By Bob Donalds
Air-Cooled What?
As anyone with any experience with air-cooled engines will tell you, this is a much different beast from the water-cooled engines that drive most cars today. Because these engines are no longer being put in new cars, it can be very difficult to find mechanics who are knowledgeable about the idiosyncrasies of these machines. Furthermore, the Bosch fuel injection system that delivers fuel to many of these engines is not always understood or properly maintained.
The main distinguishing characteristic, and cause of many of the problems associated with air-cooled engines, is heat. Because there is no water jacket, the heads on these engines run at a much higher temperature then their water-cooled cousins, and thus there is a much smaller margin of error in which these engines operate.
Because air-cooled engines do not have water jackets around the cylinder heads to keep the temperatures lower and more uniform, it is easy for hot spots to form and for cylinder head temperature to become a problem. Because air-cooled engines have only the air moving over the fins and oil to keep them cool, they run at a much higher temperature then a water-cooled engine, leaving a much smaller margin of error for sustained engine performance. Adding to this challenge is that these engines were designed in a time of much higher fuel octane, so today’s fuels force engines to work harder to produce the same power. One common condition caused by the lower fuel octane is pre-ignition, which occurs when the head temperature gets high enough to ignite the fuel before the spark plug fires. This can result in a snowball effect in which pre-ignition causes higher head temps, which in turn cause more pre-ignition, etc. Also adding to the heat problem is that most of these engines are pushing around heavy vans with a lot of wind resistance, which puts a heavy load on the engine. All of this can result in blown head gaskets, seats falling out, valves burning, poor performance and shortened engine life. Another common problem with these engines is oil leaks. Since we normally are rebuilding used engine blocks, it is easy for a small fracture or oil leak not to appear until the engine is assembled and running. There are several steps that can be taken to minimize the chance of a surprise oil leak when the engine is finished and in the car.
Disassembly and Inspection
Its time. Your engine is dead or in need of a serious overhaul, and you’re ready to get that sucker out of the car and breath some new life into this tired old horse. You can learn a lot about your engine’s life from this part of the process, and a good diagnosis of what killed your car will determine the best way to heal it. I’m not going to cover exactly how to get your engine out or take it apart, because that is all covered in any decent manual and would require pages of instruction, but I will highlight the things you should be looking for as you go.
Before you remove anything from the car, take a look in the engine compartment and make sure all of the cooling tin is in place. Many people think this is unnecessary, but it is absolutely essential in keeping the air moving over your engine to prevent it from overheating. If ANY of the tin is missing, put it on your parts list because its got to be in there. Its not so easy to find anymore, either, so expect to hunt around a little.
Pull the engine from the car and remove the cooling tin and intake and exhaust system, keeping a list of what is missing or no longer serviceable. For instance, injector seals become brittle and are a common source of vacuum leak. Fuel lines become brittle and crack, and I replace them with every job. They are typically original parts, and fuel isn’t something to mess around with. A fuel leak can cost you your vehicle.
Check out the flaps in the fan shroud, and make sure they are still in place and moving easily. I replace the thermostat, but if you don’t have the cash, you want to at least inspect it. They are designed to fail in the open position,. so if it is not expanded, its okay. This is another essential feature, because it allows your engine to warm up on cold days and stay cool on hot days. A working thermostat and flaps will increase the life of your engine, and too often people simply leave it out. If you opt to leave the old thermostat in the engine, check it periodically after you drop in the new engine, because the new engine will create higher temperatures then the old one while its breaking in, so if its going to go, its going to go then. The cylinder heads will tell you how hot your motor has been. In most cases, they are cracked and have experienced some valve recession. This means the valve is lower in the valve seat and it tells me the seat is soft. Black, sooty burnt oil around the exhaust valve springs indicate valve failure. New seats, valves, and guides are a must if you plan on having this engine last. You probably can’t do this work yourself, but a specialty shop such as Boston Engine can rebuild your heads or exchange them for new ones. This job requires a lot of skill and attention. This is not the place to save money, so make sure that these are done well and the correct parts are used.
Inspect the pistons for burns and the rings for wear. This can tell you about the life the motor has lead. Piston rings that are worn to nothing with extremely sharp edges are the result of unburned gas washing the lubrication off of the cylinder walls. Burnt or scored pistons tell you that the motor has been too hot. We’re not looking to reuse these pistons, even with new rings. This is just your engine’s autopsy. New rings on old pistons will not prevent oil consumption.
Before you split the case, make sure every last nut and bolt has been removed. There are nuts and bolts hidden all over the place on this block. Don’t forget the one on the flywheel side of the case. Never force the case apart. This requires some patience, and I would recheck it several times. When you think you’ve got them all, take a break and then recheck. Once you’ve got her open, its time to inspect the crankshaft for wear. Some indication of crankshaft wear can be seen from the bearing surfaces. If the copper color is showing on the main or rod bearings, there was a lack of lubrication. This is probably because of gasoline diluting the oil, or lack of oil changes. Remove the connecting rods and have the crank measured and the gears pulled by a somebody who does this all the time. Its not wise to go past the first undersize, because you go past the surface hardening. Make sure to find out if you need a new crank or simply a regrinding.
Any grooves or pitting in the camshaft lobes means it needs to be replaced or reground. Again, a machine shop should be able to tell you which of these options is right for you. There are two different styles of camshafts, one is made of cast iron, and is forged. Each of these require a different type of valve lifters, so this is the time to figure out what type you need to order. Only the original forged steel camshaft will have a gear riveted on. If the gear is bolted on, its most likely cast iron. Bob from Boston Engine wrote an article that explains this in detail, and he will gladly send you a copy if you mail a SASE.
Cleanin’ and Machinin’
Once you’ve got everything out of the case, including the dowel pins (save ’em!), the oil pressure relief valve, and the oil galley plugs (if you’re swapping them). pack up your case, heads, crank, cam, rods, and anything you want to clean, and head for the machine shop.
First thing to do is to degrease the case. They’ll have big chemical cleaning tanks to do this in. You also want to clean your crankshaft, camshaft, connecting rods, rocker assemblies, heads, oil cooler, tin work, and hardware at this point. Unless you are a real expert, you probably just want to hand your crank, cam, heads, rods, flywheel, and case over to someone who does this all the time. I already told you what has to be done to the cam, crank, and heads. In addition, you need to get your rods balanced, rebushed, and checked for size and straightness, your case checked for crank shaft bore size, and your flywheel resurfaced. Depending on the condition of your engine, you could be told you need to replace any or all of these parts. Don’t try using parts that don’t meet specs, because you’ll just end up wasting a lot of money on an engine that won’t last, and you’ll lose all the money you put into this rebuild. Remember that the aspects that make a successful rebuild are proper clearances and good parts.
At this point you should decide what you want to do with your engine sheet metal. Depending on its condition, you may want to derust and seal it with a zinc paint or some primer. I like a nice semi-gloss Krylon paint. It ads a nice shine to the project, and can protect hard to find tin work.
Selecting the right person to help you with your machine is vital. You don’t want somebody learning on your motor, so find someone who has been doing air-cooled you are in over your head. Volkswagens for a long time and knows what to look for and how to handle the job. Bob Donalds at Boston engine has been doing exclusively Volkswagen machine work since 1968, and he will machine or exchange any parts that need it. He also does a complete long block assembly if you think you are in over your head.
Get It Together!
Again with this section, I’m not going to tell you exactly how to reassemble your engine. Rather, I’m going to describe the changes I make and the points I’ve found that need special attention. Any good manual will walk you through reassembly. I recommend the Bentley manuals for this.
Before you put anything back in the case, you may want to drill and tap out the oil galley plugs behind the flywheel. These are push-in aluminum inserts, and they can pop out. To deal with this problem before it happens, I replace them with threaded brass inserts and then seal them with 3M weather-strip adhesive. The brass expands at about the same rate as aluminum, so it makes for a good combo. This is a lot easier to do now then on that cold morning when your engine starts gushing oil. If you’re going to do this, you might want to get them out and do the tapping before you degrease the case to get those aluminum shavings out of the oil passages.
When I’m ready to put the case halves back together for good, I seal them up with a thin layer of Permatex #3D form-a-gasket. I’ve found this helps form a good seal that will not crack with heat. I use the 3M weather-strip adhesive to seal the 6 large case bolts at the washers.
There is a technical bulletin published by Volkswagen in 1990 that describes a few changes they make upon reassembly. First, eliminate the head gasket and replace it with a 1.6mm aluminum shim (#071 101 34), or a steel one, at the base of the cylinder between the cylinder and crankcase. Increase the piston / cylinder clearance to .045mm +/- .005mm. Cut a 1.5mm deep oil groove in the large end of the connecting rods into the thrust surface on both sides to splash oil on the bottom of the pistons. For a more detailed description of these modifications, send an SASE to Bob Donalds, and he’ll give you a copy of the bulletin.
There are some parts that I replace on every rebuild, because I have found that they need to be new for the engine to run reliably. Remember that a system is only as strong as its weakest link. I replace: pilot bearing, pistons & cylinders (not just rings!), bearings, hydraulic lifters, valves and guides, redone or new heads with upgraded seats, OEM valve cover clip, oil pressure switch, FI head temp sensor, FI seals, valve adjusting screws, cylinder shims, gaskets, push rod tubes, flywheel seal, crank noise seal. Make a parts list and begin getting the parts as soon as you can, because it can take a while to get them all and you don’t want this project to be put on hold while you wait for parts.
In order to get the pistons into the cylinders, you’re going to need a collapsible type ring compressor. The solid type won’t do it, so don’t get surprised.
When reassembling, I use silicone at the base of the cylinders and on the four head nuts in each valve cover under the washer. I also use silicone to seal up the oil drain plate. This plate, if overtightened, will break off and turn your entire case to junk. In the life of you’re engine, you shouldn’t ever have to touch this, and its too risky to mess with it.
Final Touches
If you have the resources, you can save yourself some time and aggravation by performing an oil pressure check with the engine out of the car. Bolt up the bell housing to the long block and install a starter motor. Crank it until oil pressure is achieved, and you can spot any oil leaks and compression leaks. I use soapy water around the cylinder heads to see if the cylinders are sealing. This also gives the lifters a chance to pump up.
You need to let the engine run at around 1500 rpm for about 20 minutes in the car before you drive it, to get everything seated and happening and check for leaks. Remember that this engine needs to be broken in just like a new engine, so keep it at 55 or under for the first thousand miles and change the oil at least twice in that time. I do the oil at 300 miles and again at 1000 miles total. Oil breaks down quickly, and a lot of contaminants tend to be suspended in the oil, such as residues from the cleaning solvents we used.
The finishing touches to the job are the timing, and a full inspection of the fuel injection system. No engine will last with a bad air/fuel mixture. First, do the timing with a strobe. Make sure to do the timing throughout acceleration and not just at an idle. Inspect the air flow sensor door and the centrifugal weights on the distributor and make sure they’re working correctly. Also lubricate the pad under the rotor with a single drop of three in one oil while the distributor is open (this should be done twice a year). Check your vacuum advance. A skilled technician can do a more thorough test of your fuel injection system if you suspect problems. When you think you’re done, take your car for an emissions test. Test it before the catalytic converter. This will assure you that everything is working correctly and your engine is running efficiently. You’ll sleep better knowing the job was done right.
