Monday, November 20, 2006

VW - Align Boring

What is the correct procedure for checking alignment of shaft bores in a crankcase?

There’s more than one way to do this and the method I use isn’t very precise.

After checking that each of the re-machined bores is a true circle, I dismantle the crankcase and measure the depth of each half-bore using a plunger-mike that reads to tenths (i.e., .0001"). Because of the amount of ‘crush’ inherent in the design, you can have the bores asymmetric by up to three thou or so and still have a usable crankcase, assuming they are all the same. You’ll see this kind of asymmetry even in new cases. But what you can’t live with is to have the depth of one of the bores radically different from the others.

The problems I’m looking for usually show up on used crankcases that have been improperly align-bored but it’s worth your time to check even a new case.

On re-manufactured cases, you want to focus your attention of the #2 main-bearing web and bore. If the web has been severely pounded its re-machined bore will usually be asymmetric, so much so the case often isn’t usable.

In an ideal world all of the bores would be perfectly identical and symmetrical. That is seldom true. Tolerance is about seven tenths (i.e., .0007") for bore diameter so they should all fall within a thou of each other. I measure each bore at three or four points and record the measurements. The crankcase is torqued to spec with all of the fasteners in the plain of the crankshaft installed. Extremes of temperature should be avoided and if the case has just been machined it would be wise to put off any measurements until it has cooled off.

The half-depth is compared to the average of those measurements. After you’ve measured and recorded all eight half-depths a couple of times, any asymmetry should be obvious. If the asymmetry is consistent, it may be ignored so long as it’s under three-thou or so. Anything more, in either case-half, will lead to problems with the mesh of the distributor driver-gear. And of course, any single bore which is not in the same plain as the others is grounds for rejecting the case.

You should already have checked the run-out of your crank. Tolerance for run-out is about the same (i.e., .0007") but you have to take into account the diameter and allowed out-of-roundness of the journals on the vee-blocks at the time you check for run-out on the journal between the vee-blocks. See the Bentley manual for the spec, which I can’t recall . . . but it’s about a thousandth of an inch (.0010")

These problems are seldom a worry if you start with a good crank and case. As I said in an earlier message, Gene Berg’s cranks are the best I’ve seen, and any align-bore done by Larry Pauter’s shop (Pauter Machine Company) was always dead-on. But many one-time rebuilders have to work with what they have, using whatever machining services are locally available. It’s important to note here that I am not doing anything unique or unusual, nor am I looking for some exotic, one-in-a-million fit. All I’m doing is trying to ensure the components going into the engines I build meet Volkswagen’s published specifications. Assembling the parts is an entirely different subject. But as sure as God made little green apples, if you start with parts that are out of spec, there’s no way you’ll ever come up with a reliable engine.

-Bob Hoover

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