This is courtesy of: http://www.gassprings.net/ HOW TO CHOOSE THE RIGHT GAS SPRING WITH THE RIGHT CHARGE Please find the following data: 1. The weight of the part (hatch) to be lifted. 2. The length of the hatch; this is the distance from the hinges to the lock (or locking) side. 3. The degree to which the hatch will be opened. (90 degrees or more, up to 160 degrees) To find the actual weight of an existing hatch, place a hanging scale on the side of the hatch in a position as close as possible to the horizontal position and to multiply the measured weight by 2. Keep in account the friction of the hinges, if any, and the weight of locking devises that can upset the center of gravity of the hatch. Adjust for these variables in the formula. See page 3, 4, and 5 for calculations and examples. The stroke: As a rule of thumb the stroke of the gas spring is at least 1/4 of the total distance from the pivoting point to the locking side of the hatch. In case of weak structures or Plexiglas choose the next size up. This rule is based on the hatch opening of 90 degrees; gas spring mounted in the rim (application A). If a different opening of the hatch is desired, the stroke has to be recalculated accordingly. Here are more vendors. http://www.guden.com/itemdisplay/itemdisplay.aspx?wpn=GS1 http://www.spdhardware.com/default.asp http://www.ameritoolmfg.com/