Fully threaded spark plugs

[Jeffrey S.]

Hello All!
Can any body tell me why aluminum heads call for a fully threaded plug as opposed to an iron head that has half threaded plugs- 43XL vs. 43N on big block Corvettes for example?
Jeff

[Edward J.]

Re: Fully threaded spark plugs

Jeff just a guess but I would think that aluminum head maybe a bit easier to pull the threads out, cast iron not so much.

[Jerry W.]

Re: Fully threaded spark plugs

If i remember correctly the rule of thumb for alum. thread engagement is 1-1/2 times diameter

[Duke W.]

Re: Fully threaded spark plugs

Jeff just a guess but I would think that aluminum head maybe a bit easier to pull the threads out, cast iron not so much.

Correct! Cast aluminum has lower yield and ultimate tensile strength than cast grey iron, which is why Al heads from any manufacturer always have about double the length of spark plug thread compared to typical cast iron heads in order to reduce unit stress on the aluminum threads.

Duke

[Jeffrey S.]

Re: Fully threaded spark plugs

Thanks all for your responses. The strength issue was my first guess along with possibly a heat dissipation issue requiring more thread contact with the head to better dissipate heat through aluminum than through steel. I'm no engineer so these were just guesses and I was hoping some real experts would chime in. Thanks!
Jeff

[Wayne M.]

Re: Fully threaded spark plugs

Here's another example of the lower yield strength of aluminum versus iron:

Tech Service Bulletin 67-T-53, Sept 7, 1967: (in response to rocker arm ball stud pulling out of threaded hole, in both iron and aluminum big block heads)

"396 and 427 cu. in. engines with aluminum cylinder heads, built after May 22, 1967, incorporate helical inserts in the rocker arm stud holes"