A European customer contacted us to purchase spare parts for an existing machine. On their BOM, the specified part was a Rexroth linear guide block R165121420.

This Rexroth linear guide had been running reliably for years, so the customer’s first request was simple:

“Please quote R165121420 exactly as listed in our BOM. We prefer not to change part numbers.”

However, when we checked with our warehouse and Rexroth channels, R165121420 had no available stock and the lead time was several weeks. For a production line that needs quick maintenance, waiting that long would create a real risk of extended downtime.

Instead of immediately proposing a different model, our engineering team first treated this as a compatibility and risk question, not just a stock problem.

We reviewed the catalog and confirmed:

• Brand: Rexroth original, not a third-party option
• Series: Same series of Rexroth linear guide blocks
• Main dimensions: mounting hole pattern, block height, rail interface and bolt size were identical
• Preload: same preload class as the requested R165121420
• Accuracy grade: R165121320 offers one grade higher accuracy than R165121420

In other words, R165121320 is a drop-in replacement with upgraded accuracy, not a downgraded alternative. From a pure engineering point of view, these two interchangeable Rexroth blocks can be used in the same application without changing any mechanical design.

From a buyer’s side, cost is never just the price per piece. Together with the customer, we looked at three cost dimensions:

1. Unit price
   R165121320 is slightly more expensive than R165121420, which is normal for a higher-accuracy Rexroth linear guide block.
2. Delivery time and downtime risk
   R165121420: no stock, long lead time.
   R165121320: available from stock, ready for quick shipment.
   For a running production line, the cost of stopping a machine for weeks is far higher than the small price difference between the two models.
3. Communication and approval cost
   If every future order requires a new email chain to ask, “Can we use 320 instead of 420 this time?”, both sides lose time. A clear rule on approved interchangeable parts reduces this hidden cost.

After this discussion, the customer agreed that the total cost of ownership favored using R165121320, provided that documentation and traceability were handled correctly.

The customer’s main worry was:

“Our BOM says R165121420. If the label on the box shows R165121320, will it look like the wrong part in the future?”

To remove this concern, we made the replacement transparent in writing.

On the PI, packing list and carton labels, we added a clear remark:

“R165121320 supplied as direct replacement for R165121420 (same mounting dimensions and preload, one grade higher accuracy).”

This way, anyone who checks the documents later can immediately understand that:

• It is still a Rexroth linear guide block,
• The part was supplied according to an agreed interchangeable rule,
• It is not a shipping error, but a controlled upgrade.

To further reduce future communication time, we suggested the customer add an internal remark in their ERP/BOM system. We provided a suggested text they could simply copy:

Internal note:
R165121420 ↔ R165121320 approved as interchangeable Rexroth blocks (same mounting dimensions and preload; R165121320 = higher accuracy grade).

With this remark in place:

• The maintenance team knows that both codes are acceptable for this position.
• Purchasing can place orders using either part number when talking to linear guide manufacturers or distributors.
• Future buyers no longer need to ask, “Are you sure we can use R165121320 instead of R165121420?” every single time.

What used to be a case-by-case exception becomes a formal, documented rule.

The customer also raised a practical question:

“If we accept this replacement now, what happens if next time R165121320 has no stock and only R165121420 is available?”

Because we had already treated these models as an approved pair of interchangeable Rexroth blocks, the answer was simple and consistent:

• Both R165121420 and R165121320 are technically compatible in this application.
• Our internal rule is:
  1) Priority to R165121320 (higher accuracy) when available;
  2) Use R165121420 when R165121320 is not in stock.

We shared this priority rule with the customer so they know we are not “randomly changing models”, but working within a stable, documented equivalence.

Thanks to this approach:

• The customer received R165121320 from stock, avoiding several weeks of potential machine downtime.
• The machine was installed with no drawing changes and no additional mechanical adjustment.
• Due to the higher accuracy grade of the supplied Rexroth linear guide block, the customer even gained some performance margin.
• With the ERP remark in place, future orders can reference either R165121420 or R165121320 without confusion or repeated email approval.

This case shows that, when handled carefully, a stock problem can turn into a controlled upgrade:

• same dimensions,
• same preload,
• higher accuracy,
• and a clear interchangeable rule that saves time on both sides.

Facing a similar stock or lead-time problem with your Rexroth linear guide blocks? Share your BOM with us, and our engineers will help you evaluate safe, interchangeable options before you place an order.