There are defect cases that will cause it too. Sometimes in large numbers,
like the modular ford DOHC V motors. Large numbers of problems. But I think
you're right, usually it's under torquing that does it otherwise. I
understand it can also happen on AL heads if sparkplugs were ever removed
hot and caused damage to the threads without actually stripping them.

Jim Akiba

On 7/31/07, Scott Daniel - Shazam <scottdaniel@turbovans.com> wrote:
>
> Well guys,
> I don't know the precise circumstance of each car and cylinder that had a
> spark plug blow out,
> But.....I've seen about a hundred times where people that don't work on
> cars
> professionally and full time ..........i.e. backyard mechanics.......that
> they didn't screw the plugs in tight enough.
>
> I've seen this a *lot.* and imagine , or suspect anyway, that 'not tight
> enough' was a factor in most of these cases, if not all of them.
> Scott
> www.turbovans.com
>
>