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Engine compression

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  • Engine compression

    I have recently almost completed a restoration on my 1962 MCS with the 34A square barrel engine. New bearings have been fitted to the engine crankshaft & the cylinder has been rebored. In trying to start the engine the compression appears to be too high to produce a good sustained flowing revolution from the kick starter (or are my ageing legs lacking in muscle power!) Would it be sensible to fit a head gasket to reduce the compression even though the sleeve barrel does protrude slightly from the barrel? Has a head gasket ever been made for this engine?

  • #2
    I am surprised that those more knowledgeable than me have not replied yet, so for what it is worth, I suspect the compression is not the problem, or it would be on all of them. I can only think that the bore is very tight. What skirt to bore clearance is there? Have the rings been correctly gapped? How does it feel when you crank it with the plug out? Presumably everything is free in the gearbox, and the primary chain is not too tight.

    There are different opinions on whether bores should be well oiled before initial start-up after a rebore, or left dry, to help bedding in. Assuming all is correct as above, and if the problem appears to be a tight bore, I would go for thoroughly oiling the bores, and cranking it over repeatedly with the fuel off and the plug out, to see if it loosens up a bit. Having said that, if it is tighter than recommended, then honing to restore correct clearances would be the answer. Depends on what degree it is too tight.

    Hopefully these remarks may cause sufficient controversy to prompt others to contribute, so that you may get to the bottom of the problem!

    Good luck!



    • #3

      Difficult to add to Ian's advice. A few squirts of oil in the bore will do no harm, likewise removing the head and kicking it over May give some indication of the problem. Good luck


      • #4
        I think its may be more of "my ageing legs lacking in muscle power!"
        The high compression does not help & kick starter lever needs a positive, hefty quick kick. Back in the day most riders bump started the bike before coming to the starting line.
        Fitting a head gasket will do no harm & in fact you may need more that one to have much effect. Unless you are planning to use the bike in scrambles, it would make for easier starting.
        Last edited by John Wakefield; 16/12/2016, 09:25 PM.


        • #5

          My 1961 MCS engine has been rebuilt and has very good compression as you would expect from an engine built for racing and winning most of the top scrambles in its day when in the right hands, it will probably get a bit easier after running it for a bit.


          • #6

            Hi David, the main trouble I had with starting a Hawkstone was lack of flywheel effect caused by having an aftermarket ignition system (Motoplat) with a much lighter flywheel. I managed to add a small flywheel to the engine sprocket which helped but not as easy as with the standard villiers brass flywheel. What flywheel are you using? The kickstart hitting the footrest also restricts your kick...John P