Barrels and Pistons

We take a closer look at the barrels and pistons fitted to the VW Aircooled engine.

An Air-Cooled VW Engine is known as a Flat 4.

This is because there are 4 pistons that are not upright like traditional engines but on the side of the engine.

Two pistons are located on each side of the crankcase and attach to the small end of the con rod with a gudgeon pin.

The combustion inside the cylinder head pushes the piston down the barrel and rotates the crankshaft via the con rod.

Cool Air stock a range of standard size kits for each engine size and if you are looking for more power then we also stock a complete range of ‘Big Bore’ Barrel and Piston sets.

All 1300cc to 1600cc engines use the same outside diameter barrel and therefore you can increase your engine size quite easily by simply fitting a larger size.

You do however need to change or ‘bore out’ your cylinder head when it comes to 1300cc engines.

We also sell Slip in Big Bore 87mm pistons which when running the standard 69mm crankshaft will increase your engine size to 1641cc. This is a very popular conversion for those of you that want a little bit more ‘bang for your buck’.

90.5mm pistons and above will require both your crankcase and cylinder heads to be machined.

When fitting a long stroke crankshaft we also sell stroker pistons which have the gudgeon pin located higher up the piston. This helps to reduce the width of the engine and reduces the thickness of spacers required in order to set the deck height of the piston.

All barrels and pistons are sold in complete sets of 4 and are not available separately.

You will need to replace you barrels and pistons if 1 of the 4 reasons are true:

  • Worn piston ring grooves

You can check if your piston ring grooves in the pistons are worn by fitting a new piston ring.  Once the new ring is installed you need to measure the space between the side of the piston ring and the groove in the piston.  To do this you will need a set of feeler gauges.  If you can fit a feeler gauge that is any wider than 0.004” then the groove is worn and the piston will need to be replaced.

  • Barrel Wall Damage

If your barrel walls are damaged then the chances are you will need to replace your barrels.  Barrels can taper over time which normally occurs at the top of the ring travel.  Engines can tolerate some taper (up to 0.010”) but this will cause the rings to go in and out as the pistons go up and down the barrels.  This movement will cause your rings to wear out quicker so ideally you will be better off replacing everything.

  • Broken Fins

The fins are designed to cool the cylinders with the air passing through them.  All fins should be in tact without damage.  If any fins are cracked or broken then the barrel should be replaced.

  • Upgrading to Larger Displacement

Bolting on a set of larger pistons can dramatically increase the power output of your engine.  Remember that all 1300cc engines and above can be increased in size to 1641cc by simply installing a set of 87mm pistons (Please note cylinder heads will need to be changed on 1300cc engines but not 1500cc or 1600cc engines).  Any larger than 87mm will require both your crankcase and cylinder heads to be machined in order to accept the larger barrel diameter.

Barrels and Pistons are always sold in complete sets including barrels, pistons, piston rings, gudgeon pins and gudgeon pin circlips.  The cost of standard sets can be relatively inexpensive compared to labour charges of cleaning the old ones and installing new piston rings.


Once you have had a good inspection of your barrels and pistons you may feel that honing the barrels is all that is required.

Honing barrels is what is required to bring the barrels to a precise size and finish.

First you will need to support the barrel somehow so that it does not move while you are honing it.  If you have stripped your crankcase then the ideal way to support you barrel is to insert it back into the crankcase.  Be warned though that once you have finished then the crankcase will need a through clean before you re-assemble it.

Once you are set and your barrel is supported you will need to coat the walls of the barrel with honing oil.  This is like very runny engine oil.  Once coated you then need to run the hone from the top to the bottom of the barrel while rotating it in a drill on a very slow speed.  You are trying to create a little bit (but not too much) of roughness so that the new piston rings will wear in.  Remember you are not trying to remove too much metal but just give the barrel a new finish.  Books on honing always recommend trying to achieve a 30 degree crosshatch.  For the amateur honer then this is a hard thing to get accurate.  But just try to get some crosshatch into the walls.  Each barrel should normally be done within 30 seconds or so.  Make sure that you keep the walls lubricated with oil as you go.

Once you are happy with the finish you then need to thoroughly clean the barrels to get out all of the cast iron grit.  Soap and warm water will normally do the trick in the sink.  Wipe the walls with a cloth until all the grey deposits are gone.  Once you can wipe a clean cloth inside the barrel and it stays clean without any grey colouring then they are ready to install.