Type Half Bearings
In this section we provide an overview of the Main and Cam Bearings. See the cylinder-piston-rod topic for bearings in that sub-assembly.
The main bearings are probably the closest tolerance dimensioned part of the whole engine so our discussion of it is quite exhaustive. This section is not meant to be a complete inspection and installation reference for bearings, but rather a reference of major technical points of interest.
Keep an engine log book and always record your measurements. This is especially true for the bearings and saddles.
Any discoloration due to excessive heating, or any scoring of the journals or of the radius corners between the journals and the flanges that leaves the metal looking rough, torn or granular, is sufficient cause to discard the crankshaft. Nominal journal diameter, both mains & rods, is 55mm with two undersizes (-0.25mm & -0.50mm) of bearing shells commonly available. After-market VW retailers may also carry undersize main bearing shells as small as -1.5mm (!!) but any regrind of more than wears at an accelerated rate and must be checked. Also check for run out.
When parts are manufactured or overhauled there is a given tolerance associated with their dimensions. When we say a part if 55mm in diameter (2.1654") we're speaking of its nominal diameter. The actual diameter will be a slightly larger or smaller. For example, in grinding that 55mm journal to its finished size we allow it to be as much as four ten-thousandths larger... not very much... but a whopping twelve tenths smaller. (+.0004/-.0012). This insures a high probability the journal will mate with the majority of connecting rods.
Look for any evidence of catastrophic damage, such as gouges caused by a bent or broken connecting rod. Be especially critical when inspecting the center main bearing web. Feel the main bearing saddles. If the nomenclature of the bearing shells has been embossed into the case deep enough to catch your fingernail, the crankcase must be align-bored.
In measuring the bearing bores of a used engine your main purpose is to determine if the crankcase has already been align- bored. If it has, the next question is 'How much?' and is another align-bore even possible, if needed. Personally, if it's already been align-bored and needs another, I wouldn't consider it for use in an airplane. Measure crankshaft journals at four points. The journals must match your bearings and should be reasonably round. Use the factory spec's as your guide.
The structural integrity of the crankcase involves the three main bearing webs and their six studs. (The #4 ‘main bearing' was tacked on to the engine to deal with the side loads resulting from the belt-driven blower/dynamo.) The primary function of the edges of the crankcase and its twenty related fasteners is to form an oil seal. To insure that oil sealing surface can do its job we need to make sure it is flat. LIGHTLY press the flat face a clean, single-cut mill file to the sealing surface and slide it along the surface, skating around any studs and bridging the gaps. (An 8" or 10" file works best.) Any high spots or turned edges will be immediately apparent. Deal with them. I put about a sixty-thousandths chamfer on all these edges.
Trial Fit Main Bearings
When your crankcase is cleaned, painted and dried, find your crankshaft bearings and fit them into the case. Your main bearings must match in three dimensions. OD is the fit of the bearing into the crankcase. ID is the fit of the bearings on the crankshaft. Thrust is the fit of #1 bearing on its saddle. At this point you are especially interested in the fit of #1 bearing, the one having the thrust flange. If you're building upon a used crankcase and if the thrust face of the #1 main bearing web has been refaced, you will need a bearing set having an oversize - that is, thicker - thrust flange. If the case has been align-bored the newly machined bore should be LIGHTLY chamfered. The #1 main bearing has generous reliefs at the junction of the flanges & bearing shell; it will fit on even a perfectly square corner. The light chamfer, which may be done with #600 paper, is to insure the flanges will fit smoothly down over the saddle.
‘Crib death' is the term applied to engines which fail during running-in, typically about five hours for the VW engine. About half the cases of crib death are due to misalignment of the main bearings, an easily avoided error of assembly. Torque produced by the crankshaft is coupled to the engine via the bearings. (Actually it's the ‘anti-torque' as defined by Newton's third law of motion.) Steel dowel pins are used to prevent the main bearing shells from rotating in their saddles. The center main bearing is a split type and uses two dowel pins, one in each case half, making it difficult to install wrong but the other three bearings are full circle shells and use just one dowel pin each. Find the five small steel dowel pins, the ones stored in a Special Place. (If you are building on a new crankcase the dowel pins should be a part of the crankcase's hardware package.) Note that the steel dowel pins for the full circle bearings an NOT installed on the center-line of the bearing web but offset toward the clutch-end.
Taking the offset of the dowel pin into account, trial fit the full-circle bearings (ie, #1, #3 & #4) into the left case half. Align them with their pins then use your thumbs on the inside of the bearings to press them firmly into the saddle. The numbers, tell us the bearings should fit their saddles. But there is a manufacturing tolerance in bearings and in crankcase bores, either new or rebuilt. The numbers say it should fit. It's up to you as the Mechanic-in-Charge to insure that is true. To do so you fall back upon your sensory tools. The bearing shells should be a tight fit in their saddles. Too tight and the shell will not seat to full depth, a problem clearly evident on inspection. Too loose and there will be a gap which is equally obvious; you can rock the shell back & forth. Either problem is good evidence you have the wrong bearings or the bores are not to spec. (The latter problem, woefully common in cases that have been align-bored by non-machinists using portable tools, should have been caught during blueprinting.)
Failure to achieve a proper fit is one of those ‘unimportant' details never addressed by the popular press. Yet it is these exceptions to the norm that cause so much grief for a novice engine builder. Since these exceptions are never mentioned by the popular press the novice usually assumes he has done something wrong. At that point the novice often hands the engine over to the local VW guru which in many cases simply compounds the problem, since the ‘guru' is liable to tackle a tight bearing shell with a hammer and a loose one with a slather of J.B.Weld.
Check the alignment then mark each of the bearing shells with an arrow pointing toward the clutch-end of the crankcase. Use a marking pen; paint is too thick. When you are sure the bearing shell is fully seated in its saddle, use your marking pin to scribe a thick line on both sides of the bearing at the point it enters the saddle. When you do the final assembly the arrow will remind you as to the proper orientation while the mark at the parting line will insure the shell is seated to the proper depth. Except for #2, most main bearing sets are made of babbit. (#2 is babbit-coated steel.) Steel-backed full-circle bearings are used for special applications but are not commonly
To resolve a problem discovered while fitting your bearings you must first define the problem. Something obviously does not fit but the fix will depend on why. Verify the dimension of the bearings. Yeah, I know... they say " +0.50mm " on the box but does their OD actually measure 65.50mm? (I'm assuming the usual metric tolerances here) The same applies to the crankcase. It may be stamped .020 on the boss near the #1 bearing but what is the actual dimension of it's bores? Torque it to spec and get in there with an inside mike or snap gauges and find out. And be sure to check ALL of the bores, not just the easy-to-get-at #1.
Measure each bore at more than one point - with portable boring tools oval bores and tapered bores are not uncommon. Once you've defined the problem in numerical terms the solution will be self-evident: the case must be re-machined or you have the wrong size bearings available. Regular full-circle bearing shells are quite soft. It is possible to torque the case closed with the bearings misaligned and have the engine run. Most such engines fail, often catastrophically, in less than twenty hours. If you accidentally crush a bearing shell against its dowel, buy a new set of bearings and start over.
Trial Fit Bearings to Crank
Your crankshaft is CLEAN. It is standing on its nose or tail, plugged into a pulley or crankshaft and is covered by plastic bag. Until now we've worked with the main bearing shells dry. Once parts are coated with oil or grease they must be handled and stored with extreme care to prevent picking up contaminants. Working with one bearing shell at a time, squeeze a dab of moly-type CV grease onto your finger and work it in to the friction surface of the bearing shell. Don't just wipe it on, RUB IT IN. Wipe off any excess using clean paper towels. Do the same to the matching crankshaft journal then slide the bearing onto the journal, making sure you have the dowel pin hole properly oriented. Factory spec for main bearing clearance is about .0015 to .004" That should be a smooth, sliding fit. At minimum clearance there should be no perceptible rock. If the fit feels too tight or too loose you will have to install the bearing shells into the crankcase, torque it to spec then mike the ‘as-installed' ID.
Bearing shells are among the most accurately made of the parts you'll be handling. A poor fit is usually due to a crankcase or crankshaft of improper size. Overhauled crankshafts ground to minimum spec are a common problem due to the fact most American machine shops treat 1mm as equal to .040" when it's actually 0.03937" (approximately). This is one of those ‘unimportant' details that becomes painfully important when trying to achieve proper clearance between a crankshaft (or crankcase) machined to inch standards and a set of bearings manufactured to metric standards. Specified main bearing clearance is about 0.0015" to 0.0040" and your goal is to come up with a fit nearer the 0.0015" clearance than the .004". During your blueprinting you will have detected any gross mismatch and done something about it but there's little you can do about the stack-up of errors that occurs when the machinist treats 1mm as a full forty thou.
There is always some small variation among main bearing sets and if you build a lot of engines you will have a shelf of bearings to chose from. But even then, the best you can hope for is to improve the fit by a few tenths. If the fit feels especially tight or loose, and if on actual torqued-to-spec measurement you find the bearing will not provide the specified clearance, you will have to start over with a different crankshaft, ideally from a different shop. The reason is pretty simple. When the clearance reaches or exceeds seven thousandths of an inch, the engine is worn out and must be overhauled. The RATE of wear in a plain bearing is a function of its clearance; the wider the clearance the FASTER the wear.
If your engine begins its life with a main bearing clearance of between .0012 and .0025 it will take up to 2,000 hours for that clearance to wear out of spec. But if you begin with a clearance of .004" the engine will wear out of spec in as little as 500 hours, if not less. Install the #2 bearing shells into their respective saddles. Clean any grease from your hands and from the exterior of the bearing shells. (Make it a habit to keep your hands, tools and work area clean. Keep a pint of lacquer thinner or MEK beside you along with a roll of paper toweling.) Wipe down the bearing saddles in the crankcase and the parting line, paying particular attention to the places where the internal webs will make contact when the crankcase is closed.
Lay the crankshaft into the left hand case half. Starting with #1, rotate the bearing shell until it aligns with its dowel pin. Use the arrow and marks you made as an alignment guide. In a similar fashion, align #3 and #4. Laying in the crankcase, supported by its bearings, the crankshaft should spin freely with light pressure of your hand.
Checking Main Bearing Crush
Do a final check to insure the main bearings are properly seated in their saddles then install the right-hand half of the crankcase. Locate the six nuts and washers for the main case studs. Install them finger tight then use your torque wrench to run them up to 15 lb-ft starting with the middle-lower stud, then the upper-outers then the middle-upper and finally the lower outers. As you tighten each stud, periodically rotate the crankshaft. It should turn easily. With the studs torqued to 15 lb-ft, go over them again torquing them to 20 lb-ft, this time starting with the upper-outers and using the same vee-pattern to finish with the upper middle.
The crankshaft may be difficult to start turning but once free it should move smoothly with only moderate drag. If at any point the crankshaft fails to spin freely, STOP. Dismantle. Inspect. Finally, torque the six nuts to 25 lb-ft. Do the middle upper/lower pair first, then the clutch-end pair and finally the pulley-end pair. The crank should continue to rotate freely throughout the tightening sequence. Indeed, the crank should spin more freely when the crankcase is fully torqued since doing so should cause the bearing shells to form perfect circles.
If the crankshaft should bind, dismantle the case, remove the crank and inspect the bearings. Odds are, you've gotten one misaligned. If so, you'll have to obtain another set of bearings and try again. This pre-assembly has confirmed the fit and crush of your main bearings and determined the APPROXIMATE shim stack for your end-float. Dismantle the crankcase and remove the crankshaft. Bag them both. Update your documentation package.
In normal use the main bearing bores of the VW crankcase get pounded out. Careful measurement of the original 65mm diameter bore typically reveals a four-lobed shape having two major 'lumps' and two shallower departures from a circle. Overhaul of the crankcase involves reaming the main bearing saddles back to a true circle the so-called 'align-bore'. For a competent machinist, an align-bore is a relatively trivial task. The pulley-end and clutch-end of the crankcase casting have accurately machined surfaces which will grip and locate a bushing turned to a suitable size. The boring bar or reamer rides in the center bore of the bushings.
Given the short length of the crankcase a boring bar 1.5" in diameter provides adequate stiffness and the work is easily accomplished on a lathe having a 12" swing. If you elect to do this job yourself the major chore is fabricating the boring bar. The rake angle and nose radius of tools ground for aluminum work equally well on magnesium alloy. The depth of cut needed to clean up the out-of-round condition is typically twenty thou (0.50mm). Anything more and you'll probably have to deepen the holes for the bearing studs.
Sets of oversize bearings come in 0.25mm steps (about .010) for all valid combinations of ID & OD. In addition, the #1 bearing comes in two widths & flange thicknesses to accommodate refurbishing the thrust face, a necessity when adjusting the end-float.. Shops that do a good deal of VW machine work have tooling and machines dedicated to the align-boring task. The price is usually quite reasonable and some will sell you the required bearings as part of the package.
Visually inspect the lobes for asymmetric wear and discard if the tip of the lobe shows a pronounced slope. Using a surface plate and gauge, check to see that all lobes have the SAME lift. The total lift isn't especially important, assuming it is within spec. Inspect the eight cam-follower bores for chips and cracks. Measure or gauge their bores. (You may use a new lifter as a gauge, using the 'wiggle' test.) With the crankcase torqued to spec, use an inside mike or snap gauge to measure the main bearing bores. Stock is 65mm with the usual metric tolerance.
Installing Cam Bearings
Go find them. Dress their edges and wipe them with MEK. Wipe their saddles with MEK. Lube the bearing shells using moly-type CV joint grease and install them in the crankcase. In the stock set of cam bearing shells one half of the #3 shell serves as a thrust bearing. I buy two sets of bearings so as to provide a full circle thrust flange. Cam bearing sets having two thrust shells are available. For a low speed engine this is a minor point; the engine works well enough with a single sided thrust flange.