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Sparking plug washers.

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  • #16
    Sparking plug washers.

    Mick Broom wrote: Hi Peter,

    I did say everything else being equal meaning the only thing that has changed is less power. I consider if its producing less power it may be expected that its producing less heat and the components are under less load which in my opinion should result in longer life.

    The torque curves I was referring to is the ones issued by Rotax for both the single and twin spark engine of 503. To be honest I view both just a guide at best.

    I was making the point that I thought moving the spark position down by 1mm would make no material difference to the engine , what is your opinion on this?

    Sorry if I was not clear in my statements.
    Hi Mick and apologies to Tom- I appreciate we are drifting off topic and this aspect of the discussion is somewhat academic.

    As an example to illustrate my point: If an engine has it's ignition retarded, it may consume the same quantity of fuel but make less power. The fuel is still burned and its energy released as heat. Less of the heat is converted to power at the crankshaft (i.e. absorbed into the expansion of gas) so it must leave the engine via the exhaust and the cooling system (higher EGT and burnt piston!).
    Poor ignition may well lead to slow combustion, giving effects similar to retarded ignition.

    With regard to spark plug position; I would not be concerned about the small change in compression ratio. The ideal location for the spark would be at the centre of the combustion chamber volume. Realistically, it has to be located where there is metal to mount it. Moving the spark further from the centre of the chamber leads to slower combustion. I could not comment on the extent of that difference in this engine.

    Back to the original question: The plug seal washer, if it were retained, would only seal to the CHT washer. The CHT washer would still need to seal to the head. If the CHT washer is not soft enough to create a seal to the head, then you have a leak. If it does create a seal at the head, it would also be expected to create a seal to the plug.
    It would be logical to remove the seal washer when using a CHT washer.
    Pete T.

    "A closed mouth gathers no feet".

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    • #17
      Sparking plug washers.

      Indeed, Peter. It is not about compression but about optimising the flame front. That is why I want the gap in the right place. My cht washers are aluminium. I think I shall remove the crush washers.

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      • #18
        Sparking plug washers.

        I machined copper CHT washers* that were 1.6mm thick, so removing the fitted plug washer was essential. The rather crude CHT sensors that are thin plate with the thermocouple crushed in a projecting tab aren't so accurate, depending where the tab ends up in the cross head cooling air flow. Biggest issue with a plug not precisely filling the head thread is build up of carbon on either the protruding plug, or the unfilled head threads. In either case you run the risk of damaging threads in the future. On a Rotax two-stroke, there's no chance of pistons touching a slightly protruding plug.
        Dave
        *I also added a stainless pin projecting out from the washer. This sits between cooling fins and stops the washer rotating with the plug when fitting or removing and damaging the thermocouple wires.

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        • #19
          Sparking plug washers.

          Thanks Dave.

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