1992 f150 5.0
Are distributor caps supposed to have an Oring in them?
I was wondering cause mine seems a little loose.
And could this cause pinging?

 

My cap has never had an o-ring and doesn't fit incredibly tight.

Pinging as in a engine knock? What motorcraft/autolite plugs are you using? The incorrect spark plug heat range can cause it, especially carbon buildup.

 

So can switching from a higher octane fuel to a lower octane fuel. My cap isn't necessarily tight (its just clips that hold it down), but it is pretty snug and I'm pretty sure its supposed to be at least partially sealed when clipped down.

 

Using a higher octane fuel could compensate for pre-detonation? CR 8.8 requires 87 octane, anything higher is a waste because it burns up too quick.

 

If your stock engine can't use the 87 octane it was specifically engineered to use, you need th find out why. Higher octane is like heroin to theses engine. The more it is used, the more it will require it not to ping. Eventually, you will end up "out" of higher octane options & w/ ping.

If your using Platinum plugs, toss them for some stock Motorcraft or Autolite Copper Cores.

 

I could of swore my 93/94 5.0 and 5.8 Caps had an "O" ring

 

That's what I meant when I mentioned about going from a higher octane to a lower octane. A lot of people don't know/realize our trucks are designed for 87.

 

If our trucks need a higher octane fuel in order to prevent "ping" or similar issues, you need to find the real problem. Going up in octane is just a band aid. Like ymeski said.

 

Maybe use this as an opportunity to take the cap off and inspect it and the rotor inside, replacing if necessary.

 

Well, it addresses the symptom. but can actually exasperates the most common cause. Combustion chamber build up.