MAGLIOZZI

Dear Car Talk: What happens if you are given the wrong spark plugs by an auto parts store, and you — not knowing better — install them in your 4Runner?

My son-in-law had an awful experience after installing what he later found out to be the wrong spark plugs for his 1995 Toyota 4Runner AWD truck.

Why can’t auto parts stores get the correct parts? Is his experience common? I love Car Talk, both the original on NPR, and the Apple podcasts now.

Thanks. — Bruce

I think auto parts stores generally do give people the right parts, Bruce. All that stuff is computerized, so the correct information is certainly available to them.

Now, if the guy waiting on you just got a text from his girlfriend that said she’s leaving him for his cousin Regina, human error can always occur. But I wouldn’t want to condemn every Pep Boy because there’s an occasional mistake.

Also, if you’re working on your own car, you bear some responsibility for making sure the parts you were given are correct. So, if you’re a DIYer, always compare the old part to the new one before you put the new one in.

Anyway, I’m guessing your son-in-law’s truck ran poorly after he installed some new spark plugs. What could cause it to run poorly?

Well, let’s start out with operator error. We’ve had situations in the garage where a mechanic has dropped a plug on the ground. If it hits the ground tip-first, that can close up the gap. And a plug with a gap of say, eight-thousandths of an inch instead of forty-thousandths, will barely fire.

It’s also possible to crack the plug’s insulator when you’re tightening the plugs. If an insulator gets cracked, the plug won’t do anything, and the truck will run on five of its six cylinders.

But if they actually gave him the wrong plugs, then the consequences range from insignificant to disastrous.

If the plugs fit — if they’re the correct thread size and length, and they just have different parameters — the wrong resistance or gap size — no permanent damage will occur. In fact, the truck might even run pretty well. And in that case, even if the performance is affected, swapping out those plugs for the correct ones will completely fix the problem.

The worst-case scenario would be if they gave him plugs whose thread length is too long. There’s a threaded part of each plug, and the length of the thread determines how far inside the cylinder the electrode protrudes.

If you install a plug that extends too far into the cylinder, the top of the piston could crash into the plug, resulting in a broken piston or bent connecting rod. And unfortunately, you wouldn’t know that until you tried to start the engine, by which point a funeral dirge would already be playing on your car radio.

I’m hoping they just gave him plugs with the wrong parameters, Bruce, and not the wrong thread length. If that’s all it was, with the right plugs, he should be good to go.

Got a question about cars? Write to Ray in care of King Features, 300 W. 57th St., 41st Floor, New York, NY 10019, or email by visiting the Car Talk website at www.cartalk.com.