CMF Rear Sprocket Bolt Clearance Issue

[slvrbrdfxr]

Hi all,
I'm in the process of reassembling a relatively early CMF 125 Six Day bike frame # 10951060. This bike came to me disassembled and has a 5A engine which is original to the bike. Besides the engine, I'm discovering there are some other subtle differences between this bike and the later model CMF bikes that I'd attribute to product improvements in the later model bikes. For example, I was aware that these early CMF bikes have different triple clamps which makes the distance between front fork tubes slightly more narrow than the later model CMF bikes. I also figured out that these early CMF bikes do not have a tab welded to the frame tube under the rear of the gas tank which allows for installation of the aluminum splash shield under the seat around the top right side of the airbox intake. Made another discovery which had me scratching my head over the last couple days and wondered if anyone else had experienced this ? I installed a new 57 tooth rear sprocket onto the sprocket carrier and then installed the carrier assembly onto the swingarm being sure to use the correct chain adjuster which has the flat washer tack welded to the inboard side. Once I tightened the sprocket carrier axle nut, I found that there was barely 1/32" clearance between the sprocket bolts and the swingarm which didn't seem right. It also appeared as if there was  some misalignment between the countershaft and rear sprockets too. Double checked with the parts manual again to ensure I had all the parts installed in the correct order, which I did. Decided to compare this sprocket carrier installation to another 72 CMF bike I have and found there was at least 1/4" to 3/8" clearance between the sprocket bolts and swingarm on this other bike. Knew I couldn't run with so tight of a gap between the sprocket bolts and swingarm and had to figure out what was causing this. After looking everything over many times and doing some measuring I finally figured out the difference. The flange thickness on the swingarm where the chain adjusters ride are different between the early and later model bikes. The early swingarm flange is 1" thick and the later swingarm flange is 1 1/8" thick. The 1/8" change was added to the inboard sides of the chain adjuster flanges on the later model swingarm which moves the sprocket carrier and sprocket bolt heads slightly to the left and provides for some additional clearance. My solution to get the additional clearance on this early model bike between the sprocket bolts and swingarm would be to take the large flat washer (item #31, 51-10-038-000) and relocate it from under the sprocket carrier axle nut and place it on the inboard side of the chain adjuster. This should move the sprocket carrier assembly slightly to the left and give me the clearance which is needed at this location. Has anyone ever noticed this issue before when building an early CMF bike ? Anyone see a problem with relocating this flat washer to get the sprocket bolt clearance and chain alignment back to where it needs to be ? Let me know your thoughts. Thanks for taking the time to read this rather long post.
Dave McCullough

[firstturn]

Dave I have noticed different chain adjustor thickness over the years, but I guess Iwas always lucky that I was building the bike back original or I had changed swingarms.  In my book if you make this change and the countershaft and rear sprockets you should be good to go. So testing might help?  To answer  your question I had never noticed this.


Ron Carbaugh

[slvrbrdfxr]

Ron,
Thanks for the reply. I have another early frame serial #107xxxx which still has the swingarm attached and it measures exactly the same as what's on the bike I'm building. After seeing that, I feel very sure this is all correct parts for this bike. I went ahead and relocated the flat washer last night and it looks like it should be okay to run like that. The rear wheel with the brake plate, left chain adjuster and left side spacer all installed together nicely even after swapping the flat washer position. The wheel has been freshly relaced and needs to be trued up and so I'll still be able to center the wheel in the swingarm going forward. Sprocket choice is going to very important on this build as I'll need to install something as close in thickness to the original sprocket as possible to provide for maximum clearance. The new sprocket I purchased to install on this bike is made by Rebel Gears and is manufactured of approx .050" thicker aluminum than the original sprocket where the bolts attach it to the carrier. This .050" difference In sprocket thickness makes the bolt clearance tighter than I'd like to see and so I may have to look for something that works better. This Rebel Gears sprocket would work fine with no clearance issues on any CMF bike which has the newer style swingarm with the thicker flanges. Any suggestions as to another sprocket manufacturer to look at ? Thanks again for the reply.
Dave McCullough

[Cal Alexander]

Hi Dave, the early 1972 CMF bikes are a bit different. I've restored three or four of them and know the issues your running into.

For the rear sprocket clearance you might want to also check the rubber pieces in the rear cush hub. The rubber thickness was changed on the later CMF bikes and might be throwing off your sprocket clearance.

The front wheel is offset with the narrower triple clamps, this can be a little tricky when your lacing up a new rim as the wheel needs to be laced off center. The front fork tubes are also about 3/4" shorter on the early CMF bikes to compensate for the thinner triple tree.

Shoot me an email if you want to discuss any of this and we can talk. Just finished up a very nice early 72 Six Day that I acquired from Ron so these issues are still fresh in my memory.

[Paul Danik]

Dave,
   
   Maybe I have missed something, but have you looked for a NOS rear sprocket?  What size ?

Paul

[firstturn]

If you have access to a lathe you could turn the sprocket down to properspecs.

Ron Carbaugh

[slvrbrdfxr]

Thanks to all for the replies. Cal, yes once I got the front wheel laced up and trued I did notice a slight offset. Had to grind down the ends of few spokes that were protruding through the nipples due to the offset. The rear cush drive spacers are in good shape.

 On the rear, the original sprocket from this bike is a constant thickness of .280" across the entire width. The Rebel Gears sprocket I purchased is manufactured differently and has a step in it. This sprocket measures. 330" where the sprocket bolts go through and across most of the sprocket width. It then narrows down to .280" at the sprocket teeth. I'll save this sprocket for use on another project.

I called Sprocket Specialist in Utah yesterday to see what they had to offer. Their catalog showed part number 219Z for the CMF bikes. Their 219Z is made of 7075-T6 aluminum and is a constant thickness of .280" across the entire width. It can be ordered from them with whatever tooth count you want and can be manufactured with or without lightning holes. I ordered a 57 tooth with lightning holes from them and hopefully it will be here this week. I'll update this thread later once I have everything assembled. Thanks again to all !
Dave McCullough