these are neeeded when using a hho system or a test pipe. they are also used to lean out your vehicle for better gas mileage. they are very easy to install by unscrewing your O2 sensor from your exhaust then screw your sensor into the extender and then screw the two pieces back into your exhaust. the extender has A 7/8 hex head which is the same size as most O2 sensors and it makes them easier to install not like the straight sided extenders. fits most sensors with a1/2 inch diameter tip. the size of the extenders is M18 X 1.50 and they are 1.253 inches long and drilled out to .5175 inches. if you need more clearance you can stack them and they have a tapered seat no washer needed. Color: Silver Material: Iron Model:M18 x 1.5 Universal For Any Thread Size Of 18mm Straight Bolt-On, No Modification Needed Very Easy To Install Package included: 3PCS X oxygen sensor Only the above package content, other products are not included. Note: Light shooting and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.