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The Matco is very similar to the IR-315 straight line sander as well as the Central Pneumatic Straight Line Sander sold by Harbor Freight-shown here: The Central Pneumatic became the focus of my attention when I realized it was not timed properly after re-assembly!
Assuming you are having the same problem,(1) I recommend mounting the body upside down in a small vise as shown here:
(2)Rotate both piston gears full forward (to the left in this photo). (3)Then carefully rotate each toward the back end precisely three teeth (to the right in this photo). It is important to note: Begin counting to the precise 3rd tooth once the first tooth is near top center in step (2). Thus, if after rotating the gears forward until they stop, you ideally want a tooth visible right at the top center. If the tooth is not at top center, move the gear(s) slightly toward the back until the very first tooth is now centered up: then begin counting to the 3rd tooth. If after rotating the gears forward until they stop, a tooth is NEARLY at top center: use it as your starting point and count to the third tooth (step 3). I recommend wiping the oil/ grease off of these two centered teeth and coating them with red majic marker.
(4) Reference next picture: Hover cursor near rotary air valve gear.
The dotted gear lobe should be a 1 oclock, assuming 12 oclock is in line with the center of the body toward the knob end of unit. Note the piston gears are marked for the centered 3rd tooth. Note the two teeth on each side of the center valley(s) of the rail gears are highlighted as well.
(5) I recommend marking the valley after the tooth that follows the dotted tooth and verifying its position is at 3 oclock. Notice in the background that the center tooth is highligted (it is the fifth tooth from either end of that straight gear on the rack).Also reference the second photo after step (6):
(6) visually confirm all teeth are in perfect alignment for placing the rack upon the unit:
(7) Simply place the rack onto the unit's body making sure all your marks are centered up. If correctly lined up, you will incur no resistance or offsetting. If rack does not simply "lie onto the gears", lift back off and re-check the alignment of each of the teeth and valleys. A slight movement of one of the piston gears is usually the cure.
(8) re-attach the rack retainer bars with at least 2 screws on each side before moving the unit.
(9) you should visually be able to see about 1-1/2" of rack movement from the unit by pushing the rack to its limits. A centered rack measures approx. 15/16" on both ends of unit to the center of the bolt in the rack.
On the side of the sander there is a black triangle shaped cover with 3 screws holding it in place. Remove the screws and you will see the drive belt and 2 pullies or gears. The belt will be attached to the gears. Remove and replace the belt. Typically the belt is broken so the only thing you will need is to locate a new belt and slip in over the gears with the side with the teeth facing the gears. This keeps the belt from slipping. Reinstall the cover and you are ready to go back to work. The hardest part may be finding a supplier for the belt.
Go to www.harborfreight.com and search for your tool's item number, then download the manual -- which includes an exploded parts diagram -- to your computer or phone.
If the model number is no longer available, contact Harbor Freight Tools by phone to request a copy of your manual. They usually have them for obsolete tools. However, don't expect to be able to order parts!
try putting both cams down one tooth. i took mine apart and and drew a picture of the cam locations and they did not line up with the manuals picture. so i thought that my pic was wrong so i changed it to what the manual said, and it did what you said just back fired. so thats when i took it back apart to put the cam gears down one tooth so they looked like my picture that i drew. then i put it back togther and she ran great. you just want the left one just below and the righht just above if you see both bottom dots its not right.