A dob of thick grease on the tip of a screwdriver might be able to grab it if it's not jammed in.

The hole doesn't go anywhere, but best to get it out if possible, in case the bolt hits it and prevents it seating.

Hayter 13/30 Ride On Mower With 12.5HP Briggs and Stratton Engine

I was refitting the flywheel and the 15mm x 5mm x 4mm key fell into the bolt hole in the crankshaft.

I am assuming the bolthole is blind and going nowhere.

Because it is full of water as rain found its way in there while the bolt was out. (In spite of the numerous tarps over it).

Am I ok to leave it there and buy a new one?

Or do I need to figure out how to pull it out?

I'm going to need a very thing magnetic wand.

All mine are too big.

What do you all think?

Cheers

Dave, I must admit it's a bit fiddley, loads of washers, spacers etc, but just take your time and note down where everything goes.  Oh, and I think I forgot the art about unbolting and removing the height of cut lever, but I'm sure you found that out. Also some have a assist spring under the right hand cover, so you might notice you have a spring dangling.

Good luck with the repair. 

Mick, Thank you for that. Saved me a lot of searching. I thought there might be a Youtube video on this but cant find one. Had a struggle with the frame bolts - one is very difficult to get to - but now hopefully able to remove/swap.

Hi Dave, and welcome to TGMForum.

The gearbox removal is basically.
1: Disconnect clutch cable at the handle end. That will let you pull the cable through with the gearbox. 
2: Slip chain off gearbox sprocket
3: Remove roller frame pivot bolts and pull out of the way of the following steps.
4: Remove gearbox sprocket buy removing pin.
5: remove push on fix holding the HOC conrod and slip that off.
6: Drill out the three rivets and slide that plate off. Each rivet has a washer underneath which you'll need when fittng the new rivets.
7: Wriggle gearbox out.(making sure you keep all the washers to put back on the new box).

Fit gearbox in reverse order.

I have been struggling with this for a while (5 week wait at Honda dealer). I just joined the forum and saw Mick's post in an old thread that three rivets have to be removed! That explains it after countless session on AI which doesn't know because there's nothing on the internet detailing the procedure. I got the frame bolts out then thought it might drop out. No chance. if anyone direct me to more detailed instructions or YouTube or elsewhere? Or I just drill out the rivets and get new bits from DIY spare parts. I have a new gearbox ready to fit.

I'm not saying the key hasn't partly sheared, but to be honest it's unlikely unless the flywheel wasn't torqued down properly. It does to me sound like a fuel issue.  Those engines are prone to hunting and surging unless the carb is spot on. Especially if it's got a NIKKI carb on it.

Is your float bowl attached via two screws at the top of the bowl?  Or one bolt at the bottom?

Quote from: Ray Pooley on May 04, 2026, 08:22:52 AM

Quote from: Mick on May 03, 2026, 12:44:58 PMStale fuel can definitely cause issues like this, especially if it's been sitting in the can since last season. Always good to start with fresh fuel. 

Notifications, please check your notification settings by clicking on the Alerts button (Top Left) and clicking Settings.

The carb Id, you could perhaps attach a photo or two of the offending item.    If you manually apply a bit of choke while it's running does the surging even out?

Thanks again. I'll put some fresh fuel in it.
The Hayter manual tells you to apply full choke as part of the start.
I close the choke gradually until it starts to stall.
There is a volume control screw on the carb that I haven't tried yet.
I'll give that a twiddle next time I run it.

Quote from: Mick on May 03, 2026, 12:44:58 PMStale fuel can definitely cause issues like this, especially if it's been sitting in the can since last season. Always good to start with fresh fuel. 

Notifications, please check your notification settings by clicking on the Alerts button (Top Left) and clicking Settings.

The carb Id, you could perhaps attach a photo or two of the offending item.    If you manually apply a bit of choke while it's running does the surging even out?

I am now beginning to think it might be a problem with the flywheel key.
Broken or crushed. Apparently the TDC on these engines is 0 degrees.
Not much tolerance there.

Quote from: Mick on May 03, 2026, 12:44:58 PMStale fuel can definitely cause issues like this, especially if it's been sitting in the can since last season. Always good to start with fresh fuel. 

Notifications, please check your notification settings by clicking on the Alerts button (Top Left) and clicking Settings.

The carb Id, you could perhaps attach a photo or two of the offending item.    If you manually apply a bit of choke while it's running does the surging even out?

Just been out and had a twiddle with the volume control.
Not a lot of movement there. Only about 45 degrees.
Made no difference.
What I am seeing is backfire which suggests pre-ignition.
You can see the effect at the inlet and the throttle lever.
There is no way of adjusting the timing as far as I am aware.
So I don't know how this is happening.
The gap between the new ignition coil and the motor is correct at 0.3mm
What do you think?

http://86.153.209.5:163/Backfire1.mp4
http://86.153.209.5:163/Backfire2.mp4

Quote from: Mick on May 03, 2026, 12:44:58 PMStale fuel can definitely cause issues like this, especially if it's been sitting in the can since last season. Always good to start with fresh fuel. 

Notifications, please check your notification settings by clicking on the Alerts button (Top Left) and clicking Settings.

The carb Id, you could perhaps attach a photo or two of the offending item.    If you manually apply a bit of choke while it's running does the surging even out?

Thanks again. I'll put some fresh fuel in it.
The Hayter manual tells you to apply full choke as part of the start.
I close the choke gradually until it starts to stall.
There is a volume control screw on the carb that I haven't tried yet.
I'll give that a twiddle next time I run it.