Figured I'd take a little time to pass this info along, since I get a lot of questions when I talk about the subject in a group. You see, the Morris G5 magneto is the best invention since... uh... probably toilet paper. It's the holy grail for old bikes and makes your ignition system and scaled down electrics pretty much bullet-proof. I won't say perfect, but as long as you don't use those crappy see-through colored caps, you run into very few problems.
The big issue with the G5 is that Morris claims it fits without any alteration of the cases or removal of push rods. Well, that may be true for 1% of the cases Harley cast back in the day, but my experience is that you are going to grind no matter what. Some may just be luckier than me. Also, if you work on mid-to-late-60's Pans and Shovels, you may also find yourself missing a hole. As in the hole for the rear base mount on the G5 housing. They tell you this in the instructions, but it's still a pain when you find out "this means you".
I didn't take photos of every step, because I'm just not wired that way. Me and the "instagram" way of life just don't meld. I did grab a few, and will try to give you words in simple terms to fill the gaps. This may be old news to many, but I'm sure there's a few out there that could stand to hear it again even. So anyways...
First thing I do is get the base settled into the hole in the case and see how much clearance I need. Measure the gap between the case and the lower portion of the base to get the estimate, but you'll have to take the "small cuts and many measurements" approach to get it just right. By that, I mean grind in small amounts and reset the base each time to see how your doing, because since the case is a curved surface, your grinding point will change multiple times as you progress.
Note that I always take the front push rod out entirely, just to give myself room. Especially since this was one where I had to drill the back mounting hole as well. (66 Shovel here with OE matching cases) Tape and cover everything, because the grinding dust is super-fine and will get EVERYWHERE and into EVERYTHING.
Once you have the base set, mark out the area to be ground down. Realize that you aren't going to make a simple angle here if you want it to look good. It's a compound curve on the bottom of the G5 base, so you need to match that. If you just hit the cases with a simple slant, you won't be happy in the end. To each their own I guess. Notice I take material down much further up the "neck" than the G5 actually covers. This is to make sure the curve radius is a gentle slope and not a harsh angle just where I'm putting the mag. Makes it look like the cases were made for it. I mark my area with sharpie and start at the back (toward the head) and work my way forward checking after each little bit to see how the base fits.
Once I get it snug in there (I leave about 1/8" between the base and the case at all points if I can), I take a rock of preferably granite or quartz style, break it with a hammer and gently tap it around the ground area. This gives it the look of the original casting if you do it right. Takes some practice, so if you can find some scrap first, try there before going to town on your cases.
Now that's done and looking good, so I can move to the mounting issue. No back hole for me, so I have to make one. The G5 base is oil-filled (unless you forget to put oil in!) and so it needs the seal and you're forced to drill your cases. I do this with the motor still in the frame and as long as you have the right tools and a steady hand, you don't need to remove heads or jugs. Just make sure everything is still wrapped up tight so no dust or debris gets in the motor.
I use a right angle air grinding base for my "drill". The chucks on these won't usually fit the right drill bit, so you need to wrap the bit in tape at the end so the chuck grabs. We're not drilling stainless or at high speed, so the bit won't move around as long as you tighten your chuck down good. Then take some more tape and mark your depth to drill by wrapping the portion of the exposed bit so only the amount of metal is exposed to equal your drilling depth. We don't want to just punch through the case into the inside here for what I hope are obvious reasons. The bolts provided in the G5 box are a little long for my taste when drilling, so I took about a 1/4 off that to find my depth. It's usually about 3/4 or so. That gives you enough room to tap 5/8" of thread to grab and that's plenty.
You can't just measure the center-to-center distance of the mounting holes and randomly put your new one though. You need to set the base again so the gear mates up with the motor, and then I take another drill bit and dip it in marking fluid and shove it in the back hole of the base to make my drilling mark. Then get a center punch and make your starting mark in the case after taking the base back out and covering the hole.
Now drill. Slowly. I usually back the bit out once or twice so I'm not crowding the hole with debris and use cutting fluid to keep it cool. I'm paranoid around cases. Call me crazy. Once the hole is drilled, take your tap and the smallest hand chuck you've got and gently thread the hole you drilled. Make sure to blow the extra debris out before doing this. Again, I back the tap out once or twice on the way down and use more cutting oil.
Like I said. Lube. Plenty of it.
Now you should be good to go, so put your front push rod back in and then mount the G5 for good. Tighten the base down and follow your instructions for filling oil, setting the timing etc. Make sure the rubber seal is seated good when putting the cap on or use a cork one instead. There you have it. Best ignition around!