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
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
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.
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