78 cm400t hawk top end rebuild
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Thread: 78 cm400t hawk top end rebuild

  1. #1
    Junior Member colvinry's Avatar
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    78 cm400t hawk top end rebuild

    hey guys, just opened up my motor and in cylinder is pitted from water sitting. I have a few questions. First, will a single overbore take care of the pitting? Where do you recommend getting pistons and rings? Do I need a overbore head gasket?

    Am I going on the right direction or should I just look for a jug on eBay that I can hone? Want to keep this on budget but it's starting to nickel and dime me.

    What are your recommendations?


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  2. #2
    Super Moderator longdistancerider's Avatar
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    Until the cylinder is honed out and measured there is no way of knowing how much of an over bore will be needed. Even then it may have to be bored over some to get a clean surface. Best option is to get another cylinder.
    Your pistons may be ok, again they will need to be inspected and measured to see if they are within spec.
    If the fresh cylinder and pistons check out to be in spec then standard piston rings are available from Honda, best choice.
    If the pistons are bad then you can get used ones and hope they measure out good or there's an eBay seller, cruzinimage, that sells pistons for these models.
    Stock head gasket is suitable for up to a 1mm overbore, .040"
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration

    SALE IS ACTIVE TILL 200 SHIRTS SOLD T-shirt sales supporting 66Sprint's Land Speed Record Build http://www.hondatwins.net/forums/22-...d-attempt.html

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  3. #3
    Junior Member colvinry's Avatar
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    I did end up going with the cruzinimage pistons and rings. Went with the 1mm oversized because the pitting was significant. Do you think I should wait for the pistons to give to the machine shop with the cylinder or go ahead and have it bored while I am waiting? I think there is some risk involved by having it bored without the pistons in case they are OOS.

    Anyway this gives me some time to get the motor cleaned up and paint up the frame, do the electrolysis on the rusty tank and other odds and ends!


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  5. #4
    Super Moderator longdistancerider's Avatar
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    The machine shop won't bore the cylinder until they have the piston in hand.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration

    SALE IS ACTIVE TILL 200 SHIRTS SOLD T-shirt sales supporting 66Sprint's Land Speed Record Build http://www.hondatwins.net/forums/22-...d-attempt.html

    http://www.hondatwins.net/forums/22-...d-attempt.html and http://www.hondatwins.net/forums/1-p...r-attempt.html
    Road Trip http://www.hondatwins.net/forums/11-...here-i-go.html or "where's Jim now?"
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  6. #5
    Junior Member colvinry's Avatar
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    The machine shop is asking me for clearances for the rings and pistons (+1.0mm oversized). Will the clearances be the same as manual? I have an email out to cruzinimage for the clearances but I figure I would ask the site.


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  7. #6
    Senior Member blu3fan's Avatar
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    I feel like something is getting lost in translation here.

    Pistons.
    Go in the juggs/cylinders.
    When your cylinders have gotten worn down from the pistons going up and down (it ovals off instead of being a circle) Or Other damage. like you have.
    The machine shop. Bores * the act of grinding a cylinder until it is perfectely circular again.
    then hones. cleans up the boring work.
    then the end diameter of the hole is measured. Sometimes It can be +.01mm bigger Hence the +.01 sized pistons from stock.

    and you get rings and or pistons to fit the new holes.

    did that clear anything up ?
    cos I read this and didn't understand where you got lost.

    There is no machine shop in the world that can tell you over the phone or anyone on the internet that can say how much have to be bored over.
    It MUST be measured with correct equipment.
    Last edited by blu3fan; 03-20-2017 at 10:46 PM.
    1982 cm450e hardtail conversion.

  8. #7
    Super Moderator longdistancerider's Avatar
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    Use the specs from the FSM for all clearances.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration

    SALE IS ACTIVE TILL 200 SHIRTS SOLD T-shirt sales supporting 66Sprint's Land Speed Record Build http://www.hondatwins.net/forums/22-...d-attempt.html

    http://www.hondatwins.net/forums/22-...d-attempt.html and http://www.hondatwins.net/forums/1-p...r-attempt.html
    Road Trip http://www.hondatwins.net/forums/11-...here-i-go.html or "where's Jim now?"
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  9. #8
    Junior Member colvinry's Avatar
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    Blu3fan

    That is clear. I get you measure the cylinder to see how much you need to take out to true it up. My motor has 6k on it, with some minor pitting from sitting. So I assumed an oversized piston +1mm would take out any of the pitting.

    My line of questions were more about what tolerances the machine shop should allow once the nominal sizes are determined. The bore to piston clearances and ring gap specifically.

    Cruzinimage replied the piston - cylinder gap spec inquiry below:

    "Thank you for your message. regarding clearance.I asked my supplier about clearance.the clearance is 0.015-0.025
    Best regards,
    mitsuo"

    I think he must have messed up a decimal place in the numbers. What do you guys think? I reached out to confirm.

    So back to the FSM. How do you go about determining the piston cylinder clearances?

    This is from page 72. (7-1)
    Cylinder ID standard: 70.50 - 70.51mm

    Cylinder service limit: 70.60

    Piston OD standard: 70.47 - 70.49mm

    Piston OD service limit: 70.40mm





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  10. #9
    Super Moderator longdistancerider's Avatar
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    Measurements seem wrong from Mitsuo
    Tight clearance is .004" and loose is .016" by factory spec. However these are aftermarket pistons which may have a faster/larger growth rate due to different material.
    blu3fan likes this.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration

    SALE IS ACTIVE TILL 200 SHIRTS SOLD T-shirt sales supporting 66Sprint's Land Speed Record Build http://www.hondatwins.net/forums/22-...d-attempt.html

    http://www.hondatwins.net/forums/22-...d-attempt.html and http://www.hondatwins.net/forums/1-p...r-attempt.html
    Road Trip http://www.hondatwins.net/forums/11-...here-i-go.html or "where's Jim now?"
    Member Map http://www.hondatwins.net/forums/dcMembermap.php

  11. #10
    Senior Member blu3fan's Avatar
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    Quote Originally Posted by colvinry View Post
    Blu3fan

    That is clear. I get you measure the cylinder to see how much you need to take out to true it up. My motor has 6k on it, with some minor pitting from sitting. So I assumed an oversized piston +1mm would take out any of the pitting.

    My line of questions were more about what tolerances the machine shop should allow once the nominal sizes are determined. The bore to piston clearances and ring gap specifically.

    Cruzinimage replied the piston - cylinder gap spec inquiry below:

    "Thank you for your message. regarding clearance.I asked my supplier about clearance.the clearance is 0.015-0.025
    Best regards,
    mitsuo"

    I think he must have messed up a decimal place in the numbers. What do you guys think? I reached out to confirm.

    So back to the FSM. How do you go about determining the piston cylinder clearances?

    This is from page 72. (7-1)
    Cylinder ID standard: 70.50 - 70.51mm

    Cylinder service limit: 70.60

    Piston OD standard: 70.47 - 70.49mm

    Piston OD service limit: 70.40mm





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    Thank you.
    Specifics are everything when it comes to this stuff.
    1982 cm450e hardtail conversion.

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