The very oldest lock for micrometers was made in the 1950's. They are metal and have an eccentric cam lock. These are no longer available. The older model Etalon indicating micrometers (1960-1990) have plastic spindle locks which wear out rather quickly from over-tightening. These are no longer available. But there are metal replacements. You can buy this lock and install it yourself. Fortunately the newest and most recent models have an improved metal locking device. This won't retrofit to your older model, however. The new diameters are different. 
Right: #037905 replaces the old plastic lock - notice the rim - this is often a very tight fit* and may need customization. Left: #017857 new style lock (OD = .315") for new model micrometers only. How to replace: unscrew the spindle completely. The old lock usually pulls out easily. After you've removed the old spindle lock, look for a step (counter bore) measuring .406" x .090" in the micrometer body. The presence of this counter bore indicates that this is a new style lock. Insert the new lock and check to see that the inside of the barrel is clear so that you can screw the spindle back in. Don't force anything. And remember: hand tighten only. That's all there is to it. *If you're trying to install #037905 you may notice that it won't fit. The lock diameter is over-sized and may have to be stoned slightly or turned down. You might be tempted to open the hole on the micrometer instead but we suggest you don't tamper with the micrometer body. If you attempt to force fit the lock you may ruin everything because you might not be able to line up the opening correctly, and you may not be able to remove the lock! Proceed with utmost caution.
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