3/8-8 Chain Wheel The Marking Is the Dialect

The parts before this one drink oil or feed oil; this one is just a shape, a disc with teeth and one line of lettering on it. The 3/8-8 chain wheel, the marking is its whole identity.

What does the 3/8-8 on the face mean? The 3/8 is the chain pitch, and the 8 is the number of teeth on this wheel. The chain on a chain trimmer head is graded by pitch, and the 3/8 grade takes a 3/8 wheel; the teeth come in 8, 9 or 10, and the same grade of chain bites all of them, more teeth turn a little faster, fewer teeth last a little longer. So the 3/8-8, 3/8-9 and 3/8-10 are one family, and when you buy, look at what the old wheel's face is stamped with and take the same.

Why are the teeth on the flat face, not on the edge? This wheel sits flat in the center of the trimmer head, and the chain runs around the head, driven by the ring of teeth on the flat face, not the edge teeth of a chainsaw bar. Flat teeth bite the chain, the chain runs on the head's plane, the grass is cut out at the rim, and the wheel spins in the middle.

What is the splined hole in the center for? The wheel has to grip the shaft that comes out of the trimmer, the shaft is splined, the hole in the wheel is splined, the two mesh, the nut goes on, and when the shaft turns the wheel turns. The spline mesh does not slip, which is more reliable than a plain hole with a circlip; when grass tangles the head, the spline does not get ground round.

What are the lightening holes on the back for? The wheel is a stamped steel plate, and the ring of lightening holes takes out the metal that carries no load, the wheel is lighter, the shaft and the bearing carry less, and it spins steadier at high speed. The holes look like saving material, but they are actually balancing weight.

When to replace it? The flat teeth are worn to points, the chain skips on the wheel, the spline is worn round and slips, or there is a crack on the face, then swap it. The chain wheel is the most worn part on a chain trimmer head, and when the chain wears short, the wheel wears with it.

What to check when swapping it? Line up the tooth count, the spline shape and the face thickness on the new wheel against the old one. The 3/8 family mostly fits, but fit is not guaranteed for every one, so comparing against the old wheel is the safe way. On install, mesh the spline with the shaft, sit the wheel flat, tighten the nut the right way, and spin it to check that the chain runs true in the teeth.