chain gets super loose after like 5 minutes of cutting, i tighten it and it happens again. bar nuts are tight.

Diagnosis

The rapid loosening of the chain on your Ryobi RY405110 is almost always a result of "thermal expansion." When the chain runs without adequate lubrication, the friction between the drive links and the guide bar generates extreme heat. This heat causes the steel of the chain to physically expand and lengthen, which manifests as a loose, sagging chain. If you tighten the chain while it is hot and then let it cool, you risk over-tensioning it, which can warp the bar or damage the drive sprocket. To confirm this is a lubrication failure rather than a mechanical failure of the tensioner, look for "blueing" on the drive links or a lack of oil residue on the bar rails. If the bar looks dry and polished while the chain is hot, the oil is not reaching the cutting surface. This is typically caused by a clogged oil delivery port or an air lock in the oil reservoir, preventing the automatic oiler from pushing lubricant through the narrow channel in the guide bar.

How to Fix It

Follow these steps to restore proper lubrication and stabilize your chain tension:

1. Perform the "Sling Test": Place a piece of clean cardboard or a sheet of paper on a flat surface. Start the saw and hold the tip of the guide bar approximately 2 inches above the cardboard. Rev the motor for 5 to 10 seconds. You should see a distinct, consistent spray of oil dots forming a line on the cardboard. If the cardboard remains dry, your oiler is blocked.
2. Clear the Oil Port: Remove the bar nuts and slide the guide bar off the saw. Locate the small circular oil delivery hole on the chassis where the bar mounts. Use a thin sewing needle, a safety pin, or a blast of compressed air to clear any sawdust, resin, or solidified oil clumps from this port.
3. Clean the Bar Groove: Use a thin piece of plastic or a dedicated bar groove cleaner to scrape out the packed sawdust and oil "cake" from the length of the guide bar groove. If the groove is packed, oil cannot flow to the chain even if the pump is working.
4. Reset Tensioning: Remove the chain and clean it thoroughly. Reinstall the chain and tighten it while the tool is cold. Ensure the chain snaps back against the bar with a slight "click" when pulled from the bottom, but does not sag more than 1/8th of an inch. Tighten the bar nuts firmly to prevent the bar from shifting during operation.
5. Verify Oil Level: Ensure you are using a high-quality bar and chain oil. Fill the reservoir to the max line and ensure the cap is seated tightly to prevent air leaks.

If That Didn't Work

• Replace the Guide Bar: Over time, the oil hole in the bar itself can become clogged with oxidized sap and resin that cannot be cleaned with a pin. If the pump is working but the bar is dry, replace the guide bar with a new OEM Ryobi 16-inch bar.
• Inspect the Drive Sprocket: Check the drive sprocket for excessive wear or "hooking." If the sprocket is worn down, it may not seat the chain links properly, causing them to slip and feel loose even when the tensioner is engaged.
• Chain Stretch (Wear): If the chain is old, the rivets may have worn down (mechanical stretch). A worn chain will never stay tensioned regardless of lubrication. Replace the chain with a new 3/8" pitch, .050" gauge chain.
• Internal Pump Failure: If the oil port is clear and the bar is new but no oil is slung during the test, the internal oil pump may have failed. This requires a teardown of the motor housing to inspect the pump diaphragm.

Heads Up

• Never tighten a hot chain: Always let the saw cool down completely before adjusting tension. Tightening a hot chain can lead to permanent deformation of the bar.
• Avoid "Dry" Cutting: Never operate the RY405110 without oil. Doing so can ruin the guide bar in as little as 10 minutes of heavy cutting.
• Check Tension Frequently: New chains "stretch" significantly during their first few uses. Check the tension every 10-15 minutes for the first hour of a new chain's life.
• Use Correct Oil: Only use dedicated bar and chain oil. Do not use motor oil or vegetable oil, as these can clog the small delivery ports of the brushless system.