HIR 9011 and 9012 bulbs are exactly identical in all dimensions to 9005 and 9006 bulbs, except for two areas, the second of which is unimportant for most cars. The first area is the "center" tab on the mounting flange. When trimming, you should always look at the bulb from the back, with the plug socket facing down. This prevents trimming in reverse, where you look at your old bulb from the front and then trim your new one from the back. There are 3 mounting lugs on the circular base flange, the ones that fit into the slots in the housing and then rotate home. The ones at 4:00 and 8:00 are large; they are the same on all bulbs and don't get trimmed. It's the center or top ones, at 12:00, that you trim.

On a 9006 low beam, the top tab occupies a space roughly from 12:00 to 1:00. On an HIR 9012 low beam, the tab is both double wide and double deep. You need to make it shorter (trim it to a smaller radius) and also remove the section from 11:00 to 12:00.

On a 9005 high beam, the top tab occupies a space roughly from 11:00 to 12:00. On an HIR 9011 high beam, the tab is the same depth as the 9005 but runs from 11:00 to 1:00. All that's needed to trim it is to remove the portion from 12:00 to 1:00.

The bulbs will come marked with white paint. You want to REMOVE THE WHITE PART. This will give you an exact duplicate of a 9005 or 9006 OEM stock bulb.

The tabs on the HIR bulbs can be regarded as universal, or blank, or androgynous. They have an excess of top tab, and it is centered on 12:00, so they could be trimmed to fit in either a high or a low housing, depending on whether you take off the right or the left side of the tab. Remove the left for a low beam, right for a high beam. Again, you're looking at the back of the bulbs with the plug down.

Untrimmed HIR 9012 low beam

Trimmed HIR 9012 low beam

Untrimmed HIR 9011 high beam

Trimmed HIR 9011 high beam

The plastic on the bulbs is a little bit brittle if you try to cut too much of it at once, and it doesn’t cut cleanly like PVC would. A very good tool to trim with is ordinary toenail trimmers, the ones that are a large version of fingernail trimmers. BUT, don’t try to remove very much plastic at a time; just take small 1 millimeter bites. A Dremel tool is great if you have one. A cutoff disc at medium speed does well, although you’ll get some melting onto the disc. A hand file does well also, though it takes a little longer. Side cutters or end nippers don’t work well on this plastic.

The trimming doesn’t call for extreme precision. All you need to do is remove enough material so that the top tab will fit in its slot on the housing. If you deliberately ground it off altogether, the bulb would still fit OK and function with just the two base tabs. It’s a quick and easy job for anyone with basic eye-hand coordination, and the chance of error is close to zero.

No modification to the housing is ever required, only the bulb. Also, no modification to the electrical plug section is required for stock replacement trim.