Spark plug question

oldhemiguy

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Thank you guys for the help always my car I bought not knowing nothing about has ngk fr4 plugs in it 360 1973 1979 heads they are 5/8 plugs can you just change plugs from 5/8 from 13/16 plugs in it now are fr4 supposed to be gr4 or anybody want to tell me what to use thanks again for help haven't had a small block since 74
 

Bewy

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Unless the engine is worn &/or burning oil, the correct heat range is 5, not 4.
 

cudamark

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If the question is whether you can use either 5/8 plugs or 13/16 plugs.......the answer is yes, as long as you choose one with the correct length (thread reach), heat range, and sealing configuration. If you pull out incorrect short reach plugs, I would run a thread chaser tap into the hole first to clean out any carbon build up before installing the correct long reach plugs. Otherwise, they may not seat all the way in and work themselves loose, or, they might jam so tight they might be very hard to remove at a later date.
 

Bewy

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Not familiar with those heads, but with some, going to the bigger hex might make it difficult to use a socket for tightening because of clearance problems.
 
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