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Norton Big 4 query...

WD Big 4s used an 18mm spark plug. I would imagine post war variants changed to a 14mm type.
Did this occur while the Big 4 still used a cast iron cylinder head? And if so can anyone tell me the champion plug grade used?....thanks....Ian

email (option): julie@wright52.plus.com

Re: Norton Big 4 query...

Hi Ian, on Robs 16H site he lists 18mm Champion D6 K7 & K8 for the 16H, the number is presumable something to do with heat range? The wartime spares list's show the 16H and Big4 to use the same plug.

But even if the postwar big4 did go down to a 14mm plug, the engine also changed to a shorter stroke 595cc so the plug could be different in any case??

email (option): robmiller11@yahoo.co.uk

Re: Norton Big 4 query...

Hi Rob, Stroke wouldn't affect the plug I shouldn't think..they are generally specified by heat range, thread length etc.
Later Big 4s had an alloy head I think and that may well have lead to a plug grade change (and possibly the thread length)
I don't have an 18mm plug but judging by the thread in the head these had a fairly long reach when compared to the short reach type used in iron head M20s.
Norton seemed to be very reluctant to change anything much but I am guessing they must have changed to a 14mm plug after the war.
So, to summarise...The question is was there a post war Big 4, with an iron head..and if so did it use a long reach or short reach plug....also what was the plug number?.
Finally, what was the length of the thread on a wartime 18mm plug?....Bloody Nortons! ...Ian

email (option): julie@wright52.plus.com

Re: Norton Big 4 query...

Ian

All of the following is from Roy Bacon's Norton Singles
-In 1948 the side valve models went to an alloy head and 14mm plug. That is the same year the Big 4 went to reduced stroke/capacity.
-He records pre and post war models as 4.5 to 1 comp ratio but WD models as 4.8 to 1.
-The WD Big 4 differed from its civilian predecessor in a combustion chamber redesign - incl reduced clearance (squish) over the piston for more power. The squish design reportedly continued post war.
-I cant see a specific plug reference.
-No other changes in heads are recorded, although i have 2 16H iron heads and one is much thicker/heavier than the other - the timing plug over the piston is flush with the inside face on one and the same plug is a good 1/4" clear on the other.
I have also checked E M Franks Norton Maintenance, he is specific on prewar spark plugs but nothing on post war, other than confirming the dates above.
Hopefully that's of some help.

Iain

Re: Norton Big 4 query...

Ian on a 16h it's a 1/2 inch reach...

email (option): jennycook163@hotmail.com

Re: Norton Big 4 query...

Hi Jenny/Iain..thanks for the info....Ian

email (option): julie@wright52.plus.com

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