Hey there! That's a really smart question - you're hitting on one of the biggest challenges in modern manufacturing. Balancing those carrying costs against potential downtime is like walking a tightrope, especially in JIT environments where every minute of production stoppage can cost thousands. Here's what I've found works best:

First, you need to start with a solid criticality analysis. Not all spare parts are created equal. Use tools like ABC analysis (based on value), VED analysis (vital, essential, desirable), and FSN analysis (fast, slow, non-moving) to categorize your parts. The most critical ones that could cause major downtime get priority stocking.

Next, implement predictive maintenance strategies. By monitoring equipment health in real-time, you can anticipate failures and order parts just before they're needed, reducing the need to keep large inventories on hand.

Strong supplier relationships are crucial in JIT. Work with key suppliers to establish consignment inventory or vendor-managed inventory programs where they hold the stock and you only pay when you use it. This transfers carrying costs while ensuring availability.

Consider regional pooling arrangements with other manufacturers. If you're in an industrial park, you might be able to share certain critical spares with neighbors, splitting the carrying costs while maintaining access.

Technology is your friend here. Modern CMMS (Computerized Maintenance Management Systems) can optimize reorder points based on actual usage patterns, lead times, and criticality ratings. Some systems even use AI to predict when you'll need specific parts.

Finally, calculate the true cost of downtime for each piece of equipment. When you know that one hour of downtime costs $10,000, it becomes much easier to justify keeping a $5,000 spare part in stock versus risking production stoppage.

The key is being strategic rather than reactive - don't just stock everything 'just in case,' but use data-driven approaches to stock the right parts in the right quantities at the right time.