Two nails being shot at once.

There are 4 main causes usually with this tool:
1) Check the seal in the bottom of the cylinder (under the bumper) #BF0070 (#12 on the breakdown) If this seal is missing or worn the piston will not always return fully.
2) The pusher spring #GD0253 in the magazine is not moving the nails forward
2) The driver is bent or the tip is worn / chipped
3) The check valve ring #LB0077 on the cylinder is not sealing properly
Let me know if this helps Senco SFN30 15 Ga Finish Nailer Parts

Worn trigger valve orings will cause gun to fire twice, and /or attempting to single fire nail with bump-fire trigger. Easy test, look closely at firing pin under trigger. It should fit snuggly an not move from side to side in valve. If loose, replace valve. If your nailer has bump-fire trigger, do no attempt to place gun then fire one shot because the gun will most likely double fire. Good luck.

This is a bottom fire tool. (NV83A2) You need to hold the trigger down and then as you place the nailer to the wood the safety will depress and it will fire once. This is usually done in one, swift action. If you do this in the opposite way by placing the tool to the wood and then pulling the trigger they often will double fire.

Nailers that use strips of nails (one nail directly behind the other) will sometimes drive two nails at once and is caused by damage to the driver. The nails are so close together that the driver will hit the first and part of the second nail head. Coil nailers, as your an451, use nails seperated by welded wire making it very difficult the the driver to hit the second nail. Usually when a coil nailer shoots two nails, it is acutally the trigger valve that acutates twice (two very rapid fires). Two very rapid fires is caused by faulty trigger valve. I have seen a roofing coil nailer fire one nail and a second nail drops out of the gun. Caused by weak / malfunction of nail feeder or wire used to weld nails together is too thick.

Worn trigger and or worn firing valve. Remove the trigger and look for wear on contact surfaces (worn pin that holds trigger, worn inner lever inside trigger, worn or loose firing pin in valve ). Easy test for worn trigger. Remove the nails, remove trigger, fire the gun against wood with finger to test for proper action. If it works better without trigger, consider replacing trigger with new (I would also order the oring repair kit that includes the firing valve )

Not sure about Senco but most have latch you can open to expose firing mechanism. Nail sometimes gets jammed and will not allow driver to fire or if not nail push driver ****** all the way back into gun then see if gun will fire.

Bostitch nailers have two triggers available. Black trigger is rapid fire while the gray is single shot. Both triggers are usefull depending on type of work. Most people say that there is a degree of accuracy with single shot allowing for better placement of nail. If speed is your goal then use the black trigger to bump fire. More information and parts can be found at www.bostitch.com , look for "Support" tab, then "service / parts " find your model number from the pull-down list and search for the manual and parts list for needs. Good luck on your projects.

if its oiled, then take all the nails out of the track and chamber, check to see if your firing valve is stuck( it is suppose to go up and down , opening and closeing the cylinder, but sometimes it gets stuck) first press the safety in all the way ( you may have to pull the feeder down ) then press the trigger , then hook the hose to the gun in that order this will sometimes unstick the firing valve

It sounds like you have a contact trip trigger that you are trying to operate as a sequential trip. Sequential trip is when you have to put the tool against the work and pull the trigger every time, contact trip is when you pull the trigger and tap the nailer on the work and it fires. If you operate a contact trip like a sequential it will fire but it usually fire more than one nail. Your instruction manual will explain this, sometimes they give you two seperate triggers for the two modes or there is an adjustment on the nailer.

Make sure you have at least 100 psi available and I would pretty much drown the sliders and other moving parts in something like Marvel Mystery Oil.