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Discussion Starter · #1 ·
This weekend I'm going to be replacing the head gasket on my '81 CM400. Manual description looks pretty straight forward and I didn't find anything unusual by searching the forums here. Before I jump in, wondering if there is some collective wisdom I should I hear before jumping in?
Thanks!
 

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Premium Member
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I pretty much followed the manual when I had the head off my CB400. Its pretty straightforward, but just take your time with it. Especially, when putting it back together and getting it timed, don't want any bent valves. When I removed the head bolts I numbered them with a piece of tape so I knew what hole them came out of. Also, make note of what side you take the cam bits from and put them back on the same side.

Let us know if you get stuck.
 

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When reassembling, put rags into the holes where screws can fall in. It'll make your life less painful. Dont screw the valve/cam/lining up procedure. When you have the cam back on and head, turn the engine over with the wrench on the stator. Make sure you can turn it all the way around with no collisions before you proceed to any next step.
 

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Discussion Starter · #4 ·
Thanks for the tips and suggestions. Once I got in there it was much easier than I had initially anticipated. The reason for the change was a growing amount of oil seepage from the rear of the head gasket. Here are some pictures of the project:

Ready for dis-assembly:


Marked and numbered:


It's off! Look at the nice carbon build-up:


This could use some cleaning:


Much Better:



The Work Area:


While I had things apart figured it was a good time to upgrade the front sprocket from a 16 to 17:


All back together:


Finished the whole process in about 4 hours. After reassembly I ran the engine to warm it up and then changed the oil and filter to remove any contaminates that may have gotten into the engine while it was open.

After dinner had a chance to go on a shake down ride. Happy to report complete success. Not only did the seepage stop as hoped, I also enjoyed smoother idle and acceleration. I suppose the decarboning of the cylinder head and pistons was the primary reason behind these improvements. Also happy to report RPM's dropped about 400 at highway speeds making the bike much more enjoyable and less buzzy due to the changed front sprocket.

Items replaced:
Head gasket
o-rings on locating dowels
valve cover gasket
Left side cover gasket (PO had used rtv, yuck)
intake o-rings
spark plugs
 

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How did you clean piston heads without removing? i just got the head off and need to decarbon too

tks!

Thanks for the tips and suggestions. Once I got in there it was much easier than I had initially anticipated. The reason for the change was a growing amount of oil seepage from the rear of the head gasket. Here are some pictures of the project:

Ready for dis-assembly:


Marked and numbered:


It's off! Look at the nice carbon build-up:


This could use some cleaning:


Much Better:



The Work Area:


While I had things apart figured it was a good time to upgrade the front sprocket from a 16 to 17:


All back together:


Finished the whole process in about 4 hours. After reassembly I ran the engine to warm it up and then changed the oil and filter to remove any contaminates that may have gotten into the engine while it was open.

After dinner had a chance to go on a shake down ride. Happy to report complete success. Not only did the seepage stop as hoped, I also enjoyed smoother idle and acceleration. I suppose the decarboning of the cylinder head and pistons was the primary reason behind these improvements. Also happy to report RPM's dropped about 400 at highway speeds making the bike much more enjoyable and less buzzy due to the changed front sprocket.

Items replaced:
Head gasket
o-rings on locating dowels
valve cover gasket
Left side cover gasket (PO had used rtv, yuck)
intake o-rings
spark plugs
 

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Sensei
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27,180 Posts
Ryan...The good:... It reduces the number of engine revs necessary to acheive or maintain a certain speed...(less buzzy as well)
The bad:...It also reduces the initial torque applied when taking off...Every change is a compromise.....
 

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So its a low end torque vs top end hp tradeoff... thats sorta what I was figuring.
I'm running the stock 16/35 ratio on the 400 right now, yeah the 450 6 speed is in there. I'm finding that the gearing is really quite good and close to the power/torque curve. If anything I would actually change my rear to a 36 since i'm running loaded.
 
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