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An inexpensive substitute for the hard-to-get Opel locking rocker arm nuts is the locknut from a 2.8 litre, 60 degree Chevy V6. They're the correct metric thread, 10 mm x 1.00, and they are more readily available, cheaper by far, and they seem to hold their locking capability longer. I've run them over 60,000 miles with ZERO adjustments.
One word of warning, they do not have a base flange like the Opel nuts, so if you have a reground performance cam with a smaller base circle than standard, they MAY bottom out on the shank of the rocker stud. If this is the case, use 7/16" HARDENED flat washers to shim the lock nuts up (above the rocker pivot ball). Do NOT use regular washers, they will get destroyed and allow metal into your engine!
Bob Legere
One word of warning, they do not have a base flange like the Opel nuts, so if you have a reground performance cam with a smaller base circle than standard, they MAY bottom out on the shank of the rocker stud. If this is the case, use 7/16" HARDENED flat washers to shim the lock nuts up (above the rocker pivot ball). Do NOT use regular washers, they will get destroyed and allow metal into your engine!
Bob Legere