Is the trimmer head plastic or metal?

Plastic with the high stress points made of metal. I'm an engineer and there is nothing wrong with plastic if the correct plastic is used and the design is made for using plastic. This head looks like an okay design but I have no idea if a correct plastic was used. So far mine works fine but it does use up a little more line than a regular string head since the short ends are thrown away when you reload. I recommend the trimmer head for the price.

Please; i have a ryobi 4 stroke with a curved shaft. has anyone had any luck installing this unit on my 4 stroke ryobi ?...thank you !

Not sure but they do fit a lot of different ones.

Will .135 line work with this head

Yes. It is very flexible in terms of thickness of the line.

Will .155 line work with this head?

No, the Weed Warrior EZ Lock Trimmer Head is not compatible with the 0.155 diameter trimmer line. If any further assistance is needed, please give our Weed Warrior Customer Support Team a call at 866-212-7466.