what modern spark plus for 1955 xk140
NGK BP6ES if most driving is countryside. Should you have slower driving or experience fouling, then the next stage hotter plug is the BP5ES. This plug shows being replace by NGK with the BPR6ES…a resistor plug. You do not really need or want the resistance…but it will work if you don’t have modern hi resistance spark plug wires.(should be low or no resistance plug wires). A web search will still find the BP6ES or 5ES…but be wary of prices…some are way too hi.should just be a few dollars each. If your distributor is still points/stock…and you time at 7 or 10 D BTDC, I suggest plug gap at 025 to 028.
You can easily cross check the NGK # to other brands on the web…Bosch WR8DP or Champion…but most seem to feel the NGK has a broader heat range and works the best in the early XK engine.
Does anybody has ever tested/used Iridium plugs? If yes experience results would be appreciated.
iridium plugs will “work”, as in the will fire, the engine will run. The main feature and why they are used is long life. In most new cars the plugs are near impossible to get to, maintenance as to plugs is l00,000 miles. Iridium plugs are a little less conductive than copper. Iridium plugs run hotter…Ok in new cars, not so ok in older cars. Getting the right heat range is difficult with iridium plugs…they are kinda one heat fits all. The gap is not adjustable…and in the XK engine may not be ideal. If 100,000 miles on your plugs is not important…no need to use them. Copper will fire just fine, transfers heat better, you can adjust gap depending on your ignition system. The NGK BP5ES or 6ES has wider heat range than say Champion within one plug. If there is ONE really important thing about spark plugs it is do NOT overtighten. Champion website has a nice graphic that shows how much to tighten…it is not much… Spark Plug Torque Recommendations | Champion Auto Parts (championparts.ca)
Flat seat 14mm: Turn Finger Tight. A new or reinstalled plug with a new gasket will require ⅜-⅝ turn to effect a gas tight seal depending on head material.That is not much…just a few flats. LESS with a used gasket. Nick