Friday, November 24, 2006

VW - Free Horsepower II

I was sorting some pulleys today, looking for one dinged-up enough to sacrifice for an assembly crank . . . you weld a handle on an old pulley, use it to turn the crank during engine assembly. A couple of the pulleys had serial numbers stamped on the hubs, meaning they are part of a balanced assembly for that particular engine. (There’s still a few engines-in-progress around the shop.)

The sight of the serial numbers reminded me that I’d failed to mention that critical detail in my sermons about engine balancing (‘Free Horsepower’). When you get your parts back from the balancer it’s wise to mark them with the serial number of the engine they are destined for. Use cut-steel stamps to stamp the numbers onto the hub of the fan pulley, the rim of the flywheel and the flange of the pressure plate. A vibrating carbide scribe is best for marking the crankshaft.

A set of steel stamps is about five bucks from Harbor Freight. They come in all sizes, from 1/16" characters for gun work to 3/8" European-style numbers for stamping the serial number on engines rebuilt on a replacement crankcase. In some states such crankcase re-numbering is a legal requirement, and is always done on aircraft engines. You’ll also want to stamp the #1 main bearing flange and the crankcase parting line with the over-size if the crankcase has been align-bored. The good shops do this as a matter of course but some of us can’t afford the good shops. The bearing flange is marked at the 3 o’clock position looking into the bearing from the flywheel. The parting-line marks are put on the upper surface just opposite the generator tower but enough toward the flywheel so they can be easily seen.

Half a millimeter over-size is .020, a full mil .040.

If you overhaul an engine it’s polite to add a dash-number to the serial number: -1 for the first overhaul, -X2 for the second, and so forth. In some places it’s not only polite, it’s a legal necessity. (I’ve seen aircraft engines with six dash numbers after the serial.) Another method I’ve seen is to punch an asterisk after the serial number: -* meaning the first overhaul, -** for the second and so forth. In some circles this is considered less positive than dash-number markings since it’s harder to disguise -xxx4 (four overhauls) than - ****.

(Aircraft engines use aluminum crankcases that are easily welded; it’s fairly simple for a weldor to ‘vanish’ a couple of asterisks.) Various engine modifications are marked in the same way, with special codes or characters for each.

When rebuilding an engine so badly out of spec that the crankcase is no longer usable, the original serial number is stamped onto the Universal Replacement Crankcase, usually followed by the letter 'R'.

If you manufacture an engine using all new parts you are legally entitled (and required, in some cases) to use a unique serial number. The last four digits of the serial number will be found somewhere on every major part of the engine. If you ever encounter a serial number that starts ‘HVX’, it’s one of mine.

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