NGK Spark Plugs

[Chevy454]

They both used 14mm threads...in fact, if it's an original carb'd sbc/bbc then it's gonna be 14mm threads...the seat design, thread reach, hex size, etc, may be different, but the threads are gonna be the same. If the 5671s don't fit, I'm betting the GR5s won't either...

As for the switchover from the gasket seat to tapered seat design, iirc it was in May...I'll have to check my notes to see exactly which week, but we have a set of 840s that share the 1.88 exhaust valve casting size but have the gasketed plug seat...and I've heard of 291s that are just the opposite.