Going to revive this post.....same thing just happened to me. Had my headlight assembly in the oven at 270 for 15 min and the outer "Hard-coating" spidered. I suspect this is a protective UV resistant coating that is applied over the clear lens cover. I can sand the coating off, but this will be too much work and the lens will fade quickly compared to the other side....so I have some new ones ordered up.

This time, I'm going to put the housing with lens downward facing in the oven, on a dampened towel...don't want to make this expensive mistake again!

 

270 is far too hot. You should be more around the 230-240 degree range if you have it in for 15 minutes. For OEM lights try 240 at 13 minutes, then put it back in the oven if you need to heat it back up. When I opened my father's OEM lights for retro-fit I did that.

If you have aftermarket lights that don't use permaseal, then just a quick 7-8 minutes at 240 and they will pop right open like the one's I used for my retrofit (ebay clear corners).

 

Always use a moist towel guys! This will protect the lense

 

Thanxs for the advice...will follow it when the new lights arrive.