Timo,
I was very interested to learn that two different reaches of plugs were needed for a twin plugged head, and I agree with you that it would be far too easy to wreck an engine by putting the wrong plugs in the wrong holes.
So, what I'm wondering is- would it be possible to use an insert (I'm not talking about the Heli-coil type of insert here) with an extended thread length to acommodate the longer-reach plug. The extra length would be on the outside of the cylinder head, naturally. I had an engine (not a bike engine) that used a phosphor bronze insert in the alloy head for the plug and these gave no problems at all, so I wouldn't expect any problems on a bike engine. I would imagine heat transfer from the plug to the head would not be a problem, phosphor bronze being a better conductor than the stainless steel Heli-coil inserts commonly used.
That make sense?
Regards,
John
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Johnef
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