Let's first examine the stock pistons. The 1700cc used a 90mm bore, 1800cc used a 93mm and the 2000cc(2.0L) used a 94mm bore. All of these pistons feature 24mm wrist pins and standard skirts. The cylinders are generally interchangable, but the top of the 1.7 cylinders is smaller than the 1.8/2.0 cylinders. So retrofitting a 93mm set of piston/cylinders to a 1.7 only requires flycutting the head to match the top of the 93mm cylinders.
The pistons were manufactured in various configurations. There were flat-top, dished and domed pistons for use the wide ranging applications of these engines. The domed pistons were used to increase the compression ratio in applications such as the 914 and the Euro spec. 411/412s. The dished pistons were used to decrease the compression ratio for use in the Transporter and in the 411/412 variants (station wagens). For the most part the high compression ratio was 8.6:1 (domed pistons) and the low compression was 7.3:1 (dished pistons).
If you are looking to build an engine for a Bus, your safest bet is to stick with the 2.0L bore of 94mm. The extra weight and unyielding aerodynamics make the engine work over time. You will need all of the extra cooling with the thicker cylinder walls. Save your money and get a longer stroke crankshaft to get the low RPM torque that your Bus needs.
Plan carefully if you are thinking about mixing and matching stock pistons, cylinders, connecting rods, and cranks. The wrong combination can give you too much deck height or move the piston out the top of the cylinder. For instance, installing 94mm piston/cylinder set on a 66mm cranks with stock rods will yield way too much deck height. Installing 90mm or 93mm pistons on a stock 2.0 bottom end will push the piston out the top of the cylinder.
If you are looking to get the most power out of a Type 4 and are unwilling to sacrifice it's legendary reliablitiy, this is the bore range to consider. These big bore kits give you added displacement without the problems of sealing at the head and cylinder distortion associated with the larger bores.
Many manufacturers offer a slip-in 96mm piston/cylinder set for both the 1.7 and the 1.8/2.0 engines. The only difference between the two is the smaller outer diameter of the top of the cylinder where it mates to the head. Some even offer pistons that have the wrist pin moved up for larger stroke cranks.
It has been found that the 96mm bore is the best combination of increased power with stock longevity in a Beetle. This bore, combined with a 2.0L stroke crank will yield a 2056cc displacement. Throw in a longer stroke crank, like a 78.4mm, and you'll have 2270cc of long term reliability. This has been found to be a great investment for a street engine.
Some builders have noticed that the quality of the cylinders that are supplied with the 96mm piston/cylinder sets have been substandard. They have found that the bore wasn't within specs, the top/bottom of the cylinder weren't parallel and level, etc. They claim that the amount of work involved has made their use quite limited.
The trend now seems to be going towards using the stock 2.0 (94mm) cylinders, which are then bored to specs of the pistons being used. The use of German cylinders, bored to 96mm, has made the 96mm bore the choice for most street cars. They offer a long life like the stock bores but with increased performance.
EMPI and CB Performance have both sold a 100mm set in the past, but as far as I know, they are no longer available. In fact the EMPI sets had aluminum cylinders with steel liners. These kits will require the crankcase and the cylinder heads will need to be machined to match the larger cylinder. They are a highly desirable find, but I can not speak if the aluminum fins cooler quicker.
LN Engineering is now offering 102mm piston/cylinder sets that, if they live up to the hype, will at last bring BIG bore performance to the street crowd. These kits feature cylinders that are CNC billet nikasil-plated solid aluminum. This is the same material that is used in the large bore factory Porshce 911 engine.
This kit looks like it could provide 100,000 miles of trouble free motoring with excellent power. It seems that once you get beyond 96mm, using cast iron for cylinders becomes a weakness. Cast iron just isn't up to the job, and it has problems of staying round and true. Many tuners have used five and six head studs per cylinder to keep the cylinders from warping too much and leaking at the head.
Due to the extraordinary work involvement in the manufacture of the cylinders, this kit is not cheap. They are actually more of an investment, as the Porsche parts are known to last through multiple rebuilds before needing servicing. As these cylinders become more wildly used, you can be sure that I will post how they work out.
For the engine whose primary purpose to make horsepower, and a lot of it, with little regard for high mileage, the larger piston/cylinder sets are the way to go. The consensus among most engine builders is that the 103mm and larger bores are only good for about 20,000 - 30,000 miles before the cylinder will need truing, honing, and the pistons will need re-ringing. In other words, it's great for a weekend cruiser, but not ideal for a daily driver.
Numerous suppliers sell the popular 103mm piston/cylinder set, along with 103.5mm, 104.5mm, 104.75mm and 105mm sets. All of these require the case and heads to be machined in a similar manner to the 100mm set. They are available in pin heights for 71mm (stock 2.0L) to the most common stroker crankshafts. Commonly seen on the pistons for the larger stroke is the 22mm wrist pin. This allows the center of the wrist pin to be closer to the crown of the piston and more stroke.
Most Type 4 tuners and builders recommend that bores of 103mm or larger require that the case be at least "fifth studded". They weld a block on top of the case, drill/tap it, machine the cylinders, and weld/machine the heads to accept a fifth head stud on top of the cylinder. This is said to keep the head firmly in place with the large sealing area of these cylinders.