When a QB78-series CO2 rifle starts losing piercing pressure or leaking around the cap, the o-rings inside the piercing cap and valve are common suspects. For anyone searching in seals gaskets o rings, a direct replacement set can be confusing because some manuals list two part numbers that actually describe the same seal. This guide reviews one such set.
Quick verdict
The QB78, QB78D, XS78, TH78 Piercing Cap & Valve TWO (2) O-Ring Seals URETHANE is a straightforward maintenance set priced at $5.75. The listing provides two identical urethane seals, and it clarifies that the #3401 and #0409 part numbers shown in the manual are interchangeable. This set covers both the piercing cap and the valve on models that use a piercing tube cap.
Who it suits
This replacement set is for owners of the QB78, QB78 Deluxe, TH78, XS78, or other "78"-based rifles who have already confirmed that their piercing tube cap uses the #3401/#0409 o-ring. It also makes sense if you want a spare seal on hand, since the pair gives you an extra without requiring a second purchase.
What stands out from the listed details
Urethane is the chosen material, and the listing emphasizes its strength, tear resistance, and abrasion resistance. The seals are new and white, and the country of origin is the United States. The part-number situation is the biggest point to understand: the manual may show separate part numbers, but the listing clarifies that the set is usable in either the cap or valve position.
Meaningful trade-offs
The listing does not give the o-ring dimensions, so if you are not replacing a known #3401/#0409 seal, you need to confirm that your rifle uses this size. The compatibility statement explicitly names the QB78, QB78 Deluxe, TH78, XS78, and "78" variants with a piercing tube cap, but it does not cover other valve designs. No temperature or pressure rating is listed, which may matter if you run a heavily modified setup.
Bottom line
For a targeted piercing cap and valve seal replacement on the listed QB78-series models, this set supplies the factory part numbers without the guesswork of choosing between #3401 and #0409. It is a low-cost maintenance purchase that keeps your spare-seal drawer stocked.
QB78, QB78D, XS78,TH78 Piercing Cap & Valve TWO (2) O-Ring Seals URETHANE
This is the product under review. The listing sells two seals as a set for $5.75. The key attributes are the compatibility wording and the clarification that #3401 and #0409 are identical. A concrete reason to choose it: the listing explicitly names the QB78, QB78 Deluxe, TH78, XS78, and all "78"-based models using a piercing tube cap. A concrete limitation: the lack of listed dimensions means you must verify your existing part number if you are not certain about the size. As long as your rifle matches the listed models, this pair is a sensible replacement.
