Timing Chain Sprockets

This is not an indictment of aftermarket reproduction parts, but rather some things to watch for. This article is addressed specifically to the timing chain sprockets.

Just about every Triumph mechanic who ever lived has built an engine out of a box of parts. My "Waterloo" was a 1959 TR3A unit delivered in three large cardboard boxes, one of which contained new parts. As I began to assemble the engine I came upon two unmarked boxes that contained the timing sprockets. The lower one fitted too loosely on the crankshaft snout (about .002") and slipped freely back and forth. It obviously relied on the crank pulley’s main bolt to hold it steady. Now our experience has shown that without a tight fit on the cankshaft the keyway in the gear quickly wears as does the Woodruff key itself, and soon the engine and cam timing will be all over the clock.

The upper sprocket, or camshaft chainwheel as it’s known in the shop manual, had other tricks in store. It looked like a good reproduction until I went to time the camshaft. I tried the usual adjustment method of turning the chainwheel front to back, then 90 degrees in either direction, but regardless of the position of the sprocket, the cam remained 4 degrees retarded. One of my associates at the Bureau of Standards made an elaborate test jig and after completing the tool and the test, proudly announced that the four holes in the repro chainwheel were drilled concentric, defeating any possibility of adjustment. Depending on which pair of holes was used, the timing was either 4 degrees retarded or 12 degrees advanced, neither of which was acceptable. The whole problem (so to speak) was ultimately solved by going through my used spares until I found an acceptable used original chainwheel. Once installed, the timing instantly became adjustable again by changing the position of the chainwheel in the usual manner. I don’t know who made this particular sprocket, or who sold it, but my advice is to accept a little wear and use an original chainwheel. It was clear that whoever made this particular repro just drilled four holes in it without any understanding of what they do.