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We serve the VW parts and VW Tech needs of owners of aircooled VW Beetle, Volkswagen Bus, Karmann Ghia, Type 181 Thing, VW Type 3 Squareback / Fastback / Squareback, and Type 4 VW. Tech Tips, project advice, and quality parts for VW Restoration, VW Performance and Custom Aircooled VW projects.

VW Engine Piston & Cylinder Operation

Increase your VW Engine Power with Proper Piston, Cylinder, and Ring Operation

VW Piston and Cylinder OperationThe way that pistons, cylinders, and rings work together to support engine power is often misunderstood or mysterious to enthusiasts. But anyone who wants to get the most fun and drivability out of their VW engine should take the time to understand how pistons and cylinders operate in relation to the air flow through their engine system.

Oxygen availability is a well known limiting factor in the combustive process. Maximize the volume of air and exhaust that can be moved through your VW engine, and you increase the potential for more power. Performance engine builders and do-it-yourself VW engine enthusiasts tend to focus on modifications designed increase airflow volume, or overall air capacity of their engine system. These modifications include cylinder head work, increasing camshaft size, improving carburetor intake/exhaust, running the engine at higher RPMs, etc.

But there is more than one factor that impacts the amount of air flow in an engine.

Airflow system integrity is an equally important, but  less obvious component that contributes to maximal power. Think about it. Let’s assume you have made every possible modification aimed at pulling large quantities of air into your engine. How much improvement will you see if an equal or significant amount of air is escaping via leaks in the air flow system? It’s comparable to trying to fill a leaky bucket! Where the air goes and doesn’t go once it’s inside the engine compartment is just as important as getting the air into your VW engine in the first place.

One way to maximize the amount of air available to your VW engine and increase engine power is to pay careful attention to the piston/cylinder/ring seals. Even if you only focus on this aspect independently, and don’t make any airflow volume modifications, you will successfully increase engine power and efficiency! A time investment in properly prepping the piston sealing area will pay excellent power related dividends.

If the pistons/cylinders/rings are not doing their job (sealing), you have a problem called “Blow-by”. Blow-by occurs when hot combustion gasses leak past the rings and into the crankcase. Besides the obvious loss of power and efficiency, many people aren’t aware of another consequence, which is the damage caused by hot exhaust and flames into the crankcase! You are literally heating your engine’s oil with a blowtorch. Yet another complication is the contamination of the lubricating oil with exhaust and gasoline. Based on these negative side affects alone, blow-by should be avoided – the added incentive of increased engine power will be a bonus!

Obvious signs of blow-by are carbon build up (nasty hard, black chunky stuff) and/or heat discoloration on the piston land (between the top and second ring). Bad cases of blow-by will even discolor the piston skirt to a black/brown color.

Let me explain even further, so you’ll be totally convinced. Ideally, as the piston is beginning to move upwards on it’s compression stroke, the air/fuel mixture is compressed above the piston and rings. The rings are sitting on the bottom of the ring lands as the piston drags them upwards. As the pressure increases, the rings (especially the top one) are pushed downwards even more against the ring land. The combustion pressure spreads across the upper ring surface and down behind the ring, and forces the ring out against the cylinder wall. The radial pressure of the ring tension is only used to START the sealing process, and cylinder pressure is what really pushes the ring outwards against the cylinder wall. For this to occur, the ring gap and piston to ring contact surface must be perfect.

There are multiple problems that can result when the piston and ring mating areas are ignored. Excessive clearance between the backside of the ring and the ring groove in the piston results in pressure building too slowly behind the ring, and leakage occurs because the ring is not being pushed out against the cylinder. This gap should be as small as possible.

Also, if the ring is not sealed against the bottom of the ring land, the pressure will leak past this area. Once the ring is leaking, the hot gasses from the chamber are like a torch to the metal surface of the piston land and the ring itself. This excessive temperature bakes the oil in that area and turns it to carbon, which reduces ring clearance to nothing and grabs the ring solid. Can you say, “ring replacement”? You don’t want to have to replace those on a regular basis! Although there is a second ring, it is primarily an oil scraper ring, not a compression ring, and if the top one leaks for any reason, your engine loses power.

So, now you know why it’s important, and you understand the power advantage that your engine will enjoy when you’ve minimized blow-by. You are likely anxious to find out, how do I do it? Most engine builders focus on prep work done on the cylinder wall, and have a tendency to overlook the contact areas of the piston and piston rings. Well, that’s the ticket! A properly prepped piston and ring contact area effectively creates an internal pressure system that is a self-sealing force. No more blow-by!

Now for the nitty-gritty:

The top and bottom surfaces of every ring groove must be completely smooth so the ring has something to seal against. The vertical clearance between the ring and the ring land should be between .002-.004″, preferably on the tight side. More than this, and the rings will leak. Less than this, and the ring will not float in the groove freely. Piston rings are designed to rotate in the groove during engine operation, and they must be able to rotate freely so that any carbon particles are cleaned away and out of the groove. (It’s kind of like brushing your teeth regularly so that plaque never has a chance to build!) Also the ring must be free to move so it can easily push out against the cylinder wall as combustion pressure falls behind it, and it can make an effective seal.

Piston rings also play an integral part in two other aspects of engine operation. The rings transfer heat from the piston to the cylinder, from which the cooling system can carry it away. The rings are at the top of the piston, where it is hottest. The rings are the PRIMARY contact between the cylinder and piston, since the piston is floating on a layer of oil. If the rings don’t properly seal against the cylinder, the piston temperature will increase, and you have even more overheating! You can see now how even just a little blow-by can become a runaway train of lost power and cooling!

The third piston ring is responsible for scraping oil off of the cylinder wall, and returning it to the crankcase (via the small oil return holes in the piston behind the ring). If the compression rings are leaking, then those nasty hot combustion gasses are reaching the oil scraping ring and turning the oil to carbon. Of course, carbon build up leads to ring seizure. When the oil ring seizes, it can no longer effectively keep the cylinder free of excess oil. That excess oil now can reach the compression rings, turns to carbon and causing their failure too. This excess oil can also bring the engine into detonation. How many people considered that a bad piston ring could cause a rod bearing to go bad? It can.

Phew! Don’t you wish you could forget all this? Well you can if you “prep” the piston for the rings you are going to install on your engine.

PREPPING THE PISTON-RING SEALING AREA

Many a “mechanic” can be seen scraping the carbon out of the ring grooves with an old ring (cringe!). I assure you that using this makeshift “tool” will result in the ring land sealing area being far from ideal, and will virtually ensure excessive leak down regardless of cylinder condition. The piston should be chemically cleaned (for example with an immersion style carburetor cleaner). Or, even better, go hog wild and buy new pistons! (They only range in price from $120-$200. Volkswagen piston sets are very inexpensive compared to other cars).

Next, ring end gaps must be kept within specifications, which vary according to cylinder bore size. Engines built to run very hot, i.e. for racing, should set end gaps on the loose side, since ring gap will tighten up as engine temperature increases. This ensures that the gap will not go to zero, which would result in binding, and “scoring” of the cylinder (goodbye ring seal).

During assembly, stagger your ring gaps. If you line all of the gaps up, you are creating a leak! I recommend staggering each ring gap by 120 degrees, but don’t be too particular about placement because the rings will rotate and move from their original placement during engine operation.

New rings are made too tight, and the builder should set the piston ring gap during engine assembly. Remember, you can always make the ring gap larger by filing or grinding, you CAN NOT add material if you take off too much or if your cylinders are too big (worn). Set the ring into the cylinder, and use a piston to push it square to the bore. Measure the gap with a feeler gauge, and increase as necessary. You can always increase the gap, but you can not make it smaller! Measure, and re-measure. Then as you are grinding the ring gaps, you should aim to make the ring gap ends perfectly parallel. Since some combustion pressure can and does leak down through this ring gap, it must be kept as small as practical. Too small and the rings can seize when hot, too large and the blow-by through the large gap is excessive.

There are other piston rings on the market that offer an engineering solution to the problem of a large ring gap. The gap is “eliminated” by using two thin rings that fit in one groove, staggering the ring gaps. These “gapless” rings are available in a Top or 2nd ring version. Aircooled.Net recommends the Total Seal gapless 2nd ring for street use, and the Total Seal gapless Top ring for racecars. Never ever use gapless rings in BOTH the Top and 2nd position in the same engine! The Total Seal gapless ring design definitely helps in ring sealing, but don’t expect it to eliminate all the other cylinder preparatory work! If you’re just replacing your rings (and not buying a piston set which includes rings) the best quality replacement piston rings are Deves brand. They are really easy to install and break in quickly, without headaches.

The cylinders should be round and groove free (not groovy!). If you are reusing cylinders, “honing” them will prepare them for a good ring seal. Be SURE to clean them with lots of Comet (or similar) and a Scotchbrite pad, along with liberal hot water — and lube them up with a teaspoon of oil immediately after drying to prevent rust. A light coat of black paint (high temperature, of course) on the outside of the cylinders will aid in cooling.

Once the cylinders are clear, I prefer to oil the piston skirts with just a touch of engine oil, leave the rings dry! The use of Quickseat Ring Powder will also aid in ring break-in.

Be sure to read our Engine Break-In article for the proper break-in steps when you first run the engine. The cam and rings need to be broken in as quickly as possible for optimal engine life and performance.


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