This from another board explains why new /old mix is doomed.
John RGS
Golden BritBiker
Member # 3677
Icon 1 posted June 09, 2006 09:54 AM Profile for John RGS Email John RGS Send New Private Message With Quote : Though Panic is correct, the pitch will be the same, I should have put the following in to elaborate further.
New chain, new sprocket
When a new chain meshes with a new sprocket, every roller that is in contact with the sprocket is pressing more-or-less equally against the corresponding tooth of the sprocket, so the load and stress are shared out equally. From the center of each roller to the center of the next is known as the "pitch" of the chain. The sprocket teeth are made so that the center curve that makes up each "valley" is the same distance from the next. The diameter of the sprocket is determined by the pitch and the number of teeth.
The chain and sprocket have worn together. You can see daylight under the chain in some places. The worn chain has "stretched" so that it no longer matches the original pitch of the sprocket. The sprocket has worn so as to effectively increase its pitch to match the worn chain
. On a new sprocket tooth, the surface that the roller presses against is perpendicular to the pull of the chain. The worn teeth have become ramps, causing the chain to ride up under load.
The rollers ride up on the sloped teeth until they reach a radius that corresponds to the longer pitch of the worn chain. The effective diameter (and thus, the effective pitch) of the sprocket has become larger, since the chain is no longer riding in the bottoms of the valleys.
New chain, worn sprocket
A new chain on a worn sprocket. Most of the driving is happening on the front side, where the chain first engages the sprocket. Due to the mismatch in pitch, the rollers in the part of the chain that is about to leave the sprocket is doing very little to push their teeth forward, instead, they are being forced upward by the slope of the teeth.
Since the teeth/rollers on the front are doing most of the work the load is concentrated on them.
In addition, as the roller follows its tooth around the sprocket it rolls up the "ramp," while under load. This promotes wear to the insides of the rollers and to the "bushings" they roll on. With a properly meshed chain, the roller only turns a tiny bit as it rolls onto and off of the chain.
Due to the pitch mismatch, the chain will not reliably mesh with the sprocket under load, and will tend to jump forward, skipping over the teeth.
Worn chain, new sprocket
A worn chain on a new sprocket. Due to the pitch mismatch, the load is only carried by the teeth/rollers on the back, the chain hangs slack on the front. The new sprocket will wear rapidly to match the pitch of the worn-out chain.