Signs of Mechanical Mistakes

[Hugh Janus]

Kevin Cameron (Robert Martin/)

According to what we’ve read, some older experienced surgeons have resisted the use of checklists. It’s fine to have pride in one’s skill, but the appearance of a clamp on a post-op X-ray, inside the patient, crushes that pride. A deadly fire in a routine test of the Apollo 1 command module resulted when an accumulation of scrap bits of wire insulation and other build detritus caught fire in the pure oxygen atmosphere then being used. Among changes made to prevent future fatalities, a continuous inventory of everything that went into or out of the module was required. In similar fashion, mechanics make rules for themselves through experience to prevent bad outcomes.

If you work on older bikes, you will see that other mechanics before you have made unnecessary work for themselves or have even damaged the equipment while working on it. This may have been part of their learning process, as it’s easy to fall into one’s usual mistakes; the result of ignorance or lack of thought, though let him/her who is without sin cast the first stone; or from backyard methods taken to extremes. I will review some of the incriminating evidence you may find in engines.

1) Inappropriate Use of Case Sealer

Case sealer (Yamabond, Hylomar, &c) is used to seal case halves together. But it is not necessary, or even harmful, when applied to gaskets, seals, or O-rings. The mating surfaces of crankcase halves are finely machined to fit closely together, so all that is needed to assure good sealing is a thin application, sufficient to fill the fine irregularities left by machining. The mating surfaces must be clean and oil-free if the sealer is to adhere and seal properly. On modern assembly lines, robots apply sealer from a kind of pen, which leaves a uniform line of sealer about 2mm wide. That’s all it takes. If you don’t like sealant on your fingers, get a box of inexpensive acid brushes and use them to apply the stuff (throw away after use). I just checked and got a price of $9.99 for 36 of them.

Modern gaskets, oil seals, and O-rings have soft surfaces which intimately conform in order to seal to other surfaces (do we use case sealer to make tires stick better to pavement?). I have seen a case in which dry cylinder base gaskets remained in place but the same gaskets, coated with sealer, oozed out and leaked. The sealer in this case acted like a high-viscosity lubricant, allowing the gasket to glide out of place.

2) Crankshaft Threads or Tapers Ruined by Hammering

When I was rebuilding a lot of pressed-together two-stroke crankshafts, every now and then I would get one whose ends had been hammered into junk. Sometimes it was a threaded end, mushrooming the threads beyond use, or the end with a taper for the ignition rotor. In the latter case, the end of the taper was mushroomed by the hammering such that the rotor could no longer be mounted.

I do understand that sometimes, not having a rotor puller, a person may resort to “Maybe I’ll get lucky” methods such as tapping on the end of the shaft in hope that the rotor will pop off from its own inertia. My uncle taught me this method but I soon learned otherwise (he learned during the Great Depression). No banging on parts with a steel hammer! This is why soft hammers are made, with heads of brass, plastic, or rawhide, so as not to damage what is being tapped. Aluminum, being much softer than steel, needs our compassion especially.

When I didn’t have the required clutch-holding tool to remove the very tight clutch nut from a Honda CBR600 engine, I was tempted to use the bush method of “feeding a rag into the primary gears” (I actually read this one in a British bike manual). This does keep the clutch from rotating, but it exerts tremendous wedging force, tending to push the crank and clutch shafts apart. Knowing that I could order the $14.95 holding tool on the internet and probably have it in a day or two, I did other things until it arrived.

Therefore it makes best sense to order the correct rotor puller or other special tool you need rather than bang on expensive parts—crankshaft!—with a hard steel hammer. If bang you must, use a soft hammer.

3) Breaking Things While Trying to Separate Crankcase Halves

I recently put a fresh crankshaft into a Yamaha TD1-C 250 race engine. As I was removing the screws holding the case halves together (this is a vertically split engine) I noticed that some previous mechanic had given in to the temptation to pry between crankcase fins to separate the cases. Pieces of fin had broken out and there were gouge marks from the long-ago moments of mechano-drama. Embarrassing!

We don’t want our work to look like it was done with explosives. So, some suggestions regarding separating crankcases. First, be absolutely certain that you have removed all case fasteners. Check and check again in good light. Next, cases separate best when warm. Setting the unit in hot sun or next to the shop stove for some hours softens the case sealer, just as for removing stickers from fairings. Another point is that some crankcases have a deliberately designed point or points for applying separation leverage—usually at the back of the gearbox. If not, a few moments of inspection will usually reveal some way that you can apply separating force without levering against weak sections like fins. Take a moment and it will come to you. Impatience is the enemy!

Usually there is a pair of large dowel pins that align the case halves to each other, and they too resist separating. Often tapping with a plastic hammer—aluminum is soft—can keep things moving. Once the cases begin to separate, Beelzebub infiltrates your mind with the idea of poking a screwdriver between them and prying. Fight it! Carried to an extreme, this gouges up the sealing surface where the cases touch each other, leaving incriminating evidence against you and possibly creating a leak.

4) Things to Check Before Closing the Crankcase Halves

When it comes time to reassemble the cases of a horizontally split engine, it’s easy to overlook small features included to prevent unwanted spinning of ball or roller bearing outer races in the case. In some cases there are C-rings or snap-rings to positively locate shafts endwise: Be sure they are present and in their grooves. Are the connecting rods free to move? Sometimes pegs, dowels, or little balls are present in bearing outer races, to be placed in corresponding little retaining slots or holes provided for them in the case. Go from bearing to bearing to be sure all are in their correct positions. On a couple of occasions I was brought DIY customer engines whose cases had been permanently bent by closing and torquing them up without putting all the locators in their proper orientation. Check also to be sure that all seals are in place.

5) Check for Free Rotation of All Shafts, and for Gear Selection

Another point to be made before closing the cases is that this is the time to check for free rotation of all shafts and for gear selection. In an assembled engine, both gear shafts are located axially by being pulled up against a ball bearing, either by the output sprocket nut or by the clutch retaining nut. This is necessary to guarantee that shift dog engagement will be correct; some of the gears are located by the shift forks, and some by snap-rings on the shafts themselves. Therefore, to make the above checks before closing the cases, both shafts must be correctly located by being pulled against their respective ball bearings. This is easy with the output shaft because the only parts required are the seal spacer, drive sprocket, and nut. The nut doesn’t have to be torqued, just run solidly into place to locate the shaft where it belongs. On the clutch shaft the whole stack has to be present to allow tightening the nut to pull the shaft firmly against its bearing. Do all shafts turn easily? Does the gearbox shift all its speeds? (Sometimes you have to rotate one or the other shaft to get the dogs into position to engage the next gear.)

I once ignored this rule while assembling a TZ750 Yamaha race engine at 4 a.m. When I did up the nuts after the cases were closed and torqued, I had a tight shaft, and no amount of tapping or fussing with the nuts freed it up. So it had to come apart again so I could do it right. Pride for dinner.

Don’t ignore your own rules. Do have service and parts books at hand. Special tools are only a few keystrokes away. Just do everything perfectly, and the equipment you build need not bear witness against you.

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