Hello, I have a 1990 Gtu and I have had the same problem for awhile. The flywheel nut. Originally, I thought I could get away with a 54mm axle socket and a Chicago electric corded impact and call it a day... boy was i wrong...
I ended up going the more proper route of getting the 3/4 drive impact 54mm socket from mazdatrix, along with the flywheel holder bracket and the racing beat flywheel breaker bar, a case of pb blaster, and a couple extra tanks for my torch. The last few nights i have completely doused the nut and threads of the flywheel in numerous cans of pb blaster, torched it multiple times, and tried using the Chicago electric and my dewalt impact. And yet again im down a case of modelo’s and hope.
Tonight i ended up getting a snap-on impact gun rated for “700 foot pounds”. So i gave it another shot. Torched it over and over, had multiple people give a shot at it, and nothing. Hand full of beers later, i got back underneath and put on a youtube video to watch in the background and torched the nut at the treads for 20 minutes. (Now when i say 20 minutes, i mean a literal 20 minutes). The flywheel was so hot the teeth could burn you. So i hit it with the fully charged snap-on impact again, let that mf sing like no other. I sat the gun on there for 3 minutes flat. nothing. I got the racing beat breaker bar, ratchet strapped the motor mount to the subframe, and lifted it with a 3 ton floor jack. Long story short. The motor popped out of the mount and i have no more beer.

what in the hell do i do from here

 

I needed a heavy duty air hammer for mine. You're forgetting its a 3/4" drive nut. That thing was probably installed at the factory with some crazy drop down 3/4" drive heavy duty air impact gun.

 

How big a boy are ya?

Whenever you buy power tools, regardless of manufacturer, there is some stretching of truth as to what the measurements actually refer to. Ave has done quite a few videos debunking inflated power tool claims.

As far as the flywheel nut is concerned, you are possibly fighting sealant on the back of the nut. This is even more likely if someone has already rebuilt your engine, and resealed the nut according to the manual.
Assuming this to be the case, an impact likely wont help you as much, unless you use one with enough movement per strike to break both the threads and sealant loose.
I've only been able to break them loose with a 6' breaker, and a lot of effort. In my case, I needed to maintain the rotation of the nut, or it would go back to the way it was.
A word of caution with this method, as I chained my engine to my workbenches. Safety glasses, ppe, hard hat. You will be storing a LOT of potential energy in this system. Once the nut breaks loose, it will all turn into kinetic energy. Ensure that nothing can fall on you, and you can't be tossed into something.

 

I needed to get creative when removing mine.

I have a Racing Beat flywheel nut wrench as well as their flywheel stopper. They're basically just long metal bars which secure to the nut and flywheel respectively. The only thing that would get mine off is by arranging them so the bars made a "V" shape, then me and a buddy used a floor jack tilted on its side to spread apart the V and finally it came off! But let's get these priorities in order here, first things first, get some more beer man!

 

Cut a notch in both sides of the nut with a dremel and hit it with a chisel to split the nut, dont cut into the eshaft threads tho. Then buy a new nut

 

Last time I did mine, I used a Craftsman 3/4" drive 2-1/8" 6-point socket (53.98 mm) and a Craftsman 3/4" drive sliding T-Handle with a ~4 feet cheater bar attached. I bolted a 6' piece of steel flat stock to 2 clutch hold down bolts on the flywheel. Obviously, the 6' piece of flat stock served as a stop against the ground to prevent the flywheel from turning. IICR, it took a little of effort but the nut broke lose without any issues. The car was on a lift, which made it a little easier for me. I have never used Loctite when reinstalling the flywheel nut.

 

I bought one of . I don't know what the Mazdatrix socket is like, but the long thin-wall axle nut style is likely to have too much flex in it. I already had the 1/2 to 3/4 adapter around (not sure if it was impact rated) and used an old pneumatic impact wrench I've had around for ages. It banged off in about 10 seconds; it was actually easier to remove than the front main bolt. I may be an outlier, but it definitely wasn't that bad.

I've found that the best results I've obtained for stuck bolts (although usually due to rust) was alternating the temperature quickly. So I would get the nut really, really hot and then use a wand on the air compressor pointed at the threads to try and shrink it quickly. A canned air duster works if you don't have a compressor, but make sure the torch is off (obviously). Do this a few times. If that doesn't help, open all of the windows for ventilation, then get the nut really hot again and spray the PB Blaster on the threads while it's still hot. It will cool quickly and hopefully suck some of the PB Blaster into the threads. Keep alternating heating it up, then dousing in PB. Make sure to wear a respirator, it will get smokey and the smell is nauseating.

 

I've used all kinds of different tools. 6ft pipe with a 3/4 breaker bar on a few. A $89 Kobalt 700 ftlb 1/2" impact on a few. My Dewalt 1200 ftlbs 20v impact gets them off in about two hits. I use the Dewalt as my main impact at work multiple times every day and I've only had a couple times where I had to give up.

 

I might be a lucky outlier too - never had a problem breaking loose the flywheel nut. Just used my Mazdatrix flywheel lock tool, a Horror Freight 1/2 drive "earthquake" impact gun, impact rated 1/2" to 3/4" drive adapter (also Horror freight) and the same 54mm 3/4" drive socket the other guy linked from Amazon. Blasted that sucker right off every time in 10 seconds or less.

Now my luck with the front E-shaft bolt is another story - had a few episodes busting 1/2" drive breaker bars trying to get that damned bolt loose, but that was before I got a compressor & air tools.

 

I have a 19mm inertia socket at work that I use on Honda crank bolts for doing timing belts and it gets those off in a couple hits with my Dewalt 1200. I was told never to tighten the crank bolt with that socket as it will likely break it off in the crankshaft.


Mine is Cornwell, but you get the idea. It's just a very heavy socket. Maybe they make one in 54mm.

 

+1 on the Earthquake...despite being a cheap brand, the gun is an absolute animal. I had absolutely no drama whatsoever breaking a flywheel nut with an old Earthquake gun over 5 years ago.

And FYI, 700 ft lbs is not very impressive for an impact gun...especially for the job we are talking about here. Likely part of your problem.