BOCI BLT421 / BLT421S Consumables: Which Parts Actually Fit

BLT421 and BLT421S are not the same head

Most consumables do not interchange. The BOCI BLT421 and the BLT421S look alike and both sit at the ≤8 kW end of the BLT4 line, but BOCHU changed the protective windows, the drawers that hold them, the collimating unit and the capacitance sensor module when it upgraded the head. An order placed against the bare “BLT421” name is the most common reason a BOCI window arrives and will not seat.

This page sets out what changed, what still carries over, and the three photos that let us match your head without you needing a single part number.

BLT421 vs BLT421S: what changed

The safe reading: one letter changes almost the whole optical stack. Only the ceramic body, the M11×0.75 nozzle thread and the main lower spring seal carry over. Even there we match against your old part before shipping, because heads in the field are often rebuilt with a newer drawer.

Protective windows: the upper / lower split behind wrong orders

The 421S has four protective-window positions — upper, 2nd-upper, 2nd-lower and 1st-lower. Three of them take a Ø25.4 × 4 mm window. The one closest to the nozzle, the 1st-lower, takes Ø34 × 5 mm.

That is why two “≤8 kW” sizes both show up in marketplace listings labelled “BLT421 lower protective window”:

• Ø25.4 × 4 mm is the upper / 2nd-upper / 2nd-lower window on the 421S — and the 1st-lower window on the 3-series BLT310 (≤3 kW), not on the 421.
• Ø34 × 5 mm is the 1st-lower window across the BLT4, BLT5 and BLT6 families. It is not a 6-series-only size: BOCHU’s own BLT462 manual, a 4-series head, specifies a Ø34 × 5 mm lower window.

The -2 / -4 / -6 suffix on a window is a power and coating grade, not a head model, so an “8 kW machine” does not automatically mean the “-2” window. And same diameter never means same position — a Ø25.4 × 4 mm window made for an upper drawer will not seal in a lower drawer. Order by position, then size, then focal length. Never by diameter alone.

Which head do you actually have?

Read the whole nameplate, not just the first five characters:

• Model suffix — BLT421, BLT421S, BLT421T or BLT421TS. The T versions are tube-cutting heads with an M8 ring (see below).
• Focal length — F150 or F200. The sensor module and the collimating drawer change with this even inside one model.
• Power — the 421 family is rated ≤8 kW.

Model lineage runs BLT421 → BLT421S → BLT422. BOCHU’s current BLT4 page lists the BLT422 as the entry model, so the BLT421S is an upgraded replacement rather than the current head. We found no official end-of-life notice, and BOCHU has never published a definition of the “S” — treat any claim that it means a “QS version” as reseller wording, not a specification.

What to send us — no part numbers needed

1. Photo of the head nameplate, showing the full model including F150 / F200.
2. Photo of the old window drawer or cartridge, showing which position it came out of: upper, 2nd-upper, 2nd-lower or lower.
3. Photo of the ceramic ring and nozzle, so we can confirm the M11 / M8 thread.

Add the laser power and the material you cut, and we will identify the right windows before you order. BOCHU’s own BLT421 manual makes the same point: it states that maintainable parts differ in specification between products, and asks users to confirm the exact model with after-sales service before replacement. Send the three photos here.

M11 vs M8: where the tube-cutting versions split

Both planar 421 heads use M11×0.75 ceramic bodies, locking rings and nozzles. The tube-cutting BLT421T / BLT421TS use M8×0.75. The two threads are not interchangeable. Our BOCI brand page carries the full ring-thread chart by series.

FAQ

Are BLT421 and BLT421S consumables interchangeable?

Only partly. Ceramic bodies, locking rings and M11×0.75 nozzles carry over, as does the main lower spring seal. Protective windows, their drawers, the collimating / focusing unit, the 2nd-lower seal and the capacitance sensor module do not. Confirm the full model first.

Can I order a protective window by diameter alone?

No. Ø25.4 × 4 mm appears in three different positions on a 421S and at the lower position of a BLT310. The position on the head and the F150 / F200 focal length decide the part; diameter only narrows the list.

My nameplate says BLT421 but the machine is 6 kW — which ring?

A planar 421-family head at 6 kW takes the M11×0.75 ring. Check that it is not a BLT421T tube head, which takes M8×0.75. The power figure does not change the ring; the T suffix does.

Why do you ask for photos instead of a part number?

Because BOCHU revised the drawers mid-series and field heads are often rebuilt with a newer drawer. A nameplate plus the old drawer photo is more reliable than a number copied from an old catalogue — and it saves you having to decode part codes at all.

Sources

Sizes and model mappings on this page are cross-checked against BOCHU’s public original lens selection tables and consumables tables, plus the BLT421 (V1.1) and BLT421S (V1.0) product manuals. Where an official table and a reseller listing disagree, we follow the official table.

Related categories

• Laser nozzles
• Protective lenses
• Ceramic rings & cutting parts
• Collimating & focus lenses
• Sensor cables
• CO2 optics

Have a BLT421 / BLT421S head to maintain? Send the nameplate and two photos and we will match the windows, ring and sensor module before quoting.

Ruiying is an independent supplier of aftermarket laser consumables. BOCI, BOCHU and BLT are used only to indicate compatibility and remain the property of their owners; we are not affiliated with, authorised by, or endorsed by these manufacturers. Head revisions vary — confirm the full model before ordering.