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Thread: Bent pushrod

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    Bent pushrod

    After a major engine rebuild (mostly done by professional machine shop), the engine started and ran beautifully for about 100 miles.
    Suddenly things went south - diagnosis.. a bent pushrod on cylinder #2 exhaust valve.
    Most obvious cause would be a stuck valve but that does not appear to be the problem. No other cause that I can think of??
    Installed a new pushrod (from Moss) and, again, the engine is running great, about 100 miles so far. I'm keeping my fingers crossed, hoping it was a freak event.
    However, would very much appreciate any input.
    The ignition timing was quite retarded (my fault) and an old timer at a local repair shop mentioned he vaguely remembers this sometimes causing damage to a pushrod on older engines.
    Again, any input very welcome.
    Thanks

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    Re: Bent pushrod

    Anything different from stock...shaved heads, bronze guides, high-lift cam, new valve guides, nylon valve seals, etc?
    John

    1955 TR2

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    Re: Bent pushrod

    Usually a stiff guide. Often ends up with a bent valve.
    Do a compression test.

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    Re: Bent pushrod

    Bronze guides with insufficient clearance would be at the top of my suspect list.
    Berry

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    Re: Bent pushrod

    Adding to that - the offending guide may not bind until high enough engine heat occurs.

    As I recall, depending on the choice of material for the guides - they may need to be reamed to a specific tolerance to allow for heat.

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    Re: Bent pushrod

    Hi John.
    All standard stuff. Parts from Moss.. valves, guides, pistons, tappets, camshaft etc.

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    Re: Bent pushrod

    Did compression test after fitting new pushrod and all cylinders good at around 145-150

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    Re: Bent pushrod

    oh and I should have said, the machine shop guy did not install new pushrods which, in hindsight, seems daft!

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    Re: Bent pushrod

    I have to assume and hope!!! the guides etc were checked for clearances. I used an established/reputable machine shop with lots of experience on old engines of all types.
    I'll put on some miles/hours over the next few weeks and report back. Thanks.

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    Re: Bent pushrod

    Quote Originally Posted by chassutton View Post
    Hi John.
    All standard stuff. Parts from Moss.. valves, guides...
    Moss sells guides in both cast iron and manganese-bronze. Which did you use?

    As I recall, the clearances are different (due to differing expansion) and some (possibly the manganese-bronze) may have to be reamed to work properly.

    Perhaps someone with a better memory or more recent experience can comment.

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    Re: Bent pushrod

    There are 2 different types of pushrod diameters out there; the 5/16 pushrod that are stock and older and maybe did it all time--- BEND. They also have 3/8 pushrods; they are beefy and do not bend; we used to talk more about it.
    Pease out
    steve

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    Re: Bent pushrod

    I was fortunate that the bronze guide seizure became apparent as soon as the engine reached operating temp after rebuild. It involved 2 exhaust valves and was caught before the push rods were damaged.
    Berry

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    Re: Bent pushrod

    I remember doing LBC's in shops I ran years ago...and guide sizing was a big issue. Machine shop that knows old bow-tie engines hasn't a clue on how to set up an LBC head.

    We would have to search (and try) machine shops and machinists who were skilled to do the work and size guides correctly.

    Then you'd get the FBN (fly-by-night) hairballs who sleeved the guides..

    I remember doing heads myself where guides were just a bit loose and knurling guides, sizing...surprisingly those worked really good if cast guides.

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    Re: Bent pushrod

    Coming from Chevy's that amazed me too. On Chevy you can run the bronze guides very tight with no problems. The valves in the Triumph must get much hotter than the Chevy's.
    John

    1955 TR2

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    Re: Bent pushrod

    On bow-ties they are tight...but they don't shrink up when hot. Have to be tight when all they depend on is a square cut o-ring in the spring retainer for oil control.

    Seen so many LBC issues like that. Usually the valve hung and hit the piston. Not sure I would be happy with removing the built-in safety of a skinny pushrod over a thick one that would not bend.

    Of course, my engine of choice never bends a pushrod and cannot have piston-valve interference unless a valve head comes adrift.

    And no seals, since oil runs uphill harder than down.

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    Re: Bent pushrod

    Quote Originally Posted by titanic View Post
    I was fortunate that the bronze guide seizure became apparent as soon as the engine reached operating temp after rebuild. It involved 2 exhaust valves and was caught before the push rods were damaged.
    Berry
    Out of curiosity, what were the symptoms that had you shut the engine down when the valves tightened up in the guides?

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    Re: Bent pushrod

    The engine started to run rough as the temp increased. With the valve cover removed, I could see the valves sticking. I removed the head, disassembled it, and could see traces of bronze on the valve stems. The machine shop appologized, re-reamed the guides, and no further problems.
    Berry

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    Re: Bent pushrod

    Sounds like your valve train is binding. Find someone that knows what they are doing and have them check it. Poolboy had this happen with his after a rebuild. Turned out to be spring bind if I remember right.
    Tom
    76 TR6
    74 Midget
    www.madbrits.org/
    www.mg3club.org/

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    Re: Bent pushrod

    The car's PO built that engine, Tom.
    The cam and tappets got wiped out.
    The problem must have been a combination of valve springs, cam selection, and rocker arm ratio...anyway it ran like that for 20,000 miles....
    The same head, the same valve guides, the same seats, the same push rods and most of the same valves are in the current version of that engine, going on 30,000 miles.
    DRIVE 'EM IF YOU GOT 'EM

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    Re: Bent pushrod

    Hi Ken, I couldn't remember all the details and hoped you would weigh-in.
    Tom
    76 TR6
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