Your plugs are 0.705" reach according to Autolite. This is good for an aluminum head. You will need a spark plug repair kit to restore good useable threads. The best thread repair system I have found over the years is the Lock-N-Stitch system. Here is a link to their website =>
Lock-N-Stitch
This is the Ford recommended vendor for all the four-thread plug hole failures they have had on their three and four-valve Mod motor heads — until they switched to a 9 thread head casting. An interesting side note about the Lock-N-Stitch folks is they were the firm that was selected to repair the cracking cast-iron dome on the US Capitol building because they could do it without welding.
I have used them on a pricey set of ported 4V Modular heads and couldn't be happier. I didn't fully realize how robust the repair was until I read their torque specs for the repaired thread. Their instructions call for a 35 ft/lb torque spec for the repaired plug.
Notwithstanding their repair instructions, I do not use anywhere near that torque level on mine. I spin the plug down until it bottoms in the head and give it a light ⅛ of an additional turn to snug them up. That is all that's required. The plugs never loosen in service and come out easily when I change them. If you want an extra measure of removal safety, you can put a
very light coat of anti-seize on the plug threads.
This repair process will save your heads, end the potential for a repeat performance and put a smile on your face.
Pretty amazing guys and stunningly good product.
p.s. The install kit you will want is part # FFT514125LK, which includes five inserts. You will also need another five inserts part # FFT514125L-5 to do all eight plug holes — highly recommended. The kit has the drill, reamer, and install tools yada, yada that you will need to do the job. The equipment and pieces are not inexpensive, but there is none better. You will spend $500 plus shipping and tax, but your plug threads will be better than OEM, and your plugs will show the correct heat signature when you read them. Call their number at the top of the webpage. They are not only very helpful but you can only buy the repair kits directly from them.
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Threaded Mode
