Service

SERVICE

PAUL DEAN

A "true" story

Q HOW do the "pros" assemble and true press-together crankshafts? I can start them straight and true, but the closer they get to complete assembly, the harder it is to true them. Is there a secret? Dave Hughes Washington, Illinois

A There are no secrets involved in crankshaft truing, Dave; it's just a matter of combining experience with the right tools. Most professionals use special fixtures that hold the separate pieces in alignment while they're pressing a crank together so that by the time everything has been fully pushed into place, only a minor truing is necessary. Such fixtures are available either from the bike's manufacturer or one of several aftermarket sources.

After assembly, the crankshaft is put in a truing stand, which holds the crank by the outer ends of its shafts and has mounts that hold two dial indicators, one running on each of the crank's main-bearing surfaces. As the crank is then rotated slowly, the dials indicate whether or not the flywheels are both concentric and parallel. If they are not concentric (the degree of acceptable runout usually is listed in a bike's workshop manual), the crank is removed from the stand and, with a soft-faced metal hammer (usually lead), given a solid whack on the outer diameter of the affected flywheel. The impact should always be delivered at a point 90 degrees from the crankpin. The crank is then placed back in the stand and checked for trueness. This process is repeated until the flywheels are concentric.

If the flywheels are concentric but not parallel to one another, either of two techniques is used to align them. If the wheels are pinched closer together opposite the crankpin than they are at the pin, a hardwood or aluminum wedge is inserted between the wheels 180 degrees across from the pin and then given a solid blow to spread them apart. If the flywheels are spread apart opposite the crankpin, they are aligned by placing them in a stout C-clamp or a large vise—again, 180 degrees from the crankpin—and squeezing them together. In both cases, the crankshaft should be put back in the truing stand after each alignment attempt so they can be checked for correct trueness.

Much like lacing wire-spoke wheels, pressing built-up crankshafts together and truing them is an art that is only made easier over time. Patience is a valuable ingredient in the process, as, of course, is having the proper tools.

Chain letter

QMy 2002 Suzuki GSX-R750 with 23,000 miles on it didn't seem to be running as sharp as it used to, and when I took it to a local shop for a tuneup, they said it needed a new cam chain. They said the compression was good and that nothing else was worn out but the engine wasn't running properly because the cam chain was stretched. I asked how the cam chain could affect the way the engine runs but didn't get a very good explanation. The cams are working, because the engine runs, so I don't see what the chain has to do with the way it runs. I don't want to get ripped off, so can you tell me if the shop is being honest with me?

Trace Ross

Birmingham, Alabama

A Since I can't inspect your GSXR in person, I have no way of knowing if it needs a new cam chain. But I can tell you how a cam chain affects the way an engine runs.

When viewed from the right side, the crankshaft on practically all overhead-cam inline-Four engines spins in a clockwise direction. This means the chain has a straight pull at the front between the crank and the cam (or cams, on dohc engines); a tensioner pushes on the rear of the chain to take up slack and keep the chain taut. For an engine to deliver the level of power it was designed to produce, its cam timing must be set very accurately so the valves open and close at precisely the right instant. With overhead cams, that timing is dependent upon the chain having a specific length—determined by a certain number of links—between crank and cam. But as the chain stretches over time, the distance between each link increases very slightly, just like on a drive chain, so the chain effectively gets longer.

With a drive chain, that added length results in chain slack that can be taken up by adjusting the rear wheel, a process that moves the rear sprocket a bit farther from the front sprocket.

On an engine, you can't move the cam farther away from the crank, so the slack is taken up at the rear by the tensioner. But when a cam chain is stretched, the same number of links that, when new, kept the crank and the cam in perfect timing now span a slightly greater distance. As a result, the cam is gradually reoriented very slightly backward relative to the crank as the chain stretches. The cam timing, in other words, becomes retarded.

If the degree of retardation is small, the effect sometimes is a slight increase in lower-rpm power with an equally slight loss of top end. But past a certain point of chain stretch, engine performance will be lost throughout the rpm range.

If the chain is not soon replaced, it possibly could jump a tooth on one of the sprockets or break as it tries to skip a tooth. Either way, the results can be catastrophic—bent valves, broken pistons, scored cylinder walls and even worse.

If you don't trust the shop that claims your GSX-R needs a new cam chain, you should have another shop or a reputable mechanic look at the bike. A second opinion would be a whole lot cheaper than a complete engine rebuild.

Old new vs. old used

Q How does "Blue Book" pricing work when considering a "new leftover" motorcycle? The reason I ask

is that a local dealership has a leftover 2008 Kawasaki Concours 14 on their showroom floor that they're offering at $9995. That seems like a good deal to me. But if the Blue Book price for that model is $9020 as listed in the "LongTerm Wrap-up" article on the 2008 Concours in your May issue, is it really a good deal? Does that bike's $9020 value take into consideration that it is truly "used" with 11,500 miles on it, and so the Blue Book for the bike with zero miles is higher? Cliff Mason

FeedbackLoop

I was surprised at Mr. Cameron's reply about piston ring rotation

(April, 2010, Service), but as a race engine builder since 1966, I'd like to offer my two cents' worth.

In those years, I have yet to find a bore that stayed round and straight through its thermal cycle. To do that, a cylinder would have to weigh as much as an engine! The worst I encountered was the factory Triumph/ BSA three-cylinder's middle pot. To make the rings live, you strapped on honing plates and honed at 170 degrees Celsius (338° F) with a locked hone. It was oval when cold but almost round at racing temps. Mick Boddice at BSA taught me that one.

Almost as bad were the Manx Norton and big Velo Singles. Uneven cooling and casting mass meant you needed .012" piston clearance just to avoid piston nip.

The results of all these oval/out-of-round bores? In my experience, the ring gap always migrates to the apex of ovality. Why? Uneven ring pressure means the gap "cams" in to the oval; after all, a piston ring is just a beam under compression ended with two cantilevers. Want proof? Look at ring wear after two meets, and the ends normally will be worn the greatest. If the bore is not straight, the process is faster; ring "flutter" will unload the ring and find the out-ofround quickly.

On my wife's A/Fuel dragster, we "barrelhone" the bores .0008-inch bigger in the center. Combustion heat and hot crank oil straighten the bores at running temperature. Together with gapless split second rings, we get reasonable ring seal; less nitro in the pan tells us that.

It's not just the old-tech engines that suffer. I have found bore distortion on modern big Twins, screaming Fours and outboard big Vees, mostly due to aesthetics and compactness constraints that compromise thermal stability, leaving no place for large mass and efficient cooling paths. Pat Neal Submitted via America Online

A That is the most lucid explanation of piston-ring rotation I've ever seen, read or heard. A light has finally been turned on in what previously was a very dark room. Thank you very much for sharing your experiences with us and all of our readers.

East Aurora, New York

A The $9995 your dealer is asking for that Concours is a killer deal. You're talking about a motorcycle that originally had a price tag of almost $13,000 (add another $700 if it's the ABS version) and is still brand-spanking-new with a full factory warranty; never mind that it's a 2008 model. If you don't want it, let me know and maybe I'll schedule a one-way flight to East Aurora. Blue Book prices indeed are for used—oh, wait, excuse me—"preowned" bikes, car, boats and AT Vs, and they vary depending upon the vehicle's condition, mileage and the region in which it is being bought or sold. In addition to retail prices, the book also lists trade-in values, which are lower, obviously, since they are geared for dealers who hope to make a profit once the bike is resold.

In the end, Blue Book numbers are just a guide to help sellers, buyers and dealers have a reasonable basis that helps them establish a used vehicle's value. The prices I provide in the "Best Used Bikes" segment of Service are Blue Book's figures for motorcycles in excellent condition with a reasonable number of miles sold in or around the Southern California area. The prices are adjusted every month or so as a vehicle's age increases and the market conditions in the selected region change. In the three months since our May issue, for example, the suggested retail value of a non-ABS 2008 Concours 14 has dropped from $9020 to $8565, but that still does not affect the bargain that a brand-new '08 model represents. □

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