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The factory spark plug for a 1966 B18B Amazon is Bosch W175T1, correct? What number to use if I want to go one step hotter? I have gone to internet guides for comparisons but am not confident that I am reading these correctly.
 

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Don’t know how it compares to the original spec plug, but I’ve been satisfied with NGK BP6 HS.
 

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I think the W..... numbering system from Bosch is no longer in use. So, I think you would have to find a Bosch application guide and cross reference it to find the current Bosch part number. In the current 1 or 2 digit Bosch numbering system, the middle number is the heat range and in the current system the plug gets hotter with a higher number.

The Bosch part number you listed cross references to the 6 heat range in NGK with the BP6 HS or BPR6 HS (if you want resistor) being the kind of go-to plug for vintage B18, B20 engines. BP5 would be hotter and BP7 would be colder. As an observation, I would be hesitant about going to BP5 unless you are doing strictly in town driving. If you are getting some plug build up, consider the NGK BPR6hix iridium. Safe 6 heat range and the fine wire electrode makes them much more resistant to fouling and build up than conventional plugs. They are a ridiculous price from your local auto store; but, if you order them from RockAuto or the likes they are less than $6 each. I don't drive my 142 B20E a huge amount; but, I have been running the same set of BPR6HIX in the engine for either 4 or 5 years now without issue.
 
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