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To Reassemble, Simply Reverse the Process – Witness Mark

Witness Marks

To reassemble, simply reverse the process. Infamous and maddening bits of misdirection found in nearly every shop manual. If only it were that easy. Here are some other ideas.

Someone asked me the other day, “What the heck is a witness mark?” So here we go. In my broad estimation anything that gives “witness” as to how parts fit together or were improperly fit or shows clues as to why a part failed can be termed a witness mark. Sometimes we don’t have a manual or a photo showing how parts fit together or the orientation of those parts so these “clues” help us solve reassembly questions such as how do these parts fit together. 

A dab of pen for a witness mark can identify how a shift lever should line up on a spline. For doing valve adjustments on a multi cylinder four-stroke bike: a witness mark using a permanent marker can show which valves can be adjusted before rolling the engine over 360 to do the remaining half. This way you save much time not trying to do each cylinder individually.

In some cases the witness mark can also identify assembly mistakes. Pulling valves from the head of a diesel engine in one of our shop classes revealed lots of bad score marks on the stem. To me that screams improper assembly; the valve should have had a coating of engine oil or assembly lube. 

In fact, as a general practice, any metal parts that can rub each other should be lubricated. Engine oil is usually fine except for high stress areas such as camshafts, which need heaver viscosity assembly lubes. I like the GM cam lube. STP works well and can be found almost anywhere. There are lots of similar products out there and I think they all will work fine.

The score marks were not fatal but there is an area of study known as Failure Analysis, which looks into the root cause of failures. Books have been written but the long and short of it is finding out why things break. Sometimes, not often, it is bad design or materials. Much more common is “overload” or a service condition: this is a nice way of saying operator error.  

 In the case of my Suzuki that I call George W GIXR, a smoking problem was not overfilling as I had originally thought. On disassembly I found marks in the cylinder indicated that a glycol-water mix had been sitting in the cylinder leaving a corrosion scar. This upset the ability of the oil ring to properly wipe off excess oil on the wall thus causing a very bad smoking issue. 

Finding these clues is obviously important because if warranty is involved it may be denied if there is proof of abuse. Even if there is no warranty involved it is nice to know what went wrong so we don’t make the same mistake again. I think it is human nature to blame the broke part (it’s a piece of *&^ $%^$) but… 

Failures can be identified by inspecting the parts only visually (sometimes that’s enough) but also by using various destructive and non-destructive technologies. It would take a lot more than one issue to go into this stuff but it is interesting indeed.

When it’s time for assembly photos are great, and marks work if applied carefully. In the old days folks made sketches, which worked on two levels: you got a drawing and probably remembered drawing it. 

The Sportster primary case uses longer and shorter fasteners. If laid out on a towel in the general pattern it’s easy not to get them mixed up. Along this same line of thinking, keep parts in groups such as the clutch cover bits. This works if you are putting it back together fairly soon.

Happy assembly!

by Rich Burgess Canadian Biker Issue #331


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