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Ticked off

Paul Dean

My 1990 Yamaha FZR1000 has a problem that I’m afraid is serious. The engine makes a fairly loud, heavy ticking noise when it’s accelerating or on a long uphill pull. Any other time, the noise is not there. The ticking is steady and goes faster as the rpm increases, and it gets louder if I accelerate real hard. I’ve taken the bike to two Yamaha dealers and one repairs-only shop, but they don’t agree on the cause. One place said the bike needs a valve job, one said the cam chain is shot, and the other said nothing is wrong and the noise is normal. The engine still runs real strong and doesn’t smoke or anything, but this noise has me so paranoid that I’m afraid to ride the bike. What do you think? Craig Karr

Latrobe, Pennsylvania

I hate to be the bearer of bad news, Craig, but it sounds like your FZR has either a cracked or collapsed piston skirt or a damaged connecting-rod bearing. Without hearing the ticking myself, I can’t tell you with certainty which condition is the culprit, but your description of the symptoms points to these two as the most probable causes.

If the problem is with a piston skirt,

that cylinder will have excessive piston-to-cylinder clearance. If it is with the con-rod bearing, that rod will have too much bearing-to-journal clearance. If the clearances in these areas are not extreme, they won `t cause audible noises when the pressure in the combustion chamber is relatively low-such as when the en gine is idling, running at steady rpm under light load, or on trailing throt tle. But when the engine is accelerat ed or put under heavy load, the combustion pressure increases dra matically; and under high-pressure conditions, excessive freeplay be tween piston and cylinder or rod and journal is taken up abruptly and with great force, resulting in what is com monly called either "piston slap" or "rod knock." I believe this is what you are hearing in your Yamaha engine.

The cure is straightforward but not cheap. If the cause is a damaged pis ton and the cylinder bore has not been seriously harmed, a rebore and a new piston (four new pistons would be bet ter if the bike has been ridden hard or has high mileage) should get you back on the road. A damaged rod bearing, however, will present a considerably bigger repair bill. At best, the affected crank journal will need to be turned (remachined) and fitted with an under size bearing insert; at worst, the entire crankshaft will need replacement.

Lastly; if at all possible, avoid hav ing the work done at any of the shops that gave you bogus information. If they can `t properly diagnose the prob lem, they probably can `t properly fix it.

More crank calls

Please explain what is meant when a crankshaft is said to be a 180-de gree crank or a 360-degree crank or a 120-degree crank. I've asked a lot of people this question, but nobody has yet been able to give me a sensible and credible answer.

iiii..~ a ii~iui~ Erwin Garzaro

South San Francisco, California

Most engines of two or more cylin ders use crankshafts that have separate crankpins (or "throws ")for each cylin ders connecting rod. (The exception is Harley-Davidson `s V-Twins, which have just one crankpin for both cylin ders.) The "degree" designation sim ply describes how those crankpins are oriented around the crank diameter

Parallel-Twin engines, for instance, either have 180-degree or 360-degree cranks. On those with 180-degree de signs-such as a Kawasaki EX500 Ninja or a Suzuki GS500-the two crankpins are 180 degrees across from one another, an arrangement that always keeps the two pistons moving in opposite directions. When one piston is going down the cylinder bore, the other is coming up; when one piston is at Top Dead Center, the other is at Bottom Dead Center.

Parallel-Twin engines with 360-degree cranks-such as those in British Twins-have their two crankpins located side-by-side. This causes the two pistons to move up and down together and arrive at either Top or Bottom Dead Center at the very same time.

A 360-degree crank in a parallelTwin poses a greater potential for vibration than a 180-degree crank; all the piston mass in the 360 engine always is moving in the same direction, whereas the forces created by the pistons’ opposing movement in the 180 engine tend to cancel each another out. On the other hand, given that any four-stroke engine requires two complete revolutions to fire all of its cylinders, a 360 arrangement offers the lower-rpm smoothness of evenly spaced power strokes (one power stroke for every revolution of the crankshaft), while the power strokes on a 180 Twin are unevenly staggered at 180-degree/540-degree intervals.

The 120-degree crankshaft design is used on inline-Triples as well as on inline and opposed six-cylinder engines. It is so named because its crankpins are evenly spaced 120 degrees apart around the crank ’s diameter. If the engine is a Triple or an opposed-Six (on which each crank journal is shared by two connecting rods), there are three crankpins; if it is an inline-Six, there are six, with one half of the crank being a mirrorimage of the other. The 120-degree arrangement provides one power stroke every 240 degrees on a Triple, and every 120 degrees on a Six.

Incidentally, you didn’t ask, but today’s most common engines-inlineFours-use what is essentially a “flat” crankshaft, sort of like two 180-degree cranks made into one. Thus, the two outer pistons move up and down together, and the two inner pistons also move in unison but in the opposite direction of the two outer ones. So, when the inner two pistons are at TDC, the outer two are at BDC, and vice versa.

There is insufficient space here to discuss all of the pluses and minuses of various crankshaft designs; we’ve merely touched on a few of the more basic ones-and, hopefully, answered your question in the process. □