Head gasket replace flywheel question

I'm assuming your talking about removing the harmonic balancer; because the flywheel is on the other end of the engine and inside the transmission/transaxle bell housing?
You'll need to use a 1/2 inch impact wrench to buzz the bolt off and possibly a harmonic puller. You can borrow a puller from any chain auto parts store - don't know if they'll loan you an impact wrench - but I believe the sockets are available.

 

The impact gun shouldn't rotate the harmonic balancer, but it can move it off the timing mark.

I highly recommend you get someone who had done work like this before to help you. Watching videos isn't going to stop you or the video from missing something and ruining your motor. This is basic, so it seems that you've never used an impact wrench before.

 

He put a long handled wrench on the socket, braced the handle on something solid, the driveshaft IIRC, then had assistant momentarily start engine. The pulley, nut and crankshaft all start to rotate clockwise, but the restrained nut can’t, so…

This’ll get the nut off, but still leaves the dilemma of subsequently torquing it. Simplest for both directions something like this.

 

There are flywheel locks or harmonic balancer wrenches that will hold the crank in place; so you can manually bust the bolt loose. You'll need something like that to properly tighten that balancer back-on to OEM specifications.

PS: if you haven't replaced your electronic water pump before; you should probably do that while the engine is apart - saves you downtime, since the reports are that they get lazy after 150K miles.

 

Last link in my signature* is the engine section from Repair Manual, has torque values and so on. There's a couple of other links in my signature, head gasket related.

If I were in your shoes, I'd get the genuine Toyota Engine Overhaul Gasket Kit. It includes the head gasket, should be the latest iteration thereof. Here's an AI summary:

The part number for a Toyota 3rd gen Prius (2010-2015) engine overhaul gasket kit is 04111-37316. This is the genuine OEM part, though it may also be listed as 04111-37310 as a superseded number, so it is best to verify compatibility with your specific vehicle's VIN

Also, a lot of head gasket videos mention a "triple-square" bit for the head bolts, which works, but is NOT optimal. Toyota recommends a double hex bit. Google this for both 8 and 10 mm such drivers; believe it's the 10 for 3rd gen:

CTA Tools 9295 8mm and 10mm Toyota Head Bolt Wrench Set

Head bolts are best replaced, just in case. Use genuine Toyota to be sure. AI summary:

The genuine OEM part number for the Toyota 3rd generation Prius (2010-2015, with the 1.8L 2ZR-FXE engine) cylinder head bolts is 90910-A2011. This number replaces the older part number 90910-02166.

Definitely also watch @NutzAboutBolts head gasket replacement videos too.

Note: you should verify all mentions of part no's with a dealership, check against your VIN.

* on a phone turn it landscape to see signatures

 

the pulley has two threaded holes, for a proprietary Toyota tool (believe it’s shown in aforementioned repair manual engine excerpt), but something could be jury-rigged I think. Definitely useful for torquing: breaking the nut loose an impact ( or the engine start trick) will work, but IF you want it properly torqued on reinstall you have to restrain the pulley somehow.

 

@NutzAboutBolts Prius had around 300k miles IIRC.