Light Bulb Question

I'm brand spanking new here, so please excuse me if this thread is in the wrong section or if the question's been asked already.

I was in a pretty serious accident a few months ago, and I had to replace a tail light fixture. Since my car is 21 years old, I had to buy the part on ebay, and in the process I wound up with a few extra 1157 bulbs that work fine.

Since then, my center tail/brake light has burned out. That bulb was a 1156, and I was wondering if there is serious harm in replacing it with one of the 1157s I have lying around. (I'm poor and don't want to spring for the new bulb unless I absolutely have to.)

I know it says not to do that, but is it that serious? And if so, could somebody please explain the reasoning to me?

Welcome to AF

The 1156 bulb has 2 studs (those little knobs on the side of the brass housing) that are the same location.
The 1157 bulb has staggered studs, to ensure the bulb fits into the socket in one specific orientation, so the filaments work properly.

Therefore, an 1157 bulb will not fit an 1156 socket unless it is forced in or modified.

Even if you do get the 1157 bulb jammed in, you will find the 1156 socket will illuminate both filaments on the 1157 bulb at the same time. This may overheat the brake light housing and melt it, especially if you get stuck in heavy traffic, where the brake lights are on a lot...............
(unless you force in an 1157 bulb with only one working filament, but you may still damage the socket and possibly create a dangerous electrical short.)

How do I know this??? Well, 20 years ago, I wanted an extra - bright center brake light and modified an 1157 bulb in place of the 1156........ and promply melted the housing.

Thanks! That's really helpful. I'll spring for the new bulb.