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CT70 Piston hitting cylinder head?

This is a discussion on CT70 Piston hitting cylinder head? within the Z50, CT70 and JDM Monkeybikes forums, part of the General Talk category; Hey guys, first post here. Just discovered this great resource! And just so happens I have a problem. Just getting finished up on a restoration ...

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    CT70 Piston hitting cylinder head?

    Hey guys, first post here. Just discovered this great resource! And just so happens I have a problem.

    Just getting finished up on a restoration of a 1971 CT70, and put the engine back together. I thought I got it timed pretty well bang on. At TDC the timing marks for the chain matched up almost exactly. The engine spun over and all was well. However, after I torqued the cylinder head acorn nuts to spec, the piston hits the top of the cylinder now! Grr! Is it a timing problem? I should mention that I've installed a new cylinder. It's not a Honda original either. Is that the problem? Or is it valve timing?


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    Welcome. Hmm just double check that the cylinders are exactly the same length. And that the head gaskets are as well. I know some are thinner causing the valve clearance to drop.

    Also pull the cam cover off and just double check the timing and try to get it exactly spot on. At high RPM with a little valve float and timing out things can go terribly wrong.
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    You didn't install a non-original cylinder head, did you? Non-Honda (Chinese) heads I've seen are patterned after the later, 12-volt CDI Honda motors, which have a shallower combustion chamber and won't work with a early, 6-volt-style piston and its higher domed crown.

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    Quote Originally Posted by mexicanyella View Post
    You didn't install a non-original cylinder head, did you? Non-Honda (Chinese) heads I've seen are patterned after the later, 12-volt CDI Honda motors, which have a shallower combustion chamber and won't work with a early, 6-volt-style piston and its higher domed crown.
    I think this may be the case. I've sent back the Chinese cylinder head and ordered a time-sert kit to fix the original Honda head. I'd rather use the quality Honda head anyways, so we'll see if that fixes the problem. The combustion chamber on the chinese copy and the original do indeed look a little different. They measured about the same but the original is definitely tapered a little deeper. When I swap them out I'll report back!

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    What's the deal with the stock head...stripped spark plug threads or something? If it turns out your original head is toast, a rebuildable core (or one that could just be slapped on and run after cleaning it up) should be easy to find and inexpensive.


    Regarding the compatibility of early/late-style parts, the later 12V motors not only had a shallower head recess and flatter piston crown; they also had a longer connecting rod, so the piston pin hole is located higher up in the piston itself (closer to the ring grooves and piston crown), whereas the earlier 6V motors had a shorter connecting rod, a piston pin hole lower down on the piston, the taller dome on the piston crown and corresponding deeper combustion chamber to accomodate the piston dome.

    And the 12V cranks have a different, more abrupt taper on the flywheel end, whereas there are a couple different 6V crank tapers to suit either the fixed-timing points magnetos that came on the three-speed semi-auto motors or the centrifugal spark advance points magnetos that came on the 4-speed manual-clutch motors (CT70H, SL70, CL70, XL70)...and probably some exceptions to that. I believe C70 passports had the latter type of ignition along with an auto clutch.

    Just things to be aware of if you're looking at older motors as parts sources...

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