Enginner...
Having never cracked a plug yet ...this makes me think about what a mechanic buddy said not to far back about what I was using for a socket and ratchet for installation.

I assume you are using a standard plug socket with the rubber boot inside and ratchet.??

How are the angles when you install? Are you supporting the ratchet and not over torquing the plugs? Quick reference on tightness is gently snug the plug then touch the ratchet a little..in other words just past snug.

if the angles are off an you aren't supporting the head of the ratchet..it'* possible that may be your cause of the cracking. If there'* no rubber boot...etc etc. There are many things that can contribute to a cracking. I'm not quite on board that Autolites don't work in L36'* yet. I'd like to see more people come forward wtih issues on them.

Any pictures of these cracked plugs?

And in reference to the quality of Autolites, when I was a tech in St Cloud, we'd always ask for OE quality plugs from our parts supplier when doing a tune-up. 75% of the time, we'd recieve Autolites. There were the occasional comebacks due to a failed plug, but we're talking maybe a total of two over two years, if even that many.

I have been using a spark plug socket with the rubber boot inside them and I am very gentle about installing them and keeping them supported to avoid placing any stress on the insulator. I finger tighten them and then snug them up with a ratchet. I only go a hair past finger tight using steady, not jarring, pressure with the ratchet to avoid cracking them.

Normally, I would blame installation error as well, but not when the cracks are obviously recent, as is clearly the case on the most recent plug, and the plugs have been in there for 30k miles and 2 years. If this happened during installation, they would not have waited this long to fail. That and I have installed plugs in numerous engines in my life without ever cracking a plug.

I'm not saying Autolite makes bad plugs, but something is causing this to happen, unless I just got a bad set that took a while to fail. I am still running Autolites in the Regal with no complaints and the only significant difference is that the Regal runs much cooler than the Bonneville. My thinking is that Autolites don't like higher temperatures which is reinforced by these problems frequently cropping up while accelerating up a hill when the engine is running at its hottest. If I have similar problems with the NGKs, I guess I will know its a problem with the motor, not the plugs. But until I know for sure, I will not recommend Autolite for the L36.

I threw the plugs in the trash last night, but I could pull them out for a photoshoot if you like. I only have 2 of the 3 since the first one was taken care of quite some time ago and disposed of.

I figured you'd be using the right procedure.

Which autolite plug number are you using. I know on some of the SC'* we are going to a range cooler plug. Although that in itself should still not crack them. Maybe a bad batch..but there'* too many in your situation that are doing it.

It'* an odd one for sure.

I was running the stock equivalent, the Autolite 606. Before I was informed of the NGKs, I was going to replace the 606s with 605s, which is what I am currently running in the Regal. I also had good luck the Autolite Platinum 104 in the Regal, although I have noticed a reduction in KR in that motor since going to a slightly hotter but copper plug. I went with the AP104 originally hoping the cooler nature of the plug would cancel out the poor heat disipation properties of the platinum while still offering longer life. They were an improvement from stock, but the copper 605s are better still.

One thing I did notice yesterday was that the front plug wire boots do not have heat shields. The rears do. Is this normal or did someone forget to put them back at some point in the history of this car? That doesn't explain this problem since 2 of the 3 cracked plugs were in the back with the shields, but it may explain the bad wires I found up front.

The shields are only on the back 3 on all the cars I have seen.

What are the temperatures that your engine is running? Do you still use the injector site shield?

On hills on hot days, I have seen the motor get up into the 215 - 220 range. Part of this is due to the rediculously high temperature that the fans turn on on these cars. The rest is due, I believe, to a problem with the cooling system that I intend to address during the UIM/LIM repair. Under normal conditions it sits at 195 - 200 as it should with a 195 t-stat. A 180 will be going in shortly, but that still won't fix the fan problem or the cooling system problem.

The Regal has an aftermarket PCM that turns the fans on at 180* and a 180 t-stat. It consistently runs 20 - 40 degrees cooler than the Bonneville.

I'm only asking for pictures of the plugs to see for myself that it'* truely cracked and not carbon tracking. I know I keep going on and on and on about carbon tracking, but that'* only because -- like you said -- it'* unusal for plugs to crack after 30K and not at the time of installation. If it'* a hassle, don't worry about it. If they're easy to get to and get pics of, I'd appreciate it.

Carbon tracks usually form as a result of cracks, not instead of them. I didn't get around to getting these back out last night, but I will tonight when I take the garbage out.