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Understanding Air-cooled VW Crankshafts

Crankshaft in Case

Understanding Air-cooled VW Crankshafts

Responsible for converting a circular motion into something more longitudinal for the pistons and connecting rods, the crankshaft is an integral part of your Type 1 air-cooled VW engine. Read on as we take a look at how to select a crankshaft and what different options will mean for your engine build (and budget).

Since the VW motor was intended for reliable family transport it was designed to not rev very highly, to help prolong engine life. Unsurprisingly, in its standard form, it won't live very long if you rev it much over 5000rpm and at around 5200 to 5500rpm is where things will start to break.

But don't worry, as it turns out, there's plenty of room for improvement in the bottom end. Read on…

Standard VW crankshafts explained

Type 1 cranks have four main bearings and a stroke (ie. how far the piston that is attached to the crank will move in and out) of 64mm for the 1200cc motor and 69mm for the 1300/1500/1600cc motors. While 1200 cranks are known to be a bit weak, the 69mm cranks are pretty tough, since they are generally - though not absolutely always - forged rather than cast.

Forging is a far superior way of making a crank, which involves pounding the still hot steel into a crank-shaped mold, with a giant power hammer. This aligns the internal grain structure of the metal, making it very strong, a must for any higher engines producing higher than stock output. The alternative is a cast crankshaft. This is done by simply pouring metal into a mold and leaving it to cool down. The results tend to be more brittle and consequently not so strong by some margin. That said, even forged VW cranks aren’t super rigid, but at least you shouldn’t have to worry about the stock crank actually breaking under normal use. Even with some fairly ‘spirited’ driving, it most likely won’t be the part that lets you down. Having the stock crank balanced is a worthwhile tip.

What are counterweighted crankshafts?

What the stock cranks don’t have, however, are any counterweights. These are found on the crankshafts of most modern engines and serve – as the name suggests – to counterbalance the weight of the piston and conrods. These not only help to make the motor run more smoothly, they also help to extend its lifespan since they will reduce the cranks tendency to bow and therefore try to beat its way out of the crankcase.

Now Dr. Porsche was surely well aware of this when he designed the motor in the 1930s, but since the car was never intended as anything other than family transport, then you can start to see why he didn’t bother to add counterweights. That doesn’t mean that you shouldn’t though. In the past it wasn’t uncommon to actually weld counterweights to a stock crank and balance the whole thing afterwards (and it worked perfectly well), though now it’s much easier to just buy a counterweighted forged crank with them already added. So you’ll be able to rev the motor higher without worrying so much about causing damage and it'll feel much smoother than a non-counterweighted motor. The only slight downside is that it adds some mass to the rotating assembly, which takes more energy to spin up, thanks to the added inertia.

How do stroker cranks work?

The next step up is going to a crank with a longer stroke. These are also special forgings - and will all have the added counterweight flanges - and the longer stroke is a great way to gain some torque as well as displacement. While you might understandably tend to think that bigger is always going to be better - and given that the prices of stroker cranks are roughly equal regardless of size, you’d go for the biggest one - but hold your horses. There’s something to be said for exercising a little restraint here, as the some of the longer stroke cranks are where you’ll start to run into clearance problems. 

Fitting a long stroke crankshaft

The larger ‘throw’ of the crank has to go somewhere and it, or more likely the con rods, can start to interfere with the camshaft, crankcase or cylinder skirts, in which case, you’ll be reaching for the die grinder or perhaps even having to buy a crankcase with a raised roof.

You may also need longer cylinder barrels (or spacers underneath them) to accommodate the extra stroke, to stop the pistons popping out at the top. Or you can use shorter pistons and longer than stock con rods. Longer rods will stop the angles the rods work at from getting too acute and pulling the pistons sideways, but you want to avoid not having enough support for the piston at the bottom of the stroke (remember the pistons will come in further, as well as going out further). So as you can see, it quickly gets complicated once you get past a certain point. If you use spacers or longer cylinders, then the extra width of the motor has other knock-on effects - push rods will need to be a custom length and even your engine cooling tinware may no longer line up properly. It’s all worth considering as it's an added expense and hassle that you may wish to avoid, just to try and squeeze out those last few extra horsepower.

Understanding crank pulleys

The standard item is a perfectly serviceable, stamped, sheet metal pulley, though aluminum degree pulleys are a common addition to help timing the engine. Some performance pulleys are of the heavy ‘equalizer’ type, which adds weight to the opposite end of the crank to the flywheel. These are meant to work in a similar way to the harmonic balancers that you find on some other engines, by reducing the twisting forces acting along the crank and in turn reducing vibration and unwanted harmonics. You can find a variety of opinions on the effectiveness of these heavy pulleys. Smaller pulleys can sometimes be found too. The idea of these being that you free up power by using less power to turn the fan. The downside is that you stand a good chance of overheating your motor, given that the fan is going to be turning more slowly. Probably best used for drag racing only. 

Fitting the flywheel

The flywheel is held in place on the end of the standard crank with a large gland nut and prevented from spinning on the crank by four dowel pins. This is known as a bit of a weak point and if you start revving the engine up past its usual stock limits, you can lose the flywheel – and you don’t want that to happen. To fix the problem, an extra four dowel pins can be added, which is imaginatively known as ‘eight dowelling’ of the crank. For serious racing applications, some cranks have a different design at the flywheel end. These 'flanged' cranks have an enlarged plate on the end, that will accept not only additional dowels, but has room for six bolts too, for ultimate strength.

So, there you go. As always – you can never know too much - so be sure to have a thorough read up on this stuff before you jump in, and work out what it realistically is that you want from the engine. As with all aircooled engine upgrades, you can easily (and not too expensively) make vast improvements over the stock set up. But as with all things tuning, extracting those last few horsepower can cost you - not only financially, but in terms of engine life too. 

James D

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