Hit a Rock and Now It Won't Start? The 50p Flywheel Key That Saves a £400 Engine

You're mowing along nicely, the grass is looking good, and then there's that sickening crunch as your blade connects with a rock you didn't see hiding in the lawn. The engine makes an alarming noise and immediately stops dead. Your heart sinks.

You pull the starter cord and the engine turns over, but it won't catch. It just spins and spins, sounding completely different to how it should. There's no compression, no sign that it's trying to fire up. You might be thinking you've just destroyed your engine and you're looking at a hefty repair bill or even a replacement. The good news is that you've probably just sheared a flywheel key, and whilst that sounds serious, it's actually a cheap fix that you can do yourself in about twenty minutes.

What Actually Happens When You Hit Something

When your mower blade strikes a solid object like a rock, kerb, or tree root, the blade stops instantly but the engine wants to keep spinning. That sudden jolt creates enormous forces inside the engine, and something has to give.

Briggs & Stratton engines are cleverly designed so that the cheapest, most easily replaced component takes the hit. That component is the flywheel key, a small rectangular piece of soft metal that costs about 50p. It sits in a slot between the crankshaft and the flywheel, keeping everything lined up and timed correctly.
When the impact happens, this soft metal key shears in half or gets mangled, allowing the flywheel to slip out of position on the crankshaft. The flywheel carries the magnets that create the spark, so when it's in the wrong position, the spark happens at completely the wrong time. That's why the engine won't start, it's firing when it shouldn't be.

The brilliant bit is that this 50p key breaking saves your crankshaft, which would cost hundreds of pounds to replace and might actually total the mower. The flywheel key is designed to be the sacrificial part, and it does its job perfectly.

How to Tell It's a Sheared Flywheel Key

The symptoms of a sheared flywheel key are quite distinctive. The engine will turn over easily when you pull the starter cord, probably easier than normal because there's no compression fighting against you. You might notice it spins very freely and doesn't have that usual rhythmic resistance.

If you do get it to fire, which sometimes happens, it'll backfire through the carburettor with a loud bang and might even kick the starter rope back out of your hands. This is because the spark is happening at completely the wrong point in the engine cycle.
The engine might run for a few seconds and then stop, or it might refuse to run at all. Either way, it's clear something is fundamentally wrong with the timing, even if you've never worked on an engine before.

If this all happened immediately after you hit something with the blade, you can be pretty confident it's the flywheel key. It's by far the most common result of blade impact, and it's worth checking before you start diagnosing other problems.

What You'll Need to Fix It

The repair itself is straightforward, but you do need a couple of tools that you might not have lying around. You'll need a flywheel puller, which is a special tool that grips the flywheel and pulls it off the crankshaft without damaging anything. They cost about ten to fifteen quid and you can use them on pretty much any small engine, so it's a worthwhile investment if you've got multiple pieces of garden equipment.
You'll also need a socket set to remove the flywheel nut, and ideally something to stop the engine turning whilst you're loosening it. A block of wood wedged against the blade works fine for this.

The replacement flywheel key costs next to nothing. Make sure you get the right one for your engine model, though they're fairly standard across most Briggs & Stratton engines. Get a genuine Briggs part rather than a pattern one, the few pence you save isn't worth the risk of it not fitting properly.

Getting the Flywheel Off

Start by removing the engine cover so you can see the flywheel. It's the large metal disc with cooling fins on it, sitting at the top of the engine. You might need to disconnect the kill wire from the ignition coil first, just make a note of where it goes.
Remove the bolt that holds the flywheel on. This is usually quite tight, so you'll need to stop the crankshaft from turning. The easiest way is to tip the mower on its side with the air filter facing up, then wedge a block of wood between the blade and the deck. When you undo the bolt, the wood stops everything spinning.

Once the bolt is off, thread it back onto the crankshaft but only until it's flush with the end. This protects the thread and stops you damaging it with the puller. Now fit your flywheel puller. It bolts into two threaded holes on either side of the flywheel hub, or uses self-tapping screws if the holes aren't threaded.

Tighten the puller bolts evenly, a bit on one side then a bit on the other, working your way around. The flywheel will resist at first, then suddenly pop free with a satisfying clunk. Never hit the flywheel with a hammer to try to knock it off, you'll crack it and then you really will have an expensive problem.

Inspecting the Damage

With the flywheel off, you'll see the remains of the flywheel key. It might be sheared cleanly in half, or it might be mangled and bent. Either way, it needs replacing.
Before you fit the new key, have a good look at the keyway, which is the slot in the crankshaft where the key sits. If it's damaged or distorted, you might need to file it smooth. Also check the corresponding slot in the flywheel. Any burrs or damage need cleaning up, otherwise the new key won't sit properly.

Check the tapered surface of the crankshaft where the flywheel sits. It should be clean and smooth with no scoring or damage. A bit of emery cloth will clean up any light rust or dirt. The same goes for the inside of the flywheel, it needs to be spotless for a good fit.

Whilst you've got the flywheel off, it's worth checking the magnets on the underside. If any are cracked or missing chunks, the flywheel needs replacing. Also check the cooling fins aren't broken, damaged fins can cause overheating.

Fitting the New Key and Reassembling

The new flywheel key is a tight fit in its slot, which is exactly how it should be. Slide it into the keyway in the crankshaft, making sure it's fully seated and square. There should be no play or wobble.

Place the flywheel back onto the crankshaft, lining up the keyway in the flywheel hub with the key. It should drop into place easily, with the key fitting snugly into both slots. If you have to force anything, stop and check everything is clean and aligned properly.

Refit the flywheel bolt and washer. This needs to be done up tight, the torque specification is usually around 55 foot-pounds, but if you don't have a torque wrench just make sure it's good and snug. The friction between the flywheel and crankshaft taper is what actually holds everything in place, the key is just there for timing.
Put the engine cover back on, reconnect the kill wire if you removed it, and you're done. The whole job takes about twenty minutes once you know what you're doing.

Testing and a Word of Caution

Before you start the engine, give the starter cord a few gentle pulls with the spark plug still in. You should feel normal compression returning, with a rhythmic resistance as the piston moves up and down. If it still spins too freely, something isn't right and you need to check your work.

Assuming everything feels normal, put the spark plug back in and try starting the engine. It should fire up straightaway and run smoothly. If it backfires or won't start, double-check that the flywheel key is properly seated and the flywheel is tight.
One important thing to check after any blade impact is the blade itself and the crankshaft. If the blade is bent, it needs replacing or straightening before you use the mower again. A bent blade causes vibration that can damage the engine.

The crankshaft can also bend from severe impacts, though the flywheel key usually saves it. If your mower vibrates badly even with a straight blade, you might have a bent crank, which is a much more serious problem. But in most cases, replacing the flywheel key is all you need to do.

That 50p piece of soft metal is a brilliant piece of engineering. It saves you from what could be a write-off and gets you back mowing for minimal cost and effort. Just remember to watch out for hidden rocks next time.