A short while ago, on one of the Norton groups, someone referred to 16H head casting numbers and spark plug reach. I had reason to look in a 1948 Lodge sparking plug application list today (kindly scanned for me by John O'Brien) and it has thrown up something that I don't quite understand. The recommended plug for the WD16H was initially the mica KLG KS5 and later the 'Corundite' M80. Both of these are 1/2" reach. During the 1930s, Lodge had recommended the 1/2" reach H1 which was a fairly cold plug recommended for motorcycles. However, for 1948, Lodge recommended the slightly hotter 1/2" reach C3 for "Side valve Except 16H" (that only leaves the Big 4 !) and then the 3/4" reach CB3 for "16H (Ex WD)"..does that mean 'Excluding WD' or 'Ex-WD' ?) Is there a hint here that some or all WD16Hs needed a long reach plug....but if so, why did the preferred War Office and Norton supplier, KLG not specify their long reach KS6 ? The more that I learn, the less I know.
Unfortunately; most of my pile of rusty 16H heads no longer has legible casting numbers. I shall dig them out and measure the reach though.
I've always thought it strange that all the Indian Motorcycles that I know about were fitted with a 3/8" reach plug, which doesn't reach the bottom of the thread. I've always fitted 1/2" reach to my 741's. Ron
I understood that having the plug recessed into the head would retard the ignition process....Not by changing the timing of the spark but rather the rate of combustion, with similar results....Normally it wouldn't be recommended...However, if a bike was was built like that I assume fitting a longer plug would have the opposite effect...Ian