Friday, July 27, 2007

Basic Jugs - IV 1/2

Several people wondered why I failed to mention checking the fit of the piston rings in their grooves. So I went back and mentioned it. Now allow me to offer you a whiff of Mechanical History.

Back in the Good Ol' Days, which really weren't, the Ring & Valve Job was the staple source of income for the majority of mechanics. Thanks to inadequate air cleaners and no oil filtration at all, somewhere between 20,000 and 40,000 miles of service the engine of every automobile became so worn that the compression would fall so low that the engine could not be started. And if you could get it running, it was liable to burn as much oil as gas. That's when you'd deliver Ol' Betsy up to the local mechanic who perform the Ring & Valve Ritual.

Most people don't know it but the primary sealing surface needed to ensure good compression is not the fit of the piston ring to the wall of the cylinder but the fit between the ring and the top and bottom of its groove. Pulling the pistons and honing the walls of the cylinders was part of the classic Ring & Valve job but the critical work took place in the back of the shop, where the pistons were chucked into a lathe and their grooves re-machined so as to return their upper and lower surfaces to truth, meaning perfectly perpendicular to the axis of the piston. Doing so also widened the groove, requiring the fitting of wider rings, a stock of which was usually kept on-hand. A good machinist could 'overhaul' a piston so that the grooves were a nice match for the next available over-size ring but when they missed they simply honed-down the ring to match the groove, plus the usual tolerance of one to three thousandths. There were machines that could hone four rings at a time but small rural shops usually relied upon boy-power, a surface plate, and a sheet of fine sand-paper flooded with kerosene. The boy's reward was being allowed to test-drive the 'tight' engine around the block a time or two. (Yes, that's me behind the wheel of the Model A, out behind Leduc Motors, the Ford dealer in Turlock, California.)

Things are a bit different today :-) Nowadays, when you buy a set of replacement Pistons & Cylinders for your Volkswagen engine the fact the pistons are already installed in the barrels is good evidence that the rings fit their grooves. They might be a tad too loose but you know they can't be too tight, otherwise they wouldn't be able to compress the rings enough for the piston to fit in the barrel. This reflects the fact that mass-produced replacement parts are manufactured to a looser standard - - a wider range of tolerance - - than the factory-produced original.

If you were building an engine meant to run hours at a time above 5,000 rpm you're probably using forged pistons fitted with high rpm rings. That's when you concern yourself with the precise fit of the rings in their grooves. (Indeed, you might even buy blank forgings and machine the grooves yourself.) But for low rpm applications, such as spinning a propeller, you generally don't. Other than to ensure the ring moves freely in its groove there is little to be gained in measuring the precise amount of clearance because in the practical sense there isn't anything you can do about it other than to order a new set of P&C's... with the strong probability they will be no better.

When assembling an engine from a collection of after-market parts there are many areas in which the close attention of the assembler can pay a significant dividend in terms of power, durability or efficiency. But this isn't one of them.

-R.S.Hoover
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