USING PLASTIGAUGE TO CHECK CRANKSHAFT JOURNALS
by Pete Snidal (C) 2000
The worst part of engine rebuilding, for the hobbyist, is knowing where to stop. This is no problem for the commercial shop - they usually just rebore, regrind, and replace everything they can, and then of course charge top dollar for the job when finished. But the hobbyist often wants to avoid spending money where it isn't really required, and also, in the case of the vintage stuff we know and love so well, doesn't especially want to grind away any unobtanium or other precious metal if it isn't required to. So let's deal with deciding whether or not our crankshaft needs grinding.
The main parts of a crankshaft are the "journals" - the places around which are bolted the big ends of the connecting rods. The main bearings are generally roller or ball bearing, which are of course replacable and not really part of the crankshaft. They also generally hold up very well. Inspection should be done with a magnifying glass and a bright light, as you look carefully at the surfaces of the balls or rollers themselves, and also at the "races" - inner and outer - within which they run. They want to be clean and shiny, with no signs of scratching, scoring, or the "flat" look of brushed chrome.
As for the journals, we have to ensure that they are still perfectly cylindrical - round and of the same diameter along their entire length. This is best done with a micrometer - available at your friendly local parts dealer or tool store, or at least with a vernier caliper - preferably of the dial kind, that will enable you easily to see differences of a quarter thou or so. (.0025" - .1 mm = .004", so I guess that means a sixteenth of a millimetre.) Your service manual will give you the specs within which your parts must be.
Of course, the really important tolerance which must be observed is clearance - the distance between the various spinning parts, such as between the crank journals and the connecting rod bearings. This is where plastigauge comes in. Plastigauge(TM) is precision - extruded plastic "wire" of a carefully calculated diameter. It comes in a long paper envelope, upon which may be found sample gauge marks corresponding to different widths to which the plastigauge is squished when compressed inside a bearing-journal space.
To use plastigauge, we squish it between a connecting rod and its crankshaft journal. We simply clean up the bearing shells and the journal, apply a light coating of oil to each, break off a short piece of plastigauge, which we apply to the journal along its axis, and bolt up the rod. Snugly, but not torqued all the way, since we're going to remove it shortly anyway.
The plastigauge will be squished in the joint. The less clearance, the more it will be flattened, and of course vicey-versey. The envelope it came in gives you gauge marks which will tell you what the clearance is at that point. Simple, safe, and efficient! (You will of course remove all traces of pg before final assembly, or even before taking more measurements at other spots in the bearing/journal interface.)
You will want to bear in mind that the position of the rod on the crank is important in this measuring process. The crank will of course show maximum wear on the downstroke, since this is where the power is applied, so be sure to check clearance at the obvious places -center of the rod where it contacts the crank journal halfway down the downstroke. Check also at right angles to this spot, to see if any variation shows out-of-roundness of the journal.s
If your clearances exceed specs, then you have to decide whether it's due to wear of the bearing shell surface (most likely, since it's so much softer than the metal of the crankshaft journal) or the journal itself.
Worn bearing shells will often begin to show some of the copper backing behind the white metal bearing surface. Also, if the clearances vary at different points about the journal, the wear will obviously be due to wear of the journal itself, getting out-of-round, or egg-shaped. But if the clearance is the same measured at different points about the journal, you can be fairly certain that replacing the shells will cure the problem. In many cases, slightly thicker shells can be obtained, in sizes .001 or .002 smaller, to take care of slight (and consistent) journal wear.
Plastigauge comes in various ranges - .0001 to .003, for instance, so be sure to get the range in which you're interested, although they overlap a bit. It's a cheaper alternative to micrometers and dial calipers, and it requires less skill to use, but it's just as good a way, if not better, to find the data necessary to make that all-important decision: to grind or not to grind?