The Triumph Camshaft Sprocket Saga
by Brian Mills
Putting a timing chain on an engine is usually not a big deal. In many engines, both the crankshaft sprocket and the camshaft sprocket are aligned to their shafts with a key and key way. This allows the sprocket to fit only in one position relative to the shaft. It is usually a minor matter of aligning the timing marks and installing the chain.
Triumph camshaft sprockets are different for reasons not clear to me. The crankshaft sprocket is fixed by a key and only goes on in one position. The camshaft sprocket has four mounting holes, but only two are used to attach the sprocket to the camshaft. Most Triumph cam sprockets (but not TR6 sprockets) are symmetrical, i.e., they can be flipped over and either side put toward the engine.
Most Triumph Camshaft Sprockets
The camshaft sprocket has 42 teeth, which means that for 90 degrees of rotation there are 10.5 teeth. This also means that if the other two holes are used to mount the sprocket the relative position of the sprocket to the cam is offset by 0.5 of a tooth (Figure 1).
Putting a timing chain on an engine is usually not a big deal. In many engines, both the crankshaft sprocket and the camshaft sprocket are aligned to their shafts with a key and key way. This allows the sprocket to fit only in one position relative to the shaft. It is usually a minor matter of aligning the timing marks and installing the chain.
Triumph camshaft sprockets are different for reasons not clear to me. The crankshaft sprocket is fixed by a key and only goes on in one position. The camshaft sprocket has four mounting holes, but only two are used to attach the sprocket to the camshaft. Most Triumph cam sprockets (but not TR6 sprockets) are symmetrical, i.e., they can be flipped over and either side put toward the engine.
Most Triumph Camshaft Sprockets
The camshaft sprocket has 42 teeth, which means that for 90 degrees of rotation there are 10.5 teeth. This also means that if the other two holes are used to mount the sprocket the relative position of the sprocket to the cam is offset by 0.5 of a tooth (Figure 1).
Figure 1: In most Triumphs there are 10.5 teeth in 90 degrees and the mounting holes are symmetrical; however, because the mounting holes are offset 1/8 of a tooth, adjustments of up to 0.75 of a tooth are possible.
In addition, the centre lines through the mounting holes are actually offset from the crest and trough by ± 1/8 of a tooth. When one side of the sprocket is used we have 0 (reference) and 0.5 tooth offsets. When the sprocket is flipped front to back, installed in the same holes or rotated 90 degrees you produce a net effect of 0, 0.25, 0.5, and 0.75 tooth offsets.
This reversible sprocket is found in TR2 through TR4A (dual chain) and Spitfires, early TR250s, and GT6s, which use single chain drives.
TR6 Non-Reversible Camshaft Sprockets
After having problems setting up the valve timing of a TR6 engine that had a new camshaft sprocket installed, I did some investigation. Here is what I found:
In addition, the centre lines through the mounting holes are actually offset from the crest and trough by ± 1/8 of a tooth. When one side of the sprocket is used we have 0 (reference) and 0.5 tooth offsets. When the sprocket is flipped front to back, installed in the same holes or rotated 90 degrees you produce a net effect of 0, 0.25, 0.5, and 0.75 tooth offsets.
This reversible sprocket is found in TR2 through TR4A (dual chain) and Spitfires, early TR250s, and GT6s, which use single chain drives.
TR6 Non-Reversible Camshaft Sprockets
After having problems setting up the valve timing of a TR6 engine that had a new camshaft sprocket installed, I did some investigation. Here is what I found:
- the reversible dual chain sprockets (TR2–TR4) and the sprockets used for single chains (Spitfires, TR250s, GT6s) have the mounting holes arranged in a square pattern that is offset to the tooth crests and troughs by ± 1/8 of a tooth.
- the original TR6 camshaft sprockets (which are the same size and have the same number of teeth) have a smaller mounting hole spacing, and the mounting holes are NOT in a square pattern (see Figure 2). What I found was that dimensions A and D were larger than B and C. However, they work just fine when setting up a camshaft and I have done it several times with no problem at all. However, I still don’t understand why the spacing is different.
Figure 2: Arrangement of holes in stock TR6 cam strocket. Dimensions A and D are 0.390 inches; dimensions Band C are 0.370 inches (averages of three sprockets).
In an accompanying article, Brian explains what all this mean when you try to adjust your valve timing.
- with the after-market TR6 cam sprocket that Bob Thomas bought, we could not get better than 0.25 tooth alignment, which is not satisfactory for good performance. This sprocket differed from the stock sprocket in that the mounting holes were arranged in a square pattern, with a spacing of 0.385 inches between the holes.
In an accompanying article, Brian explains what all this mean when you try to adjust your valve timing.