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CB400T Shitbox Build - Daily Driver

This is a discussion on CB400T Shitbox Build - Daily Driver within the Project Builds forums, part of the Caferacer.net Forums category; I suspect that you already found the problem with those rocker arms. Get a new or replacement set or have them refaced by Megacycle or ...

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  1. #231
    Senior Member Teazer's Avatar
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    I suspect that you already found the problem with those rocker arms. Get a new or replacement set or have them refaced by Megacycle or a local supplier. Those shell bearings are what you would expect to see after all these years and are probably still in spec. Check that with plastigauge as you mentioned to be sure.

    Typically a rocker to cam noise is a light sort of tapping and big ends tend to be a deeper noise. sort of like a big hammer against an anvil compared to a small one in terms of sound.

    You should measure the piston to bore clearance while it's in bits to see if worn bores are contributing to the racket.

    Those motors sort rustle rather rattle as a rule.

    Check parts list at CMSNL to see if they use the same pistons and parts as general export models and get a camshaft with bigger bricks on it if it will fit.

  2. #232
    Senior Member jakejensen's Avatar
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    Pistons are somewhat worn, to the point that they are out of spec I think. I mic'd them both at about 70.3mm, and service limit is 70.40mm. Piston/bore clearance is .13mm - .15mm, which is outside service limit. I don't have a dial gauge to measure cylinder OD. but it seems like the old pistons are a little worn. They show standard marks of wear, the bore is not scored. I inspected it and honed it already, it looks clean.

    Is there another way to measure bore OD without a dial gauge?
    Last edited by jakejensen; 09-25-2019 at 02:10 AM.

  3. #233
    Senior Member woodsman's Avatar
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    Take it to a machining shop and have them measure it.

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  5. #234
    Senior Member TrialsRider's Avatar
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    Quote Originally Posted by jakejensen View Post
    ...
    Is there another way to measure bore OD without a dial gauge?
    ID

    Yes, run your finger (or inside callipers) up and down on the cylinder bore from where the piston rings do contact the cylinder bore, to where they never contact the cylinder bore and see if there is a difference, you don't even need to know how to read a micrometer that way.

    If there is visible wear on the pistons you found another problem, in a perfect world pistons never ride in contact with the cylinder bore, the rings do.

  6. #235
    Senior Member jakejensen's Avatar
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    Quote Originally Posted by woodsman View Post
    Take it to a machining shop and have them measure it.
    Ugh. Finding a German machine shop seems like such a pain in the ass. But then again this isn't a walk in the park either

  7. #236
    Senior Member jakejensen's Avatar
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    Quote Originally Posted by TrialsRider View Post
    Quote Originally Posted by jakejensen View Post
    ...
    Is there another way to measure bore OD without a dial gauge?
    ID

    Yes, run your finger (or inside callipers) up and down on the cylinder bore from where the piston rings do contact the cylinder bore, to where they never contact the cylinder bore and see if there is a difference, you don't even need to know how to read a micrometer that way.

    If there is visible wear on the pistons you found another problem, in a perfect world pistons never ride in contact with the cylinder bore, the rings do.
    Okay back up here. Piston/bore clearance I measured without the rings on the piston. After that, I have a new set of piston rings, which I placed in the honed bore (compression ring) and measured the ring end gap. The ring end gap was exactly spec. I measured the ring end gap at 3 different heights in both bores and the reading was the same for all sites. I shined a flashlight under the rings and there was no gap between the rings outside edge and bore.

    This means the cylinders are serviceable, correct?

  8. #237
    Senior Member TrialsRider's Avatar
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    Certainly sounds like you have a good tolerance fit between your rings, piston and the cylinder bore, that is a good thing,
    but if I was going to measure the cylinder bore, I would be inclined to just measure the cylinder bore.


  9. #238
    Senior Member Cyorg's Avatar
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    Quote Originally Posted by TrialsRider View Post
    ID

    in a perfect world pistons never ride in contact with the cylinder bore, the rings do.
    ????? A world without offset pins and flame fronts moving across pistons etc..... or I guess you could just add more rings.

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    "Non urinat in ventum"

  10. #239
    Senior Member TrialsRider's Avatar
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    Quote Originally Posted by Cyorg View Post
    ????? A world without offset pins and flame fronts moving across pistons etc..... or I guess you could just add more rings.
    No like this:

  11. #240
    Senior Member Cyorg's Avatar
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    Short skirt. #streetwalker. Gotta wonder about longevity on that one... Don’t think my long stroke air cooled thumper would survive with one of those given the clearances I need to run. Speaking of pushing the limit.

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    "Non urinat in ventum"

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