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  1. #1
    Junior Member Awger's Avatar
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    78 CB400T -- It's dead, Jim

    I recently picked up a 1978 CB400T2 ... in really good shape, except for this gaping hole in the left piston.

    I was in the process of cleaning up the bike and going through all the normal maintenance items in the shop manual (props to all here who provided that essential info) and discovered the problem after a compression check (right side was 200, left side was zero). Obviously, the motor needs to be rebuilt.

    My question is -- should I bother rebuilding (I've heard comments about pistons being difficult to find?) or just start looking for a replacement motor?

    I'm fairly handy with a wrench; I've rebuilt smaller motors (lawnmowers, etc), done top-end rebuilds on cars, and my next door neighbor is a master mechanic -- so I'm not afraid of getting my fingers dirty. It's more a question of throwing good money after bad, and what the most cost-effective approach is for getting this bike in good running order and back on the road.

    Note to self: Next time you feel like buying a motorcycle, bring the boroscope with you.
    Steve

    1978 CB400T ... It's ALIVE - Thanks, Jim!

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  3. #2
    Super Moderator longdistancerider's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    You are faced with a number of options here. Any of the CM/CB 400/450 engines is a bolt in operation pretty much, depending on the actual engine there may be a couple of little things to do.
    Advantages of the 450 besides the obvious 50cc's are the oil cooler, 6 speed trans
    (OD 6th), solid balancers and better cam oiling. Downside is IMO a weaker oiling system.
    Advantage of the 400 is a better basic oiling system. Downside is 3 piece balancers, weaker cam oiling, no oil cooler.
    Might wander thru this topic viewtopic.php?f=58&t=19761&hilit=79+CM400T+with
    There are parts available, just have to look for them. Some sources: http://www.davidsilverspares.com/parts/by-model/
    http://www.servicehondapsn.com/fiche_se ... &mfg=Honda
    and this ebay seller has pistons http://www.ebay.com/itm/HONDA-CB400T-CB ... 1268wt_663
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    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

  4. #3
    Senior Member LashLarue's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    Well, take it out and take it apart. As is, it isn't worth anything anyway. Decide at that point if you want to rebuild or just part it out. Either way, you learn about the motor that is going back in. The only real way to tell what it needs is to open it. Why did it hole the piston? If you don't find out, it might haunt you.

    Getter another engine for a good price is not easy. Patience. A piston and rings is easier to find than an engine.
    Mike

    CT90 (2)
    SL350 - k1 (2)

  5. #4
    Junior Member Awger's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    That's quite a rebuild you did, Jim. Far more ambitious than I'd consider, aside from replacing the balancers with the one-piece units from a 450 (direct swap, no rubber, no brainer). And $52 for a pair of NOS pistons bodes well for finding parts at a reasonable price.

    Sounds like it's time to tear it apart and see how deep the rabbit hole goes (like Mike said, the question of why there's a hole in the piston needs to be answered).

    Thanks for the links, I have a lot of reading to do. And I should probably build a sturdy (er) workbench, as I don't think the table I have now will hold the weight.

    Aside: "It's dead, Jim" was just a quip, and entirely off the cuff; I recognize your avatar from half the posts on this site, but I didn't notice your name was Jim until you replied. Makes for a good plea / entre nous, though.
    Steve

    1978 CB400T ... It's ALIVE - Thanks, Jim!

  6. #5
    Super Moderator longdistancerider's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    I'm more familiar with Spock: "It's life, Jim, but not as we know it."
    The piston damage is really a question mark. I've never heard of this on these engines
    Those pistons are not NOS. They're an aftermarket piece, a few members have used them and I've heard nothing bad to date. Nice thing is they included rings and pins. He sells pistons for other bikes also and reviews have been good.
    In using DSS don't just rely on their listings. For instance they have a crankshaft listed in the CB400N but not in the CB400T even though the part number is identical. You can enter a part number on this site and it'll tell which bikes use it. http://www.partzilla.com/
    Yeah, I got a bit carried away but that engine is in the road bike and I wanted the best reliability I could build into it.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    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

  7. #6
    Junior Member Awger's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    Finally got some time to work on the bike. Judging by the state of things (one dowel missing, one seriously kinked) I'd have to say somebody's been in this engine before.

    Here's what I'm dealing with:

    [attachment=0:1m22xe0d]IMAG0246_sm.jpg[/attachment:1m22xe0d]

    I have the head and cylinder off and the pistons out... what's the best way to split the case? The book says flip the motor, but I can't do that with the cam tensioner sticking out.
    Attached Images Attached Images  
    Steve

    1978 CB400T ... It's ALIVE - Thanks, Jim!

  8. #7
    Super Moderator longdistancerider's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    She'll have to be upside down to get to all of the bolts. 4x4 or 2x4 blocks to support it.
    Interesting piston, doesn't appear to have melted but rather something punched a hole in it. Wonder if the PO did something.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    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

  9. #8
    Junior Member Awger's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    A friend of mine had a similar experience years ago, he says the spark advance got stuck, it ran lean, and then *pop*. I'll be taking a close look at the ignition when it starts going back together.

    So... my neighbor (the mechanic) drops by, and I showed him the clutch nutch, and told him I screwed up by not loosening it before I took the motor out of the frame, and he says, "I've got all the tools, you wanna borrow 'em?" Turns out his castle socket was too small, and I still wound up making one (thanks again for the instructions, Jim!), but those funky vise grips sure did come in handy. The flywheel puller and case splitter will come in handy tomorrow.
    Steve

    1978 CB400T ... It's ALIVE - Thanks, Jim!

  10. #9
    Super Moderator longdistancerider's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    Spark advance is controlled by the CDI and stator, so that is kind of ruled out unless there's something really strange going on with both of them that allowed more than 45* of advance. The picture kind of looks like there was something bouncing around in there but that may have been a piece of piston too.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    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
    Member 80hondaf's Avatar
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    Re: 78 CB400T -- It's dead, Jim

    Hey Steve, I'm not much of a mechanic but what do the valves look like, is it possible that the valve timing jumped and a valve punched the hole? Tim

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