the rear lower (blue) mounts mounted to the frame but leave the top rear mount (green) unmounted?

Otherwise I will be doing what others have done and make a C - type bracket for the back mounts that attaches to the rear swingarm pivot bolt. It just wouldn't look as clean and more of a pain to remove and install.

Mounts.jpg

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At the engine's torque peak in first gear there can be 1000 lbs of tension in the drive chain pulling the counter shaft sprocket towards the rear sprocket.  The designers of the frame used the swing arm bolt to transfer that force to the engine, Honda sub 250 engines, and the clones, do it via the frame and two rear mounts. When I installed a XR200 engine in a RM frame I reworked the frame's lower rear mount to suit the XR engine and then fabricated a box shaped rear mount adapter that attached to both of the engine's rear mounts and replicated the RM engine's rear mount.  This resulted in both of the engine's rear mounts  attached to both of the frame's rear engine mounts.  

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If you go back and look at how the early 80's cr engines were mounted at the back,  you can see how easily a cradle that uses the swingarm pivot can be made and mouted.  If you can cut and weld you can do it,  and it is much cleaner than building to the frame.  lots of frame makers who made flat track frames use the same method.  WAY stronger,  and though you won't need it on this one,  it helps keep the engine inline with the swingarm better too. 

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Thanks for the reply's.  I guess when I really think about it the centrifugal torque on the back of the motor could be enormous under certain circumstances so it looks like I'll be making a box/C type mount adapter for the back.  It's too bad I had to offset the motor for sprocket alignment making the left side rear mounts lineup in-front of the swingarm rather than a direct shot to the pivot bolt.  Just means offsetting the bracket design a bit, not a big deal.

 

Chuck, I'm pretty sure I read some of your build details from way back but I was wondering if you have any pics of the rear mount you came up with?  I have a pretty good idea as to what I'll make but sometimes seeing what others have done makes you consider things you don't think of until it's too late.

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As ossagp said it isn't too difficult to fabricate. The most important issue is getting the engine counter shaft sprocket aligned with the rear sprocket. If you have the old engine in the chassis make some reference measurement of the countershaft sprocket for vertical and horizontal alignment to the SA.

I couldn't find any pictures of the rear mount on my conversion. Basically you need a spacer on the swing arm to replicate the rear mount of the old engine, and attach to it a structure that pick up the two rear mounts on the new engine, plus the lower frame mount.  When I did mine I didn't have a TIG welder so I used bronze welding to join the parts together, 35 years later it is still working. Bronze welding is really brazing with a more viscous filler rod that allows gap filling and a filet bead. It also  provides good vibration tolerance and a fatigue resistant joint.  The bike is 40 miles away so not easy access for pictures.

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