Monday, July 6, 2009
The Volkswagen engine uses poppet valves, as do all other cars today. The exhaust valves are the weak link in any engine but especially so in air-cooled engines, be it Pratt-Whitney or Volkswagen. If we want to keep tabs on our engine's condition all we need do is keep track of the condition of our exhaust valves since they are the first part to fail. Of course, we never allow them to fail, we merely keep track of their condition and when they tell us they are about to fail, we pull the heads and give them a 'valve job.'
To keep track of them we use the leak-down test, pulling the heads any time the leak-down is more than twenty percent or thereabouts. To do a 'valve job' means to remove the valves and restore the worn parts to spec. In about eighty percent of all cases, the worn part is the valve itself but other parts associated with the valve – the valve guide, the valve seat or the rocker arm – may also require repair or replacement. Since replacing the valves is the most frequent chore, that's what I'll talk about here, with a minor mention of the other valve-related components as we come to them.
The valve is opened by the cam but it is closed by the valve spring, which we must compress in order to remove the valve. The valve spring is held in place by a retainer and a pair of valve keepers. That is, the retainer fits down over the stem of the valve and rests on the valve spring. The stem of the valve has a pair of grooves into which the valve keepers fit. When so fitted, the valve keepers lock into the grooves and wedge into a tapered bore in the retainer, which sits on top of the valve spring. So long as the spring is in its proper position and is not damaged, the valve will not come loose, even though it may be actuated several times per second.
The valve spring is a coil-type compression spring that is progressively wound; the turns nearest the head having a higher ratio of turns per inch. This allows for easier opening. But like all springs, the Volkswagen's valve springs are effected by heat and age. VW valve springs typically take several million actuations before showing any sign of weakening. In a vehicle, the stock VW engine needs a valve job after about thirty thousand miles of service but Flying Volkswagens are rarely of stock displacement (which is less than 100 cid). Flying Volkswagens may be as large as 140 cid and their valve wear is a function of the work they do, which may be several times that of a vehicular engine. These non-stock 'Big-Bore Strokers' may need a valve job as frequently as every ten hours... or as infrequently as every two hundred hours, depending on how the engine is used.
The Volkswagen is a robust little engine that, like all Otto Cycle engines, provides a wealth of precursors of impending failure. So long as the engine is properly assembled and operated within its limitations by a pilot who has learned to recognize those clues, it is as reliable as any other engine in its class. But this puts a heavy burden on the pilot, who must be able to recognize those clues.
In an airplane the condition of the valves is determined by a leak-down test, which is performed periodically, the length of that period determined by the manner in which the engine is used. The leak-down tests (ie, wet & dry) reflects the amount of wear of the valves and rings, with the valves wearing at a much faster rate than the rings.
To perform a valve job we must remove the heads from the engine and the valves from the heads. Exhaust valves are replaced rather than reground but the intake valves may be reground and re-used.
When doing a valve job we check the valve springs against the spec in the work-shop manual. The diameter of the valve dictates the amount of spring tension needed to achieve proper closure. In specifying one valve tension but with a very wide tolerance for both the intake and the exhaust, you are seeing one of the many compromises Volkswagen made to keep down the price. For example, when the spring pressure is given as 96 lbs (+/- 6lbs) it is fair to assume that the smaller figure ( ie, 90 lbs) should be used for the exhaust and the larger figure (ie, 102 lbs) is used for the intake valves.
One of the more interesting features of the Volkswagen engine is that each lobe of the cam actuates TWO valves rather than one. That is, the intake valve of the #2 cylinder is actuated by the same cam lobe that actuates the intake valve of the #4 cylinder. The action of the cast iron cam as it wipes across the face of the cast-iron cam-followers (ie, the 'tappets') is the engine's major source of metallic residue, which in turn is the main source of wear in the engine's bearings and oil pump. To minimize this wear the wear-factor of the cam must be exactly twice that of the cam followers. The pressure of the valve spring plays a critical role in the wear-factor of the valve train as a whole.
At this point you need to go to http://home.hiwaay.net/~langford/
That's the home page of Mark Langford, who has contributed about three Ph.D's worth of information to the pool of knowledge all of us are swimming in. Specifically, I want you to read about how Mark measured his valve springs. Here in the Blog I've posted an article about a tool I made for that purpose but I was dealing with forty or fifty springs at a time. Like Mark, you are building only one engine. His method is more practical than mine.
What I want you to do is to COPY the method Mark has used for measuring his valve springs.
The assumption here is that you have only EIGHT valve springs. What you'll want to do is create the best possible match from BANK to BANK.
If you are running stock heads... meaning you are using valves of stock diameter, the spec for your valve springs is 126, +/-9 at a compressed height of 1.32". Which may be translated as 117 for your intakes and 135 for your exhaust.
Doing a leak-down test, the 'wiggle' test and doing a valve job are common chores for those of us who flys behind a Volkswagen engine. Given that my life literally depends on the quality of the work done to my engine, it should come as no surprise that I'm unwilling to trust any work done by a mechanic who is NOT certified by some agency or authority equally concerned with the quality of his craftsmanship. Since there is no such agency for auto engines converted for flight it seems logical that I do such maintenance myself. There are a few tools specific to these tasks. They are available from most of the larger after-market retailers who specialize in VW parts. But it is the nature of the Volkswagen philosophy that the tools may also be fabricated by the individual mechanics. Volkswagen used to provide a booklet of dimensioned drawings for such tools but no longer does so.
The first illustration in this article shows the two most recent types of valve spring retainer used on Volkswagen engines. As you can see, we need to provide for a retainer approximately 1-1/4" in diameter. Once the tool makes secure contact with the retainer we need to provide a downward force to compress the spring. It doesn't take much -- about half an inch will do. We then use a scribe or other pointed tool -- a sharpened nail will work -- to free the keepers from the grooves in the stem of the valve. Once the keepers are freed, they are removed but kept sorted according to the valve from which they came. A magnetized scriber works best or you can use a magnetic pencil.
For those of you without a metal lathe, making a tool that fits over the valve retainer is the most difficult part of the job but as you can see from the photos there are any number of workable options. And those rivets you see started out as regular nails. Just cut them off short.
As a point of interest the Single Port (SP) head shown in the photos is a junker. The spark plugs were installed WITHOUT anti-seize compound, a necessity with regular spark plugs and ALUMINUM heads. Both of the spark plug holes have been stripped until they are almost smooth.
The shade-tree fix for a stripped spark plug hole is to install a Heli-Coil (a brand name), which is coil made of wire having a diamond-shaped cross-section. The inner diameter of the coil matches that of the spark plug whilst the outer diameter matches that of a special tap that is sold with the Heli-coils as a kit. The Heli-Coil tap is threaded into the hole WITHOUT drilling it to a larger size.
There is another type of spark plug repair kit which uses a metal sleeve having it's ID threaded for the spark plug and it's external thread of some larger diameter, usually that of a regular size. This type of repair requires the spark plug hole to be opened up to a larger size, usually with a drill. To use this type of repair kit on a Volkswagen engine that is still in the vehicle the mechanic needs to use an angle-head drill-motor or a reamer, since there isn't enough room to use a regular drill-motor. (Clearly, this does not apply to aircraft installations.)
Unfortunately, the point over looked by shade-tree mechanics is that ANY form of spark plug hole repair that involves the use of a coil or sleeve must not be used on an AIR-COOLED engine (!!) The sleeve or coil upsets the resistance -- both thermal and electrical -- of the spark plug.
The fact this type of repair is allowed on WATER-COOLED engines fitted with aluminum heads is taken by non-professional mechanics to mean the procedure may be used on ANY engine. Sadly, this is not true.
So how DO you repair a stripped spark plug hole? Working from the chamber-side of the spark plug hole you hog out a crater of generous proportions, pre-heat the head to about four hundred degrees and go at it with TIG ( or even MIG, if you've got the right equipment) ...and fill the crater with molten aluminum. The head is then put back into the oven, the oven is shut off, and the head(s) are allowed to cool to room temperature.
(Did you notice the implied plural? The plural does not refer to the fact VW engines have two heads but to the fact it is not economically practical to repair damaged heads one at a time. What you do is wait until you have about two dozen damaged heads then tool up to do them all at once.
But here in Southern California, with more than twenty-one MILLION registered vehicles(*) -- and more air-cooled Volkswagens than anywhere else in the country, there was another option.
During that period (circa 1970's) for small shops such as mine, it was worth while to find a bigger shop that regularly overhauled heads on an assembly-line basis in batches as large as 250. They would allow small shops to add their heads to the batch, inspecting them to ensure all of the preliminary work had been done, and done to their specs. They would then do ONLY the welding, charging a nominal fee.
The point here is that the proper repair of a VW head with a stripped spark plug hole is to weld it up and re-machine it. For someone FLYING behind a Volkswagen engine, if it suffers a stripped spark plug hole your best option is to replace the head, since the repair would cost more than a replacement head. But don't forget that any replacement must be an EXACT match for the old head, meaning identical chamber volume and valve train geometry.
And this time remember to apply a dab of anti-seize compound to the first few threads... and then wipe it off. The tiny amount that will remain deep in the threads is all you need.
A handy way to prevent cross-threading a spark plug is to install it full-depth using only your fingers. It will then take little more than one turn to achieve the required torque-spec (22 ft/lb). And be sure that's done with a NEW WASHER. (They've got them at the real automotive parts places; don't waste your time in those chain-store auto parts retailers.)
Finally, I recently read a post where a fellow stripped his heads because his spark plugs projected into the combustion chamber. This would NEVER happen on a properly assembled engine, where checking the projection of the plug is a standard step during pre-assembly.
If the proper plug projects too far you will want to add a solid copper washer between the regular washer and the body of the spark plug. That is, you want your new, crushable washer to be in contact with the head on at least one side. Spark plug manufacturers provide solid copper washers as well as new, crushable washers. They're usually racked in the 'Dorman's' trays (those orange & black trays taking up wall space in the back of the store) :-)
(to be continued)