the chain on my dccs620b keeps coming loose no matter how tight i get it, i tighten it and two cuts later it's sagging again.

Diagnosis

The DCCS620B is a high-torque tool, and the rapid loosening of the chain usually stems from one of three root causes: improper initial tensioning (over-tightening), "chain stretch" caused by heat or wear, or a compromised guide bar. When a chain is tightened while the bar is not under load, it can seem secure, but as soon as the bar enters a log, the wood creates a "pinch" or "push" effect that forces the bar to bow slightly, effectively lengthening the path the chain must travel and causing it to sag. Furthermore, if you are using a non-OEM chain or a chain that has been run without proper lubrication, the drive links undergo microscopic stretching. This isn't the metal actually stretching, but rather the rivets wearing down. If the drive link tangs (the protrusions that fit into the bar groove) have worn down or the bar groove itself has widened (ovaled), the tensioning pin cannot maintain a consistent hold. You can confirm this by running your fingernail along the bar groove; if it feels wide or "sloppy" compared to the base of the bar, the bar is shot. If the chain feels loose only after a few cuts, you are likely experiencing "thermal expansion" where the chain heats up and elongates.

How to Fix It

To properly tension the DCCS620B and ensure it stays locked, follow this precise sequence:

1. Loosen the bar nuts using the onboard tool or a socket wrench just enough so the bar can move, but not so loose that it flops.
2. Place the tip of the guide bar firmly on a sturdy workbench or a log. This is critical; the bar must be under a slight downward load to simulate a cut.
3. Grasp the "head" (the tip) of the bar and lift it upward slightly. This puts the chain under maximum tension at the drive sprocket.
4. While holding the bar up, turn the tensioning knob clockwise. Continue turning until the chain snaps firmly against the bottom of the bar.
5. Test the "Snap": Pull the chain away from the bar in the middle of the guide. It should pull away about 1/16th to 1/8th of an inch and "snap" back instantly. If it stays away or feels limp, it is too loose.
6. Tighten the bar nuts firmly while maintaining the upward pressure on the bar tip.
7. Re-check the tension after the first 30 seconds of cutting, as new chains always stretch during the first few passes.

If That Didn't Work

• Replace the Guide Bar: If you see "burrs" or mushrooming on the edges of the bar rails, the chain is rubbing against the sides, creating friction that pushes the chain out of alignment. Replace the bar with a genuine DeWalt 16-inch replacement to restore the tight tolerances of the groove.
• Swap the Chain for a New OEM Set: If the chain has reached its "stretch limit," no amount of tightening will help because the drive links are too short to be captured by the tensioner. Install a fresh 3/8" low-profile chain. Avoid generic "big box" chains that may have slightly different pitch tolerances.
• Inspect the Drive Sprocket: Remove the side cover and inspect the drive sprocket for deep grooves or "hooks." If the sprocket is worn, it will not seat the chain links properly, causing the chain to "jump" and loosen during high-RPM cuts.
• Check the Bar Oil Flow: Ensure the oil reservoir is full and the oil holes are clear. A dry chain heats up rapidly, causing the metal to expand and the chain to sag almost immediately. If the chain is dry to the touch after a cut, the oil pump may be failing.

Heads Up

• Never over-tighten: A chain that is "guitar-string tight" will put immense pressure on the motor's drive sprocket and the battery's electronics, leading to premature motor failure or an "overload" shutdown.
• The 5-Cut Rule: For the first five cuts with a new chain, check the tension every single time. New chains "seat" themselves and will always loosen initially.
• Clean the Groove: Every time you change the chain, use a thin flat-head screwdriver or a dedicated bar groove cleaner to scrape out the packed sawdust and pitch from the bar channel.
• Lubrication is Key: Only use high-quality bar and chain oil. Using motor oil or WD-40 can degrade the seals in the DCCS620B's automatic oiler.