Heads have a steel insert  cast in to accept the 18mm spark plug. Combustion chamber is 10.5mm deep.
FITMENT
 1940 / 1941 WLDR  1940 / 1942 WLD  1947 / 1957 WL  ACCESSORY ITEMS
49-094549-10199705-18-4515-0185
          
        
      
      
        
      
      
          
        
     
   
 
  
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