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Thruxton piston to head relationship

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  • Thruxton piston to head relationship

    question on Thruxton piston - If thruxton piston is the same as a venom item how does it all work out?
    The Thruxton cylinder is about 1/8" shorter than a Venom, the Thruxton pushrods shorter by the same amount.
    I believe the conrod is the same length or is this not the case?
    Or does the extra 1/8 of piston fit into a deeper combustion chamber on the Thruxton.?

  • #2
    Interesting question. The biggest difference with the TX is the cylinder head which is very different, with different valve angles and sizes. Possibly it, as you say, has a greater volume which accounts for the shorter barrel. It's higher, 9.0:1 compression ratio too isn't it? (I think my Clubman is 8.75:1)
    All will be revealed... As I'm visiting Guru Norm tomorrow for cakes and ale and all things Velo, so I'll see what he knows about it... stay tuned (pun intended!)

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    • #3
      Consensus seems to be that the shorter barrel is because of the higher compression ratio plus more volume in the Thruxton head compression space.
      I'd def' do a compression check while the engine is out though (if it is out).

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      • #4
        I know the inlet valve angle was inclined a couple of degrees to give the 2" inlet some additional room away from the exhaust valve. However, no mention of a larger combustion chamber?
        I will try and find out if the standard Thruxton head was machined deeper. I know some had a squish combustion chamber like the manx and they were certainly machined deeper..
        The other oddity is that the 8.75 : Comp Ratio on the Clubman velo was achieved with standard piston and cylinder just by making minor change to the shims under the barrel .
        So you would think that an extra .25 of compression could be achieved by taking a 10 thou shim out.
        Only way to tell will be a measurement from the gasket face to the top of the combustion chamber hemisphere. I can come up with the Venom dimension just need to find someone with a Thruxton in pieces!

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        • #5
          Is your motor 'on the bench?' out of the frame? If so I'd certainly measure the compression ratio. Not that hard to do once you get the hang of it, but it will forever give you certainty.
          Most of these things have been through plenty of previous owners, and who knows what they've been up to.
          I recently measured my KSS, but that's 350, someone cleverer at arithmetic than me might be able to work out the volume difference with barrel shim thicknesses on a 500.
          I'm doubtful if a simple linear measurement as you suggest will help... you need to know A:The swept volume, and B: The compression space at TDC.
          With today's fuels it's pretty important to know your comp ratio eh?
          Last edited by joolstacho; 02-14-2020, 10:59 PM.

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          • #6
            "The other oddity is that the 8.75 : Comp Ratio on the Clubman velo was achieved with standard piston and cylinder just by making minor change to the shims under the barrel ."
            No, the 8.75:1 ratio on the Clubman was achieved using a great big high domed piston as opposed to the "standard" MSS flat top piston. -Ya want pictures?

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            • #7
              sorry Jools, we appear to have a misunderstanding here , I just wanted to get a simple dimension to see if the Thruxton head was indeed machined deeper.
              Secondly when I talked about shimming to achieve differing comp. ratio I was comparing the stock venom ratio of 8:1 to the clubman 8.75 which I thought was achieved with the same lumpy piston -not the slightly curved MSS,

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              • #8
                Pardon my ignorance , why does the motor need to be on the bench to to carry out the volume measurement ?.... I would like to establish what the CR is on my Venom motor .... it has a domed piston, no plates under the barrel. My motor is loosely bolted into the frame , so, if needs be, I can lift it out .... preferring not to if possible !.

                No response needed .... I have found an article that clearly deals with the subject.
                Last edited by LImeyrider; 02-15-2020, 08:19 PM.

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                • #9
                  Edited my question.

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