Standard exhaust port shape
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  1. #1
    Senior Member Erwin's Avatar
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    Standard exhaust port shape

    I have this NOS head that I want to start using and yesterday I looked at the exhaust ports and noticed the shape. Are they always so small compared to the exhaust and exhaust gasket? Could be that I am just used to seeing them covered in soot.
    Standard exhaust port shape-kop2.jpgStandard exhaust port shape-kop3.jpg

    BTW: Judging from the combustion chamber it is not a 250 head
    Standard exhaust port shape-kop1.jpg
    Last edited by Erwin; 05-10-2019 at 11:38 PM.
    1972 CB350K3 General Export | 1976 CB200T K0

  2. #2
    Senior Member oupa's Avatar
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    Very nice score!

    The basic answer is, "Yes."
    "They're not like cars with a body that rusts out in a few years. Keep them tuned and overhauled and they'll last as long as you do. Probably longer." - Robert Pirsig

  3. #3
    Senior Member TOOLS1's Avatar
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    What he said^^^^^^^^^^^^^^^^^^^
    TOOLS

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  5. #4
    Senior Member ancientdad's Avatar
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    Is it possible it's an SL350K1 head? they had smaller ports and a more midrange grunt cam
    Tom

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  6. #5
    Senior Member Erwin's Avatar
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    According to CMSNL a CB head is the same as a CL head.

    https://www.cmsnl.com/honda-cb350k4-...r_12200287100/

    According to their site it fits:

    CB350K4 EUROPEAN DIRECT SALES
    CB350K4 FRANCE
    CB350K4 GENERAL EXPORT / KMH
    CB350K4 GERMANY
    CB350K4 ITALY
    CB350K4 SUPER SPORT 1972 USA
    CB350P GENERAL EXPORT
    CB350P0 GENERAL EXPORT
    CB350P4 POLICE TYPE GENERAL EXPORT / KPH MPH
    CL350 SCRAMBLER 1968 K0 USA
    CL350 SCRAMBLER 1970 K2 USA
    CL350 SCRAMBLER 1971 K3 USA
    CL350 SCRAMBLER 1972 K4 USA
    CL350 SCRAMBLER 1973 K5 USA
    CL350 SCRAMBLER K2 GENERAL EXPORT
    CL350 SCRAMBLER K3 GENERAL EXPORT
    CL350 SCRAMBLER K4 AUSTRALIA
    CL350 SCRAMBLER K4 GENERAL EXPORT / KMH
    CL350K3
    CL350K3 JAPAN
    SL350 MOTOSPORT 1969 K0 USA

    If someone could take a picture of a standard exhaust port then that would be very helpful. I have some extra heads but I think that most (if not all) are modified. I will do some digging in the stash, later today.
    1972 CB350K3 General Export | 1976 CB200T K0

  7. #6
    Senior Member Erwin's Avatar
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    So I looked at all the heads in my stash and I was able to find two heads with unaltered channels. I compared them to my NOS head and the channels on that actually do seem a little but smaller.
    Standard exhaust port shape-img_20190511_151441.jpg
    To compare the surface area of the channels I used an exhaust valve to measure.
    On the used head the exit of the channel is basically the same size as the valve. I can just fit the exhaust valve in the channel but it is a tight fit and I can't slide it far into the channel. Only one millimeter or so. On the NOS head the valve does not fit into the channel at all.

    Standard exhaust port shape-img_20190511_151458.jpg
    So my NOS head indeed has smaller exhaust channels. Towards the valve they look the same so I modifying them will be easy. Inlet channels also look the same. I still need to figure out if I need to enlarge them or if then can stay like this. I am building a tuned engine and from what information I have found, the consensus is that enlarging the outlet channels on a CB350k is not needed and actually does not help. I found some nice information about tuning these engines from thrustworthy sources (succesfull and well known racers) on this page: Honda CB350 Road Racer FAQ, Engine Modifications Page
    1972 CB350K3 General Export | 1976 CB200T K0

  8. #7
    Senior Member BobbyPlayhouse's Avatar
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    Thanks for that link. Odd that they would be differently sized. I wonder if there are differences between years of production; for example, isn't the duration/lift of the cam slightly different between 68/69 and 70-73? That may be a false memory...
    2x 1973 CB350G

  9. #8
    Senior Member Erwin's Avatar
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    That is correct. For the CB/CL 350 there are basically two styles of cams: Early style fat cam design and late style skinny cam design. Really early versions of the fat cam design have a hotter profile but only the really early ones. I think K0 but maybe not even all K0 bikes. Most fat cams have the same timing as late style skinny cams.

    All the heads that I have have different casting ID's and numbers on them. I guess they used quite a number of different templates.
    1972 CB350K3 General Export | 1976 CB200T K0

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