what causes bent valves

Bent valves usually happen when the piston smacks them. Common causes are a slipped timing chain or a failed tensioner. On the Sportsman 500, check your timing chain guides since they wear out.

I bent a valve on my 400EX after I accidentally bounced off the rev limiter too hard on a downhill. Over-revving can definitely do it if the valve float happens and contact occurs.

Could be a dropped valve if the keeper or retainer failed, but that's less common. More likely the timing jumped. Check your cam timing marks before pulling the head.

Don't forget a bad carb tune causing a backfire through the intake. That can bend a valve if the intake is open and the explosion hits it. Happened on my buddy's Quad.

If it's a 4-stroke, timing chain stretch is the number one culprit on older machines. I'd replace the chain and guides when you fix the valve.

Also consider debris—if something broke off inside, like a piece of a piston ring, it could get stuck and bend a valve. Rare but I've seen it.

On the Sportsman 500, check the camshaft as well. If the chain slipped just enough, it might have bent the valves but not fully snapped. Happened to mine at 8,000 miles.

I disagree with over-revving being the main cause unless you're racing. Most stock ATVs have rev limiters. Timing issues are way more common.

Make sure your valves aren't just stuck from carbon buildup before condemning them. A leakdown test will tell you if it's actually bent or just not sealing.

Had the same thing happen after a muddy ride. Water ingestion can bend a valve if it hydrolocks. Did you go through deep water recently?

If you bent one valve, the others might be slightly damaged too. Consider replacing all of them while the head is off. Cheap insurance.

On a 10+ year old machine, inspect the valve seats and guides. A bent valve often damages the seat, requiring a machine shop to cut it.

Check your valve clearances before assuming bent. A tight valve that burnt can mimic a bent one. Happened to me once and saved a head rebuild.

I'd pull the head and look for witness marks on the piston. If you see a shiny spot, that's valve contact. Then you know it's bent.

Before tearing it down, rotate the engine by hand with a wrench. If it hits resistance at certain points, valves are likely bent. If smooth, check compression again.

Could be a broken rocker arm too. That can let the valve drop and get bent. Check all the valvetrain parts.

Your best bet is a compression or leakdown test first. Then do a timing check. Saves you from pulling the head for nothing.

If the timing chain jumped because of a worn tensioner, that's an easy fix once the head is off. Lots of YouTube vids show the process for your model.

I partially disagree with the carbon buildup idea—that's more for stuck valves on Honda models. Polaris usually get bent valves from timing issues.

When you fix it, upgrade to a manual tensioner or a heavy-duty chain. Greatly reduces the chance of it happening again. I did on mine and no issues since.