Rebuilding a Batavus M48
I can't cover everything, I say unless you know you're stuff you should have a Batavus M48 manual on hand, there are lots of valuable tips and the proper methodology is laid out with pictures.
The goal here was to go inside and replace the 2 crankshaft bearings, their seals, and the various gaskets, in addition to cleaning out the engine's internals in general. M48's generally seem to be simpler than other engines, so many of the same principles apply when compared to other available rebuild wiki's. Buffalo balls.
Something I was told before I did this but didn't follow too much but now recommend: take pictures before you take it apart. Anywhere bolts are, just snaps pics and it'll be a nice timesaver and it'll put you at ease later on.
This image makes things pretty clear:
After taking off the head by unscrewing the stud nuts, you can carefully slide the cylinder off of the engine, and only the piston will be left, like shown below. After removing the piston via its snap rings and pin, you can see a bunch of buildup on the piston and stuff, clean it off if you can.
The clutch assembly (a nut is missing):
And disassembled, with only the hub remaining. You have to unscrew that central nut (either with an impact drill, or a piston stop to hold the crankshaft stationary), and then carefully heat up the clutch hub assembly (includes the bowl the clutch spring sits in, the top piece shown with that extruded gear-ish shape, and the screws that are pressed in which hold those two pieces together) and slide it off. It got heated with a MAPP gas torch and the aluminum billowed like a marshmallow in the microwave (low-quality aluminum??) and ruined that top gear-ish piece, so be careful. Note: You may be able to remove the clutch assembly without heating it up. It's worth mitigating risk from the torch, so give it a good solid shot before heating it up. Maybe a gear puller or something. idk.
This is the magneto side. The rotor (that big gray round piece with magnets) has been removed, you're looking at the stator. The coils of wire that you see produce electricity when a magnet is rotated around them (the rotor). These power all them electricalonics on the bike, like spark plug, head lights and brake lights. Each of those systems just mentioned has its own coil, which is why you can see 4-5 of them in the stator. Power!
Unscrew the screws holding the magneto to the crankcase. "Note:" Remember that the rotational position of the magneto here determines your timing. So make a note of it's position or after everything's back together be prepared to re-do the timing with a timing light or something idk lol. In the above image of the magneto, note in the background the wires that come from the stator and go through that rubber grommet in the crankcase.
In my opinion, the easiest way to pull those out is a) cut off the spade terminals at the ends of the wire, not at the stator end, the other end (and keep the terminals so you can either re-crimp them onto the wire, either with pliers or a special spade crimping tool, or so you can buy new ones at the store and have those for comparison), b) put a little oil on the wires and back them out through the grommet, and then c) push that grommet through. Kept the previous steps for reference, but you don't need to do any of that. Just spray some general purpose lubricant onto the whole wire/grommet interface and push the whole thing through. You can feed all the wires through the hole in the crankcase afterwards if you do one at a time. That grommet is flexible and shouldn't rip even if you really deform it, but go slowly - it's a prime candidate for handamage. Once you pull those wires through, you can remove the three screws that hold the stator to the crankcase.
Now that both the magneto and clutch are removed, you can also remove the two black, lobed engine mounts via their 4 bolts. Make a note of which way they're facing.
Once you have the crankcase with everything removed (stator, clutch assembly, and piston - no plastics either), you can throw it in the oven (~300F depending on the oven, for like 15-20 minutes), and then bang it apart with a rubber mallet. Be careful - it's a spicy meatball at that temp.
This is the engine totally disassembled and in pieces. You can see the crankcase halves on the left, the cylinder in the middle, and below the cylinder the crankshaft itself, with the old ball bearings still on. Note: When disassembling the engine, the bearings will likely stay on the crankshaft, and then you have to removed them with a bearing removal tool or some half-baked measure. I did it by kind of hitting two screwdrivers in towards the crankshaft from the outside, and it slowly pushed the bearings up enough that I could grab it with a gear puller or something. Once all this stuff is removed clean the crankcase and crankshaft of deposits, being careful not to scratch surfaces that mate with other surfaces or require a smooth finish.
During reassembly with new bearings (remember there shouldn't be any seals between the inner and outer races - as in, the balls and the cage should be plainly visible - this allows oil to penetrate and lubricate the bearings.... baby), you first heat the crankcase (212F is the magic number - it'll boil water sprinkled on it immediately. Just do 300F for 15-20 minutes in the oven like before.) and then slip the bearings into the crankcase first, not onto the crankshaft. If you throw the bearings in the fridge/freezer while the crankcase halves are in the oven, and then coat both the outer race of the bearings and the ID of the crankcase pocket with some 2-stroke oil, the bearings should slip right into the crankshaft - you'll only get one shot, so hold the bearing by the inner race with your finger, make sure it's lined up, and it'll fit like a glove. If it doesn't fit right away you'll probably have to reheat and refridge the parts and try try try again!
Bang (no need to heat) the crankshaft/crankcase seals into the crankcase halves. They are different on each side (see pic below).
Replace the crankcase gasket, give it some 2-stroke oil to help it seal and stick to the crankcase. Then you reheat the crankcase halves (with bearings assembled inside) again, and bang the crankshaft (use oil on the crankshaft to assist the slip-fit) into the bearing-crankcase assembly (you'll do it twice, once for each half). So remember reassembly differs from disassembly. I did it backwards (bearings on crankshaft first), and had to redo the whole thing. It sucked. Don't do it.
Bearing dimensions (42x15x13):
Crankshaft/Crankcase Seals. The smaller on the left is for the magneto side (30x15x7) and the right is for the clutch side (42x20x7) - the larger ID is because a part of the clutch hub slips through there, not just the crankshaft like on the magneto side: