Ok to use spark plugs that are not all the way through the threads in the head???

[billlsbird]

......asked a question yesterday r/e "what plugs should I use?" {I ones in there are bad & special order. Autolite # AR 3911}. combo is 413 small block Chev, 230/236 @ .050 cam, 750 Double pumper, air gap manafold, 10 to 1 compression.... Eric E. recommended the AC 44 or 45's. I got them this morning but they are 3/8" shorter than the plugs that were in their. Yesterday I FORGOT to say that I have AFR aluminum heads. May be aluminum heads have more threads than steel heads??? Anyway the plugs that I got today are so short that the electroid would just barely be inside the cylinder. Is this ok until I get the special order plugs??? If so great, If not can anyone recommened a plug that would work thats longer??? THANKS Everyone, your all life savers Bill

[brianrupnow]

A theory---aluminum is not as good in a shear situation as steel is, so the sparkplugs need a greater amount of thread engagement to prevent compression from blowing the sparkplugs out of the holes and shearing all the threads out of the sparkplug holes. I'm not sure that is correct, but if it was my engine, I wouldn't be running short-thread sparkplugs in it.

[Dave Severson]

You're going to have to use the long reach plugs. A plug firing that far up in the plug hole is not going to give a decent burn. I don't know my chebbie plug numbers at all, hopefully one of the guys can supply you with the correct number for the long reach plug with the right heat range.

[kitz]

If you have even half the threads engaged the load capacity is there. But that won't matter; as Dave says you won't have as much load anyhow as you won't get a good burn at all with the spark shrouded like that.

I would call AFR and get a recommendation or look at some modern performance factory SBC aluminum head/engine spark plug #'s for a start ..........

Kitz

[billlsbird]

.... Thanks everyone, I tried the shorter plugs & it runs ok BUT a new problem surfaced. Somethings wrong with my carb. I will start another thread..... THANKS again, Bill
ps, oh tomorrow I will call the guy who built the engine & ask him for a suitable replacement....

[firebird77clone]

the plugs will run fine, albeit you'll loose a tad of power because of their recessed location. The real problem is that one day you'll try to run in the right plugs, and the threads will be carbon plugged. A plug hole thread chaser will fix it up, but better to avoid it all together.

[billlsbird]

the plugs will run fine, albeit you'll loose a tad of power because of their recessed location. The real problem is that one day you'll try to run in the right plugs, and the threads will be carbon plugged. A plug hole thread chaser will fix it up, but better to avoid it all together.

.....ah ha, yeah it will gum up the threads with carbon! Thanks .... Bill