About Heater Box Restoration

Heater boxes are "peripheral" engine components often taken for granted in engine rebuilding and vintage VW restorations. All too often correct, functional and nice looking heater boxes are overlooked in even the best of restorations. And the available pool of good used boxes and restoration parts is quickly shrinking. Zarwerks.com has invested heavily in developing and re-producing the parts needed to correctly restore the early heater boxes. This page will hopefully answer any questions that you may have about what boxes are restored, how we restore them and what to expect from us in terms of restored boxes and parts.  Much of this applies to boxes that we might restore for you, as well as the restored boxes we sell outright.

Use the index below to navigate this page, it contains lots of information on heater boxes.

What boxes do you restore?

We restore all 25 and 36hp "Stale Air" heater boxes used on VW Beetles and Buses. "Stale Air" refers to the 1962 and earlier heating systems (25, 36 and the earlier 40 hp engines) in which the same air that was blown across the heads and cylinders was also blown into the car for heat when the boxes were engaged. In these systems, a large square flap in each side of the lower heating channel was closed off when the cable was pulled preventing the warm air from escaping out of the back of the lower engine, and instead forcing it toward the front through the heater box. The '51 and later heater boxes also had a flap that opened to the ductwork going forward into the cabin. These systems work well as long as the engine is in good shape and clean. But a dirty and oily engine makes for smelly air in the cabin when the heat is turned on. And worse case, if there were any leaks in the exhaust system, the gas is quickly vented into the cabin.

In 1963 the "fresh air" heating system was introduced. This system ported air out of the fan housing above the engine (before it had a chance to pass over the dirty engine) and ducted that to a different kind of heater box that was more efficient in heat transfer. An easy was to identify engines with these systems is the presence of the fan housing "arm" outlets and 2 inch cardboard hoses in the engine bay. These systems are still being used today in the Beetles produced in Mexico. Reproduction heater boxes for them are readily available and we do not restore these.

The availability of good, restorable stale air heater boxes is dwindling. Limited production of spares has made finding good early correct boxes very difficult. Only the latest versions of these were ever produced post-production for repair parts as they were interchangeable with the earlier versions. And of course real problem is that when the pipes rust out, there is no easy solution to replace them. The whole box must be replaced.

We currently do not restore the 40hp heater boxes

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What are the different kinds of stale air boxes and what is correct for my VW?

There are 3 basic types of stale air heater boxes, often erroneously referred to as KDF, 25hp and 36hp respectively. And within each type, there are some subtle differences that give clues to the over age of the boxes.

The first type we call the "flapless" boxes as they had no flap mechanisms inside. They were used until April 6, 1951 when the new box design with the flap came out. These boxes are very difficult to find. Like many early parts, their design was less than optimal, and in the early years they were yanked from cars, discarded, and replaced by the newer designs. The halves of the box are not held together by screws tabs or bolts, but rather spot welds, about 42 of them actually. The metal used for the box halves is about half as thick as that of the later boxes, so needless to say they didn't last long. And the boxes weren't actually welded to the pipes at all, rather an L shaped tab allowed them to be held to one of the exhaust studs with a second nut, and they fastened to the lower heater channel tin like the later boxes. Restoration of these boxes is very tedious. Many times, all spot welds have to be drilled to separate the box halves and replace the pipes . Welding repairs to the thin metal is very tricky, as is re-welding the halves back together and making them look like they were never touched. 

As there are no flaps in these boxes, there are no levers for heater cables to connect to either. The heater control cables connected to a special mechanism in the lower heater channel tin (often called "shoeboxes") that moved the flaps in the them. The shoeboxes  had 2 flaps in them.  These heater boxes and their compatible lower heater channel tin are very rare.


The next type of box is often called the "square head" box, noted by a square-ish protrusion on the top of each in which the flap mechanism resided. These boxes superseded the flapless style in April of 1951 and were used in production until March 3, 1958. The earliest of these boxes had M6 nuts and bolts with round slotted heads holding their halves together. The pivoting lever inside in the front half was also of a slightly different design than the later models of this box. 

The second generation of this box had 9mm hex head sheet metal screws with a "dog" point holding the halves together.

The last version of the stale air box had a significantly redesigned front and rear half with each being pressed out of a single piece of steel (except for the outlet duct on the forward halves), the square protrusion for the flap was now gone. These boxes started in March 1958.  Early versions of these boxes used the 9mm sheet metal screws like the later versions of the previous style. The later versions of this box (as virtually all of the post production replacement parts) had bend over tabs that held the halves together. Some versions of these boxes still had the sheet metal screw holes even though they used the tabs to hold them together.

The last style of this box was the 40hp setup used only in '61 and '62.  In an effort to improve the engine to air heat transfer, the engineers at VW modified the box and the lower engine tin. They widened the inlet opening of the heater box, it was now a large wide oval.  These 40hp boxes are very often mistaken for the 36hp boxes which have round inlet holes.  The lower tin was modified with an extra tin outcropping along its length to gather the heat radiated from the longer, exposed section of the J pipe, and make it available to the heat system inside the box.  The tab that secures the tin to the lower box was also relocated to the lower side of the box, rather than the bottom like the earlier versions.  Lastly, a small duct was set on top of the left heaterbox and allowed for hot air to be drawn into the aircleaner to improve cold weather performance.  Most of these boxes had the dog point sheet metal screws holding the halves together.

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How are the boxes restored?

The difficult part of the restoration, and the reason that the J pipes are not simply "bolt-in" replacement parts is that the pipes must be free of the flange at their ends to remove and re-insert the J pipes, and the flange is welded on to the J pipe. Of course to those people who would make big slices in the box rear half, pry it apart and then re-weld the tin after they found a decent J pipe with a flange already on it, this wasn't an issue (although it made for one horrible looking, hack job repair). The more common "repair" attempt was to just cut off the exposed (and often rotted out) straight section of the pipe, and weld or clamp on a "repair section".

But none of this is necessary any more as ZARWerks has had reproduced replacement J pipes that are of the original external specification, and slightly thicker wall thickness for durability. These pipes are custom made for us (that means you won't find them sold anywhere else!) out of mild steel, seamless tubing. Wall thickness is .083".  Laser cut 1/4" thick flanges are also available and used in every heater box restoration we do.

The restoration process starts by disassembling the box halves and cutting the old J pipe out. The weld points are broken, and the pipe is cut just as it exits the box on the lower end. This allows the pipe to be rotated out of the box with the flange still on it.

The next step is to solvent clean the box. Many of them share the layers-thick sludge of road grime and oil that the rest of the engine underside sees.

Once they are cleaned, they are assessed for re-buildability. Most dents and bends are straightened here, and the flap mechanism is examined for any problems or missing parts. Seized actuating levers can be freed up. On the square head boxes the main pivoting lever is removed and is cleaned and painted separately. Rusty or worn springs are removed and replaced later.

Then the box halves are media blasted and stripped to bare metal inside and out. After this is done, they are re-examined and any addition straightening or dolly work is carried out. Any holes or metal tears are welded at this point and dressed. The front halves are then painted inside and out and they will have no welding done to them. In the grade one restoration, each fastener is wire brushed, examined and replaced if needed, and painted also.

The rear halves are then fitted with a new J pipe, and a new flange is loosely tapped on. Then each box half goes on a special alignment jig to insure the flange angle and box housing placement is correct. This insures the pipe will slip straight into the muffler opening and that the inlet opening on the box will align with the opening on the lower heater channel tin below the heads. Once this alignment is correct, a few tacks are made with the MIG welder to hold things in place. The J pipes and box halves are then removed from the alignment jig and put in a vice where the welding is completed. Strengthening brackets inside the boxes are added if needed (the earliest square head boxes didn't have these, VW added them later in an engineering change; we put them in all boxes we restore). The flange is fully welded to the J pipe end.

The J pipe is masked off, and the housing is cleaned with solvent. Any last minute visible weld dressing is done here as well, then the housing is shot with the black epoxy enamel.

Once dry, the halves are re-assembled and the flap mechanism checked for operation. If the exposed J pipe is to be painted with the POR-20 high temp paint, the housing is masked off, the pipe surface finished and the manifold gray paint is shot. The whole assembly is then baked for 1-2 hours at 400 degrees farenheit.

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Why the grading of condition?

The final condition and appearance of restored heater boxes is determined by their original condition prior to restoration. Remember that we are talking about 40-50 year old parts here. It is rare to find restorable boxes that aren't dented, rusted, torn or otherwise mangled somehow. And by far the worst is if previous repair/restoration "attempts" have been undertaken. But we must also keep in mind that these parts are used on the engine, are not visible from the engine bay and only barely visible if you get on your knees and look up under the car. For this reason, many enthusiast's primary objective is to get a good set of boxes with function the foremost priority, and appearance taking the back seat.

But of course, there are those for which nothing but perfection will suffice. For those people we weed out the very best boxes, the ones that have not been previously hacked, or rusted to the point of pitting, or dented and mangled. Great care is spent on these boxes in the area of appearance and correctness.

Grade Two Boxes:

These boxes are restored with the same process as the grade one boxes, but are priced lower. They too are disassembled, stripped of old pipes and flanges, cleaned and media blasted. Obvious bends and dents in the housings are straightened, and new pipes and flanges are installed on the welding jig and when done, they are painted inside and out with semi gloss black epoxy enamel. What separates these boxes from the grade one boxes is that there may be imperfections in the box housings and evidence of repair. There may also be some metal pitting visible under the paint and the pipe openings in the housings may be slightly irregular. But these boxes will be as functionally perfect as the grade one boxes and should be expected to serve your warm air needs for many years to come. E-mail us if you have specific questions about a set of boxes that are available, we may be able to send you detailed pictures that show you why a particular set of boxes didn't make grade one.

Grade One Boxes:

These heater boxes are restored with the nit-picky enthusiast in mind. In addition to the basic mechanical restoration steps, extra care is taken in the finishing detail. Screw and bolt hardware will be carefully cleaned and painted and correct on these boxes. And perhaps most importantly only the best restoration candidates are chosen for grade one boxes. There must be no substantial metal tearing or previous patch work done and no substantial rust pitting. All surfaces must be smooth and uniform, even the flap mechanism inside is carefully cleaned and painted. All welding will be either contained inside the box, or carefully dressed so it is undetectable (except for the flange weld).  These boxes are worthy of the very best (and correct!) vintage restorations.

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Can I restore my own boxes?

Of course you can, but you need to have a MIG welder and be familiar with basic metal working tools and techniques.  Zarwerks sells heater box restoration kits which include a pair of pipes and pair of flanges.  Or you can buy the pipes and flanges individually if you want.  if you are restoring your early VW's engine and it is removed from the car, you can use it as your own "alignment jig" to fit up the pipes correctly and tack weld them.  You will not do any damage to your engine doing this.  You cannot properly restore boxes without the use of an engine (or jig) with heads, lower tin and a muffler in place.  If you don't get the parts aligned right, the boxes will never fit up to the heads and lower tin correctly.

Very soon, detailed instructions on heater box restoration will be completed and printed and included with each J pipe order from Zarwerks.



JH 10.31.03