Monday, November 20, 2006

VW - Doghouse Retrofit

Time and time again, people visit my garage wanting a ‘Dog House’ oil cooler on their pre 1970 Bug. I am wondering if a late model oil cooler would work on an early case with the small oil holes on top of case where the cooler bolts on? If so, which oil cooler seals do I use?


Yes, a late-model dog-house style oil cooler can be retro-fitted to the early-model engines.

Somewhere in the archives, or perhaps in one of my articles, I’ve given all the details for adding a dog-house cooler to an early engine.

Mechanically, there isn’t much too it, although you need to drop the engine to do the swap. The oil cooler adapter bolts to the case, the late-model oil cooler bolts to the adapter and away you go. But like most things, success is in the details. You will need a dog-house-type blower housing, the 10mm wider fan, new forward breast tin (for the exhaust ducting) and the exhaust ducting itself. Local junk yards were charging $65 for all the necessary tin-ware plus $35 for a used oil cooler (the price is higher now). If you do not know the provenance of an oil cooler it is unwise to use it. If it is off a blown engine the oil cooler will have trapped a lot of metal particles that will cause early failure of a good engine. Best bet is to use a new or rebuilt unit.

For a leak-free installation you need two sets of grommets, one for the adapter, one for the cooler. That means you need to specify a post-71 gasket set, or you may be able to salvage grommets from the remnants of old gasket sets used to install upright coolers. Don’t even think of using old grommets. You need the ‘crush’ of the new grommets to provide tension on the fasteners. And don’t use any sealant on the grommets . . . it will get squeezed out and ends up in either the cooler or your bearings.

The adapter . . . available from Bug-pack or a junky . . . dictates the type of grommets (seals) you’ll need. In the overhaul gasket set there are two types of grommets, one for early engines with the small hole, one for later models.

The adapter dictates the OD the grommet, the crankcase the ID. Dig through the gasket set until you find the set that match your needs . . . you’ll be looking for the THIN-WIDE grommets (those for a Type III engine are THICK/ WIDE... whatever you do, don’t get them mixed up). Be careful not to over-torque when you install the adapter to an early case. You will have to replace the stud with a bolt of suitable length. With the proper grommet installed, the adaptor will come down flush on the crankcase. This is where you need to be careful. IF you don’t have the right grommets you’re liable to strip out the threaded bore for the bolt (i.e., where the stud was) or even break off one of the ears. If the adapter is not flush the oil cooler will be too high by that amount, causing a mis-fit of the blower housing, which can lead to air-leaks, excessive vibration and so forth. The different grommets are illustarated in Tom Wilson’s book ‘How to Rebuild Your Air-cooled Volkswagen Engine’.

Having a COMPLETE set of dog-house tin-ware is vital to the success of the conversion. The piece of tin-ware most difficult to find is the little gasket bracket which bolts to the back of the oil cooler and provides the nut-plate for securing the new blower-housing. Don’t leave this piece out. It forms a vital air-dam as well as serving to secure the ‘flappy’ part of the dog-house. Without it, you’ll have a massive leak of cooling air and vibrations from the unsecured blower housing will eventually cause the oil cooler to loosen and leak.

I always install new foam rubber gaskets on the oil cooler. They get torn rather easily by removal of the blower housing and they have a critical role . . . if the high pressure cooling air can find some way around the oil cooler, it will. The foam, in conjunction with the blower housing and ‘dog-house’, forms an air seal on the sides and top of the oil cooler. The little ‘mystery’ bracket (that most people leave out), with a small piece of foam attached, forms the air seal for the bottom of the oil cooler. Without it the oil cooler is only about 50% effective . . . the air blows thru the gap and at higher rpm, lifts the tin-ware away from the cooler core making the gap even worse. If you can’t find this little piece of tin-ware you will have to fabricate something to serve as the air-seal for the lower edge of the oil cooler. There are a number of ways to accomplish this but don’t put your faith in glued-on weather-stripping or the like . . . it will come loose in time and may block the oil cooler.

After-market tin-ware is especially bad when it comes to forming a proper seal around the oil cooler. I’ve a hunch the Taiwanese or Siamese or whatever have never seen a properly assembled dog-house style blower housing with all of the bits & pieces properly installed. For whatever reason, after-market dog-house style tin-ware often leaves gaps of an inch or more around the base of the oil cooler, defeating the whole purpose of the thing.

Sealing the exhaust ducting is equally important, not only where it attaches to the dog-house and the back of the blower housing, but where it passes thru the breast tin. I use RTV and literally glue the ducting in place. Since you’ll be replacing the breast tin, this is a good opportunity to install a bulkhead fitting for the fuel line. You’ll find this covered in at least two of my articles on the ‘sermon’ page.

Finally, drill out the upper bolt mounting hole on the driver’s side of the engine and install the threaded barrel found on the ‘71 and later engines. Once the dog-house oil cooler is installed you can’t get a wrench onto the nut.

The dog-house oil cooler, which is in fact an external oil cooler, represents a vast improvement over the old-style up-right oil cooler. The wider fan, new blower housing and external exhaust serves to make the oil cooler circuit separate from the normal engine cooling.

-Bob Hoover

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