I have a magnum air 33degree gun, i removed the head cap to replace blown gasket, when i put it back on the gun will not fire.

Check the pressure cap and change the coolant if over two years old. While the cap is off see if there any bubbles or oil traces in the coolant, if so you have major engine problems, possibly a bad head gasket.

Hitachi nailers cycle the whole cylinder as the air valve when gun fires. The orings are most likely too tight around the cylinder. Easy to check. Remove the head and depress the cylinder with thumbs and cylinder should pop back up. If the cylinder is down pull it back up and reinstall head. The hack method to get the cylinder up is to rap the gun head against hard rubber pad (with head installed and air connected). If cylinder moves down then up, most likely head valve gaskets (under headvalve ) or headvalve itself or top of cylinder damaged. Good luck

Easy test, Remove the nails and remove the trigger retainer pin and trigger to expose the trigger valve (tva6 valve). With air connected and nails removed, quickly depress the trigger firing pin all the way in. Gun should fire (if gun fires, adjust safety foot to make better contact with tva6), if no joy air is leaking past orings in the head valve or head gasket vent hole and/or trigger valve. Remove head and check gasket around vent hold (small hole in the head gasket). If the gasket good remove head valve and examine surface where orings make contact for fine scratches. If scratched or worn, orings are not sealing. To check if orings are leaking, apply lots of white grease to orings in the head valve, replace the head and test fire. If the gun fires, sand contact areas with fine sandpaper and or use oversize orings or wind teflon tape under the external orings. The metal ring or washer that you are wondering about is sometimes used to space up the bumper to control depth of nail. Other nailers use a metal fitting under the bumper to help guide the driver and to seal the lower chamber helping to prevent too much air leaking past the driver. If too much air leaks past the driver, guide, the piston may not return to top. Good luck

Did you bleed the cooling system? and check operation of the thermostat, which should have been done after fitting radiator before finishing by putting on radiator cap.
If not remove radiator cap when cold get vehicle warm and check thermostat operation, while bleeding any air pockets in cooling system and if no flow and bubbles or it starts to boil (shooting water) out of radiator the thermostat could be jammed or head gasket blown.
Sounds like it could be a blown head gasket. Get it hot again and miss firing, remove and inspect all spark plugs you may find a very clean spark plug indicating water entry into that cylinder or even water/coolant droplets in spark plug. You can also hold paper over spark plug holes and give the engine a quick crank over to see if any water comes out of cylinder while cooling system is still under pressure, or use radiator pressure tester to pressurize cooling system, wait 5-10 minutes and then crank over checking for water/coolant in cylinders. Another tool to check for blown head gaskets that cause overheating is a TK Head check, which sniffs the cooling system for evidence of exhaust fumes in cooling system.

Belive it or not you can get all of or at least most of these parts at a good hareware store . Air nailers are all Orings if you can take it apart then take the Orings with you match them up exactly it may cost you 2 or 3 dollars to completely rebuild this gun i have about 10 differant nailers and do this when ever one sticks . Put a water trap on your compresser and shout some WD40 into the air inlet on the gun before you use it and this will make the gun last much longer

There are about three issues with this gun that could cause your problem. The safety contact arm not depressing the trigger pin fully, worn trigger valve (tva6), or leaking orings in the headvalve. Easy tests. Remove the nails, and remove the trigger to expose the tva6 valve. Reconnect air and with very quick and firm action, depress trigger pin all the way in. If the gun fires, adjust safety foot and trigger so that the trigger pin is depressed all the way in each time that you fire the gun. If the gun does not fire, remove the head and remove the headvalve. Examine the surface/s where Orings make contact with magnify glass looking for fine scratches. Sand with 400 grit paper. Orings must make good contact here or gun will not fire. Lube orings and surface with silicone paste lube (sold at Lowes in the plumbing department or swimming pool supply store). If no fine scratches and surface looks smooth, replace Orings in head and the tva6 valve. Good luck with your repair/

Remove the 4 screws and pull cap off .careful not to rip the gasket and pay attention to how the head valve sits. Pull driver out to make sure its not bent . If its fine put driver back in place. Remove the head valve from cap . Replace the oring around the head valve part # 180535 . If thats not a option take a thin strip of teflon tape and wrap in around head valve once then put old oring over it. Grease or oil , put cap back on and test fire. Should be good to go . Best of luck !

Gun will not fire if air is leaking past orings in head /headvalve or safety /trigger is not pushing trigger pin all the way into trigger valve. Test: Remove trigger to expose trigger valve pin. Remove nails. Place nose of gun against thick rubber mat or coiled rubber hose to absorb blow of driver. Very quickly and deliberately press trigger pin all the way in. If gun fires, safety foo or trigger is not pushing trigger pin all the way in. Make adjustments or replace worn parts. If gun still does not fire, remove head and pull out headvalve. Apply lots of silicone paste lube on the orings and the surface of the head where orings make contact. Reinstall head and test as above. If the gun fires then orings were leaking. Replace orings and sand any scratches on body of cap with very fine sand paper. Parts are at www.toolpartsdirect.com Good luck

We used nail guns for years. I still own 6 guns, but they aren't used much anymore.
You have a 'blown head gasket.'
Or there is debris from the air hose that got lodged into the gun.
There is a head gasket and a trigger gasket.
Both can blow out any time from use, lack of oil, or they can stick open from debris.
You can take apart the head and clean it up and add oil to see if that solves problem.
You can order a gun re-build kit for your model and put in the parts yourself.
You can have a nail gun repair shop do the job.

Nail guns require gun oil.
If you don't use gun for a while, oil should be put into air hose opening and then left to run down into gun. More oil is better.
If you use 3-in-1 oil or some oil besides gun oil, then it will corrode the gaskets.
When storing air hose and guns, put tape over openings so they stay clean.
Before hooking air hose to a gun, you can also put a blower on hose and blow out the line first

We used nail guns for years. I still own 6 guns, but they aren't used much anymore.
You have a 'blown head gasket.'
Or there is debris from the air hose that got lodged into the gun.
There is a head gasket and a trigger gasket.
Both can blow out any time from use, lack of oil, or they can stick open from debris.
You can take apart the head and clean it up and add oil to see if that solves problem.
You can order a gun re-build kit for your model and put in the parts yourself.
You can have a nail gun repair shop do the job.

Nail guns require gun oil.
If you don't use gun for a while, oil should be put into air hose opening and then left to run down into gun. More oil is better.
If you use 3-in-1 oil or some oil besides gun oil, then it will corrode the gaskets.
When storing air hose and guns, put tape over openings so they stay clean.
Before hooking air hose to a gun, you can also put a blower on hose and blow out the line first