The alloy used in the VW crankcase has proven to be susceptible to age-hardening and the cracks which follow. It took about thirty years (!) for that fact to become evident but tests went back and confirmed that the crankcase, which is a magnesium-aluminum alloy, became more brittle with age. So in 1971 Volkswagen changed the alloy, increasing the ratio of aluminum. Which helped. But after 36 years that's the best they can say; the age-hardening is not as bad as it was with the earlier alloy. The 'new' alloy, which is at least 35 years old as I write this, still shows a propensity to become more brittle with age. As for cracking, the jury is still out.
Which isn't a problem with a professionally built engine. Because you always start with a new crankcase, which are still available from Mexico and Brazil.
The best part of the joke is that a new crankcase usually turns out to cost less in the long run than trying to re-work an old crankcase.
Why? Well, there's a lot of reasons, some people put more emphasis on one than another, which can get you into a race to see who can type the fastest :-) But most of the problems stem from the design of the VW engine, in that it only has three main bearings. There's a fourth
bearing on the crankshaft but it was only added to off-set the asymmetrical load of the dynamo & blower and should be addressed more as an accessory item than an engine item. Engine bearings are the three which support the crankshaft and have two connecting rods between each of them. The thrust bearing is located on the flanges of the #1 main bearing, immediately adjacent to the flywheel. The #2 main bearing is in the center of the crankcase and unlike most flat fours, it has the same bearing area as the other two. (Corvair or Subaru provide good examples of how flat engines are usually designed.) The #3 main is adjacent to the last pair of con-rods. Then comes the cam gear and a scroll-gear for driving the distributor, then another bearing, this one only 40mm in diameter where all of the others are 55mm.
One of the reasons a new crankcase usually costs less is because the youngest used crankcase is so old. Old crankcase, the center-main has been pounded out by the asymmetrical load of the 'boxer' design. That asymmetry appears in the bearing shells for the center-main as well as
in the web supporting the shells. Bottom line is that a used engine with 20 or more years of service is going to require an align-bore.
Align-boring is a basic chore in automotive machining and VW provides a range of over/under sized bearings, plus the thrust flanges. One rainy afternoon I worked out how many bearing-sizes they offered: 127, although not from Volkswagen. They put the limit for over-size main bearings at half a millimeter -0.50mm for the OD of the inserts. They did the same with their crankshafts, putting the limit for under-sizes at half a mil, meaning you could have a center-main that spec'd plus 50 for the OD and minus 50 for the ID, plus you could have two ranges for your thrust flange. And since you have STD-STD as your starting size you're already up to a dozen sizes and you haven't even seen what some of the after-market bearing-makers will sell you: Oversizes as large a one and a half millimeter!! And ditto for under-sizes on
No reputable builder will use that sort of stuff of course. But the kid will have no trouble finding someone who will. Of course, you gotta ask yourself if that Someone is a competent automotive machinist? Because in most cases the answer is 'no.' It's a shade-tree mechanic with some kind with a portable cutter-bar who will argue that his dune-buggies run just as good as all those other dune-buggies... mebbe even better, with his chin sticking out to about... here. So... howz they do in an airplane? you ask. And the answer(s) can keep you smiling for a while.
So let's leave the align-boring aside for a minute. In fact, lets assume your engine doesn't even an need align-bore. (Miracles have happened, you know.. :-)
So now what you need is to open up the spigot-bores for the cast iron cylinder barrels. You got four jugs so you'll need to open up four new holes. Ideally, you'll do it on a milling machine using a cutter that's been 'proofed' on earlier work to show that it cuts a true circle with square sides, meaning the sides of the hole are perpendicular to the deck of the crankcase... which may need to be re-cut due to 30 years of shuffling by the cast iron jugs. Such shuffling is normal, by the way. It's an artifact of the thermal coefficient of cast iron as compared to non-ferrous metals such as the aluminum in the heads and the magnesium in the crankcase. Every time you start the engine it under-goes a 'heat-cycle.' It is those heat cycles that cause the relative motion between the barrels and the crankcase. And between the heads and the barrels. And of course, after enough of those heat-cycles the tension in the head-stays (most kids call them 'studs') tends to relax, which is why Volkswagen tells you to re-set the torque of the head-stays periodically. And which
damn few VW owners ever do.
But there you are, wanting to over-bore the spigot-holes for a new set of jugs. First thing you want to do is to be sure the fellow doing the machining takes the trouble to measure your set of jugs, rather than the last set that came through the shop. Or the set that came through two months ago. Or whatever. Because when you shop by price for over-size jugs, you'll generally end up with a piece-rate product, imported from a country where the size of a machinist's pay-check is determined by how many sets of barrels he was able to crank out this week.
Now, checking on the machinist in this way is a bit of a trick because you should have already blueprinted your new set of jugs. What you want is a hole that's 0.005" over. No more and no less. At least, not for 92mm Kolbenschmidt barrels. Why? Well, in the first case... getting the spigot bores too big means your 'shuffle-rate' is going to go right through the roof: Your jugs are going to be doing the fandango when what you want them to do is a slow waltz. In fact, about the only thing worse that a spigot-bore that's too large is one that is too small, because that one is going to start grabbing at your piston in a process called 'scuffing.' That is, the aluminum piston inside of your cast-iron barrel is going to expand faster than the barrel... we already know that because of the difference in their coefficient of expansion. (Remember, the spigot bore is also going to be expanding.) If you didn't provide enough allowance, it's going to limit the barrel's 'growth' relative to the piston inside of it and that's going to cause the piston to rub against the wall of the cylinder. And you don't want that to happen.
So what if you've got something other than Kolbenschmidt jugs? Well, you gotta find out their recommended spigot bore allowance and use that, since they know their product best; they know the spec for their cast-iron But since there's only about nine different makers of
after-market VW jugs -- and the odds are you'll only run into two or three of them, you should be able to work it out. That means, pulling 'Machinery's Handbook' off the shelf and looking up the range of thermal coefficients for finned, cast iron barrels and coming up with a figure. Typically it will tell you use so many thousandths of an inch per inch of bore. So you run the numbers, round it up, do a few tests and there you are. For that particular jug. (In fact, your machinist should be the one doing all this. And if he knows his onions he'll already have the right figures for whatever brand of jugs you have. The thing you really want to watch for is the fellow who thinks One Size Fits All. Because it don't. Except for dune-buggies :-) (Hint: Ask to see his copy of 'Machinery's Handbook.' Most shade-tree types have never even heard of it.)
So there you are, all set to cut your new spigot bores. And while one half of the crankcase will lay flush to the table of the milling machine, the other case-half is home to a number of studs and will not lay flat. So the experienced VW machinist will have made up an Accessory Table to hold that side of the crankcase the required distance above the real table of the milling machine. The accessory table will of course be true to the milling machine, meaning it will be true to the crankcase, which means the holes will at least be perpendicular to the deck of the crankcase. As for bore diameter, we've already addressed that. But like Machinery's Handbook, most shade-tree types not only don't have a milling machine, if they do, they don't have the required accessory table for working on VW crankcases.
Now comes an interesting test of competence. After cutting you four new holes for your four new barrels the machinist will pull all or some of the plugs sealing the oil galleries in the crankcase. This is because some of those galleries have dead-ends or 90 degree 'corners' that become swarf-traps. If you don't get all of the swarf out of the castings, they'll end up in your bearings and cost you an engine. Or your life. So part of the expense is pulling those plugs so you can
get in there with your bore-brushes and what-not and clean the crankcase. The machinist's work is limited to pulling the plugs, threading the holes to accept socket-head pipe plugs, and providing you with a suitable set of plugs to match the holes. The cleaning is left up to the Customer. And if any of this comes as a surprise, you need to look at some other engines, such as those from Continental. Or General Motors. Look for the pipe plugs. Because they are a
standard feature on a properly built engine.
If your machinist is experienced with flying Volkswagen he (or she) will have drilled and threaded a couple of other holes in your crankcase. Some are to be used to lock certain components in position, others are used to improve the lubrication system and so on. Every machinist varies a bit in this regard; you'll have to work out which holes and plugs you expect to be done and those which you do not.
Now, having read the above, you've got to ask yourself exactly where you planned to realize any savings by starting with a used crankcase?