Sunday, November 19, 2006

Pushrod Tubes and Cooling

It's hard to get excited about push-rod tubes, those eight accordion-pleated pipes lurking under the cylinders. They form an oil-tight (we hope!) conduit between the valve gallery and the crankcase, allowing multiple return paths for the oil pumped out to the rockers through the push-rods.

At overhaul time it's best to use new push-rod tubes since the bellows portion on either makes them especially difficult to clean. Not the outside; they clean up a treat after an overnight soak in carb cleaner. It's those deep folds on the inside that can't be cleaned. And who knows what kind of crap may be lurking in them? Best to start with new ones; they cost about a buck each, cheap insurance.

But whatever you do, don't even think of using two-part push-rod tubes. They will make your engine run hot.

Eh? The push-rod tubes create heat?

No, they don't create it, but the stock tubes are wizards at getting rid of it! Given a thin coat of flat black paint to preserve them and lower their thermal resistance, your eight push-rod tubes serve as eight auxiliary oil coolers. Thanks to the surface-area of the bellows sections and the fact the push-rod tubes are constantly bathed in a film of hot oil from the heads, they do a dandy job of coupling heat to the air that flows over them. And that function works in both directions, too.

Heat always flows 'downhill.' If you put a hot thing next to a cold thing, the cold thing absorbs the heat while the hot thing loses it until they achieve equilibrium. Even if the difference is only a degree or two, the heat flows toward the cooler part.

Since the push-rod tubes are bathed in the flow of air that has just passed over the cylinders, the air is hot. But not as hot as the oil coming from the valve gallery. The cylinder head is the second-hottest part of your engine (the exhaust valves and stacks are first) and the oil in the valve galleries is typically a hundred degrees or more above the oil temperature in the sump. (Fahrenheit scale used here.) But that short trip down the push-rod tubes is sufficient to suck a lot of the heat away, thanks to the slightly cooler air coming off the cylinders and the generous surface area of the push-rod tubes.

And that function works in both directions, too. (I wonder why I keep saying that...)

When you start your engine the push-rod tubes serve to absorb heat. The air coming from around the cylinders is hotter than the tubes -- and hotter than the oil in them. The design and location of the push-rod tubes helps the engine warm up quickly and reach a dimensionally stable temperature which greatly reduces the wear factor on pistons and rings.

If you replace your stock push-rod tubes with those trick two- part anodized aluminum jobbies so beloved of show-car freaks, you've just thrown away one of the more subtle gems of the VW engine design. And provided ample evidence your engine is for Show rather than Go. And chromed push-rod tubes are even worse than the aluminum jobbies.

Adjustable push-rod tubes have their place; everyone should carry one in their kit of spares, just as we carry a spare fan belt, throttle wire and so forth. If a rock gets around your skid plate and dings a tube, you're out of the race. But with a two- piece push-rod tube in your kit you need only pop the cover, pull the rocker shaft, slide out the push-rod (praying it isn't bent too badly) yank off the damaged tube, replace it with the adjustable tube and you're back in action. Very handy thing to have -- when you need one.

But seeing chrome push-rod tubes, or those colorful aluminum jobbies, on an engine does serve a useful purpose: It tells you to stay the hell away from whoever built the engine for they know not whereof they speak.

-Bob Hoover

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