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- Hex head screw, SW=11, M7, for carburetor mounting on Vespa Largeframe Dellorto SI -BGM PRO- Vespa PX (1984-), T5 125cc, Cosa
BGM PRO M7 shaft screw with a convenient 11-mm hex head for mounting Dell'Orto SI20, SI24, and SI26 carburetors on Vespa large-frame engines. Replaces the original Piaggio Allen screw and makes carburetor installation much easier.
Important: Always install with the specified washers and lock washers. We recommend the matching BGM PRO washer set BGM2261.
Easier to install, more accessible, and with an additional safety margin for the engine casing. The BGM PRO shaft screw BGM2255 is a well-designed improvement on the original Piaggio carburetor mounting for the Vespa PX, T5, and Cosa.
It replaces the hex socket screw used since around 1984 with a version featuring a practical 11-mm hex head. At the same time, the effective insertion length is reduced by approximately 1.5 mm compared to the original Piaggio screw. This minimizes the risk of damaging the sensitive rotary valve sealing surface below the mounting thread when tightening.
On many Vespa PX engines from around 1984 onward, the SI carburetors are secured with special shaft screws. Piaggio uses screws with a hex socket head for this purpose.
This generally works well, but isn’t always particularly practical during maintenance work. A dirty or already damaged hex socket can make it difficult to loosen the screw.
The BGM PRO shaft screw, on the other hand, features a conventional hex head with an 11 mm wrench size. This allows you to use the same tool required for numerous other M7 fasteners on the Vespa engine.
The comparatively slim head also improves accessibility in tight installation spaces. This is particularly useful for aftermarket intake manifolds from Malossi, Polini, or Pinasco, where conventional screw heads can sometimes be difficult to reach.
The second important improvement concerns the insertion length.
The mounting threads of the SI carburetor are located in the engine casing immediately above the sensitive rotary valve sealing surface. This is precisely where a shaft screw that is screwed in too deeply can cause significant damage.
The problem: If the specified washers or lock washers are omitted from the original Piaggio screw assembly, the screw may bottom out at the bottom of the thread before the carburetor is actually clamped tight enough.
When tightening, this creates noticeable resistance. This can be mistakenly interpreted as the correct tightening torque, even though the necessary clamping force on the carburetor is lacking.
If tightening is continued despite this, the end of the screw can exert pressure on the thin-walled housing area below the threaded hole. In the worst case, the rotary valve sealing surface will be deformed and the engine housing permanently damaged.
The BGM PRO BGM2255 has a thread engagement length that is approximately 1.5 mm shorter than that of the original Piaggio screw. This provides an additional safety margin against unintentionally bottoming out at the bottom of the thread.
Important: The shorter screw does not replace the required washers and lock washers. Only a correctly installed screw assembly with sufficient spacing ensures the intended clamping of the carburetor.
The ideal complement is the BGM PRO washer set BGM2261. It contains the washers and lock washers for both carburetor mounts.
Instead of the original combination of a flat washer and a slotted retaining ring, the set uses a preloaded spring washer.
The right combination of the BGM PRO shank screw and washer set offers several advantages:
Two BGM2255 shaft screws and one BGM2261 washer set are required for complete carburetor mounting.
On older Vespa large-frame engines, the SI carburetor is often mounted using two stud bolts and nuts.
The BGM PRO shaft screw can also be used in place of this original mounting if the thread and installation conditions are suitable. This makes it much easier to remove the carburetor during maintenance work.
Such a conversion is possible, for example, on corresponding engines from the VNB, VBA, VBB, GL, GT, GTR, TS, Sprint, Super, and Rally series.
Before the conversion, the threaded holes, the required insertion depth, and the existing carburetor mounting must be checked.
| Feature | Model |
|---|---|
| Manufacturer | BGM PRO |
| Item Number | BGM2255 |
| Part | Carburetor Shaft Screw |
| Thread | M7 |
| Screw head | External hex |
| Wrench size | 11 mm |
| Screw-in length | Approx. 1.5 mm shorter than the Piaggio original |
| Carburetor types | Dell'Orto SI20 / SI24 / SI26 |
| Other Applications | Compatible rotary valve intake manifolds from Malossi, Polini, and Pinasco |
| Tightening torque | 16–20 Nm with the corresponding original M7 carburetor mounting |
| Recommended washers | BGM PRO BGM2261 |
| Piaggio equivalent number | 239265 |
For Vespa large-frame engines with corresponding M7 carburetor mounting, specifically:
For older engines with stud bolts, as well as for aftermarket intake manifolds, check the compatibility of the respective thread and mounting design before installation.
For the original M7 carburetor mounting on Vespa large-frame engines, a tightening torque of 16–20 Nm is specified.
The following points should be observed during installation:
Caution: Noticeable resistance when tightening does not automatically mean that the carburetor is securely fastened. If a screw is touching the bottom of the thread, it must not be turned any further under additional force.
For aftermarket intake manifolds or different bolting configurations, follow the respective manufacturer’s installation instructions.
Two shaft screws are typically required for a Dell’Orto SI carburetor. The matching washer set BGM2261 is available separately.
The BGM PRO shaft screw features a practical 11-mm hex head instead of the original internal hex. Additionally, the thread engagement length is reduced by approximately 1.5 mm. This simplifies maintenance and provides additional protection against bottoming out at the bottom of the thread.
No. Even with the shorter BGM PRO shaft screw, the specified washers and lock washers must be used. We recommend the BGM PRO Set BGM2261 for this purpose.
The screw may bottom out at the bottom of the thread before the carburetor is securely clamped in place. If it is then forced to turn further, the sealing surface of the rotary valve underneath may become deformed and the engine housing may be damaged.
Yes, this is possible on compatible older large-frame engines with suitable M7 mounting threads. The screw-in depth and the correct washer configuration must be taken into account.
Yes, provided the rotary valve intake manifolds are designed accordingly. Thanks to its relatively slim hex head, the screw is easily accessible even in tight spaces. The specific fit must match the respective intake manifold.
