I use the Nordlock 3/8" washers on my engine in combination with remflex gaskets, hooker headers and Eddy heads. No header bolts coming loose and no leaks after five years.

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1970 Plymouth Cuda, 512cid on 230 400 block, 5 speed manual.(TKO 600)
1970 Dodge Charger 500, 446cid, 5speed manual.(TKO 600)
1969 Plymouth Roadrunner, 383cid, 5speed manual (Tremec TKX)