OP

Quoted from PinMonk:

So, I've posted some of this in a number of Stern threads from Ghostbusters onward when they started cost cutting on cabinet wood/glue/god-knows-what-else and cabinets started splitting. I just finished reinforcing an Iron Maiden and took pics this time so I can post more of a step-by-step instruction type thing this time, hopefully as a single reference instead of bits of this hidden in lots of different Stern threads.
The goal is to replace the wimpy almost no-support Stern leg brackets with the older, more robust Bally/Williams style brackets to give some reinforcement to the cabinets beyond what they ship with and reduce instances of splitting cab joints due to Stern's ongoing cost-cutting materials in their ever-more-expensive machines.
You'll need some supplies first. Here's your leg reinforcement shopping list:
Wood screws (#10 x 3/4" Coarse Deep Thread Pan Head Screws) $6.95/100. These are real wood screws with a nice, wide bite. Don't use sheet metal screws. The difference in bite is obvious when you compare it to these:
amazon.com link »

Titebond III glue $7.47/16oz. The best wood glue on the planet:
https://www.homedepot.com/p/Titebond-III-16-oz-Ultimate-Wood-Glue-1414/100522343

Bally/Williams Leg Plates (x4) $4.95. Marco has these too, I just get them from Terry at PBL:
https://www.pinballlife.com/index.php?p=product&id=144

Before installing for real, put the screw through the leg plate and line that up with the top edge of the cabinet to eyeball how far through the cabinet the screw will go. It should ALMOST come out the other side (like 1/16" from the other side). If you're concerned about this, use a lock washer to move the screw away from the other edge a little when you install it. Due to variation in cabinets, DO NOT INSTALL WITHOUT THIS CHECK FIRST. I don't want people putting pinholes through to the outside of their cabinets. I've done 5 machines and none needed a washer, came out the other side even a little, nor even made a pin-sized bump, but I ALWAYS PRE-CHECK, and you should, too.
Turn the machine off, and raise the legs off the ground with a pin dolly or in a pinch, a stool.
Here's the inside right of the cabinet. That vertical strip of metal is the "leg plate" from the Stern factory. Note that this brand new Iron Maiden Pro has this plate installed backwards from the factory. Those raised threaded holes should be pointing INTO the wood, and the black side of the plate should be facing out instead. What's become the usual lax Stern Q/A of late accidentally demonstrated fully when I did this upgrade.

Remove both legs from the front of the machine.
Here's the inside left of the cabinet. Note that this plate has the correct orientation with the raised thread part not visible because it's going INTO the wood as it should. It's a little more complicated because on Iron Maiden at least, there are two wire-routing clips that have to be removed to make room for the wider bracket and then re-installed slightly off from where they were originally. Remove the three small screws from the bracket and slide the bracket out from under the wire braid to remove it:

I'm only showing pictures of the inside left removal since it's the harder one. Here's a comparison of what the machine comes with, and the enhanced bracket and better screws we're replacing it with:

Spray some dry graphite lube into the threads on the new bracket so the Stern leg bolts you're re-using will glide in easier. We don't want the nice, new brackets to be cross-threaded!

Put the new leg bracket in place (note orientation of the two leg bolt holes should be situated near the TOP side) then screw in one of the leg bolts (without the leg) on the outside until it's finger-tight. Repeat with the bottom bolt. This will hold the leg plate in position while you put the screws in.
Note that I have the cabinet ground braid (the flat metal braided wire) lined up so it passes just behind the upper center screw hole in the new bracket (you can kind of see the metal through the hole before I put the screw in in the picture below). Since the braid only has to contact the bracket tightly, and the new leg brackets will press against the wood corner brace tight enough, it's not necessary to screw through it, but I did anyway, just to make sure I had a good ground. Your choice if you want to be this picky with your upgrade. Pressed behind the plate is probably enough:

Dip a screw in the titebond (I just take the cap off and dip because it's faster than squeezing some on a screw) and brush the excess off by scraping against the bottle edge, then with your drill on LOW TORQUE (so you don't overtighten and reduce the effectiveness of the grab on the screw into the cabinet), screw it into the leg plate. I do one on the top, then one on the bottom opposite side, then fill in the rest. If the plate gets a little crooked after you put the first one in, straighten it a bit, then re-tighen the leg bolts to hold it again and then repeat the process until all 8 screws are in.

Put the wire guides back on (if applicable), then remove the leg bolts, put the leg on, and put the leg bolts back on and tighten with a wrench. One side complete.

Repeat for the other side, then if you wish, the back two legs, too. The front ones are the critical ones, but I usually replace all 4 while I'm in there.
Congrats, you have reinforced your Stern cabinet to reduce the likelihood of it splitting on you!
ALL done with this project? Here's a couple other guides I've made for spike machines:
Want to put a quieter fan in your noisy Spike power supply? I made a guide for that, too. It's here:
https://pinside.com/pinball/forum/topic/replacing-stern-spike-noisy-ps-fans
Is your Spike machine occasionally resetting during hectic play, torpedoing your high score runs? This guide to fixing it might be for you:
https://pinside.com/pinball/forum/topic/fixing-stern-reboots-on-spike