6V to 12V Conversion
VWs converted to a 12 Volt system have brighter lighting, better starting and modern sound system options.
Here’s how and what you need:
When installing a 67 or later (12 volt) complete engine into an earlier car, about ½ the work is done for you. You need only those things that are not on the engine (bulbs and relays for example) and match up the Starter & Flywheel (see below)
First - a list of what's needed for a complete conversion (with a 1200 6v engine), followed by the procedure.
Parts Needed:
12V Generator or Alternator 
(and matching regulator)
Correct oil stand that supports either a Generator or Alternator. (Alternator Conversion Kits are available that include everything needed).
Present-day alternators are internally regulated, so you’re covered. But you may need the later style fuel pump which is angled to make room for the alternator. The carburetor  accelerator pump linkage can be a problem too as it will interfere with the alternator. Either grind a small hunk out of the alternator casting or raise the carb on a spacer block. Best is to simply flip the linkage upside down which is what VW did while updating to the 34PICT-3.
Other 12 Volt items you may need:
12v Coil 
12v Turn Signal Flasher Relay 
12v Choke element 
12v Headlight Relay 
12v idle cut-off Solenoid for carb 
12v Bulbs (All exterior and interior).
12v Radio (or voltage reducing kit).
12v Wiper motor (or voltage reducing kit).
12v Horn. (Optional) 
12v Starter / Flywheel Match-up:
When VW finally switched over to the 12 Volt systems in 1967, they also changed the number of teeth on the flywheel from 109 to 130. This increased the size of the flywheel by .160". With that change, the car now required a different pinion gear, pinion shaft, and a different pinion shaft bushing in the bell-housing. The bore in the bell-housing for the bushing also changed in size.
These slight changes make the job changing over to a "true" 12 volt system a little more difficult.
The easier way is to leave the 6V starter in place, and run it on 12V. It will survive for a very long time if you don’t crank it for more than 5-10 seconds at a time.
Usually, if you're keeping the original 1200/1300 engine, re-using the old 6 Volt starter really makes sense.
If you're switching over to a later 1500cc (or larger) engine, you either have to bolt on a 6 Volt flywheel or change to a 12 Volt starter 
If you do decide to use a 12V starter, you have to use the 12V version of the flywheel and Dremmel out the bell-housing for the larger diameter of the flywheel.
If the flywheel is swapped out, be sure to reset the end play to 3-5 thousands.
12 volt pinion bushing replacement:
Special replacement bushings  are available to use a 12V starter on an old 6V bell-housing.
The procedure for changing the bushing is simpler than most people think, Insert the correct size tap in the inside of the bushing, tap it all the way in until it bottoms out, then keep going. The tap will draw the old bushing out like magic. Tap the (slightly greased) new bushing in with a large brass or copper punch and a small hammer. It’s that easy. Another way is to use a starter from an auto-stick car  . These are self supporting and didn't use a bushing in the bell-housing. These starters bolt right in, and are also more powerful.
Clutch and pressure plate compatibility:
On earlier cars (both swing-axle and IRS), the throw-out bearing was held onto the shaft with 2 U-shaped clips and the bearing was free to pivot on its mounting pins.
This design is fine, and can hold up to plenty of miles and even HP increases. But, in the typical German fashion, they had to make it better...
The redesigned bearing  is more firmly anchored by new clips and is supported by a guide tube which is bolted to the bell-housing, and runs through the center of the bearing.
It also has a wide thrust surface built into it. This change not only kept the bearing straight, but also eliminated the need for the thrust ring on the pressure plate.
To say it simply, if you're installing a later (1600) engine into a 70 or earlier car, you will have to replace the pressure plate with an early one  which has the thrust ring. The earlier transmissions do not have the mounting holes for the guide tube.
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