Which tool would be least effective for tapping oil seals into place?

The ball peen hammer is designed for metalworking and is primarily used for shaping and striking hard materials. Its hard, rounded striking surface makes it more suitable for tasks that require force, such as peening metal, rather than gently tapping delicate components like oil seals into place.

When it comes to installing oil seals, the goal is to avoid damaging the seal while ensuring it is seated properly. Tools such as rawhide mallets and rubber mallets provide a softer impact, reducing the risk of damaging the seal or the surrounding components. The dead blow hammer, filled with sand or shot, also delivers a controlled, effective impact without the risk of rebound, making it safer for this application than a ball peen hammer.

Using a ball peen hammer for this task would likely lead to uneven application of force and potential damage to the oil seal, hence making it the least effective choice for this specific job.