M9 Washer Set for M7 Shaft Nut - BGM PRO - Vespa PX (1984-), T5 125cc, Cosa - used to mount the Dellorto SI20/SI24/SI26 carburetor on Vespa Largeframe models

BGM PRO Washer Set M9 – Secure Carburetor Mounting for Vespa Largeframe

Four unassuming washers that, in the worst-case scenario, can prevent costly damage to the engine casing. The BGM PRO Washer Set BGM2261 was designed for mounting Dell'Orto SI20, SI24, and SI26 carburetors on Vespa Largeframe engines.

This special design combines matching M9 washers with pre-tensioned spring washers instead of the original slotted retaining rings. This provides the necessary spacing for the carburetor mounting while incorporating a spring-loaded screw lock.

The advantages at a glance

• Important protective function: Correct spacing prevents compatible shaft screws from being screwed in too deeply
• Pre-tensioned spring washers: A spring-loaded design instead of traditional slotted retaining rings
• Original-spec fit: M9 washers for the special M7 carburetor mounting
• Universal application: For compatible stud and shaft screw mountings
• Complete set: All four washers for the two mounting points of an SI carburetor

Why the washers are so important

On Vespa PX engines from around 1984 onward, Piaggio uses special shaft screws to mount the Dell’Orto SI carburetor directly to the engine casing.

The mounting threads are located immediately above the sensitive rotary valve sealing surface. Therefore, the correct screw-in depth is particularly important.

If the specified washers or lock washers are omitted, the screw may reach the end of the threaded hole before the carburetor is sufficiently tightened.

This can have two possible consequences:

• The carburetor is not properly secured: When tightening, there is noticeable resistance even though the screw is only resting against the bottom of the thread. The necessary clamping force on the carburetor is lacking.
• The rotary valve sealing surface is damaged: If the screw is nevertheless tightened further with excessive force, the thin-walled housing area below the thread may become deformed. In the worst case, the rotary valve sealing surface is pushed downward, causing permanent damage to the engine housing.

A damaged rotary valve sealing surface can significantly impair the intake seal. Repair is costly and, in the worst-case scenario, may require replacement of the engine housing.

Therefore: Never install the carburetor shaft screws without the specified washers and lock washers!

Pre-tensioned spring washers instead of conventional retaining rings

The original Piaggio fastening system uses a flat washer together with a slotted retaining ring.

BGM PRO replaces the classic snap ring with a preloaded spring washer. When tightened, this creates an elastic counterforce and supports the preload of the screw connection.

Together with the washer, this results in a washer assembly specifically tailored to the carburetor mounting. However, the correct tightening torque and the proper insertion depth remain crucial for the security of the screw connection.

The Perfect Complement: BGM PRO Shaft Screw BGM2255

We highly recommend combining it with our BGM PRO shaft screw BGM2255. It was developed as an improved replacement for the original Piaggio hex socket screw and offers two key advantages:

1. Practical hex head with an 11 mm wrench size

Instead of the original hex socket, the BGM PRO shaft screw features an external hex head. This allows it to be easily loosened and tightened using a standard 11-mm wrench or a matching socket.

This is particularly convenient when performing maintenance on the SI carburetor. Thanks to its slightly slimmer screw head, the BGM screw is also suitable for securing various rotary valve intake manifolds from Malossi, Polini, and Pinasco.

2. Approximately 1.5 mm shorter thread length

Compared to the original Piaggio shank screw, the BGM PRO version has a thread engagement length that is approximately 1.5 mm shorter.

This provides an additional safety margin against the screw accidentally bottoming out at the bottom of the thread and reduces the risk of damaging the sealing surface of the rotary valve beneath it.

Our recommendation: Use BGM PRO shaft screws (BGM2255) together with the washer set (BGM2261). The shorter screw offers additional safety but does not, under any circumstances, replace the necessary use of the washers.

Also for older Vespa engines with carburetor studs

On older Vespa large-frame engines, the SI carburetor is often secured using two studs and nuts. The appropriate BGM PRO washers and spring washers can also be used here.

These include, among others, the classic VNB, VBA, VBB, GL, GT, GTR, TS, Sprint, and Rally series with the corresponding SI carburetor mounting.

If you want to replace the original stud mounting on these engines with the BGM PRO BGM2255 shaft screws, you’ll also benefit from a particularly maintenance-friendly solution. This requires the correct thread and installation conditions.

Technical Specifications

Installation Instructions

A tightening torque of 16–20 Nm is specified for mounting the M7 carburetor on classic Vespa large-frame engines.

The following points should be observed during installation:

• Use one washer and one spring washer per mounting point.
• First, hand-tighten the bolts and ensure that the threads engage smoothly.
• Before tightening them completely, make sure the shaft bolts do not rest against the bottom of the thread.
• Tighten both carburetor mounts evenly using a suitable torque wrench.
• After installation, check that the carburetor is securely fastened and that the connection is leak-free.

Important: Even if the specified torque is reached, this does not guarantee a properly clamped connection. If the bolt is touching the bottom of the thread, it must not be turned any further under additional force.

Compatible Vehicles

For Vespa large-frame engines with the corresponding Dell’Orto SI carburetor mounting, specifically:

• Vespa PX80 / PX125 / PX150 / PX200
• Vespa PX Lusso / MY / 2011
• Vespa T5 125
• Vespa Cosa 125 / 150 / 200
• Vespa VNB / VBA / VBB
• Vespa GL / GT / GTR / TS
• Vespa Sprint / Sprint Veloce
• Vespa Rally 180 / 200
• Other large-frame engines with compatible SI carburetor mounts

Package Contents

• 2 × M9 washers
• 2 × preloaded spring washers

A complete set for the two mounting points of an SI carburetor. Shaft screws, studs, and nuts are not included.

Frequently Asked Questions (FAQ)

Why are M9 washers used with an M7 screw?

The special carburetor mounting uses a larger shank diameter than the actual M7 mounting thread. The M9 washers fit this shank diameter.

What happens if the washers are omitted?

With these shaft screws, the screw end may come into contact with the bottom of the thread before the carburetor is properly clamped. If you continue to tighten it, there is a risk of deforming the sealing surface of the rotary valve underneath and damaging the engine housing.

What are the advantages of the BGM PRO shaft screw BGM2255?

It features a practical 11-mm external hex and a thread engagement length that is approximately 1.5 mm shorter than the original Piaggio screw. This simplifies maintenance and reduces the risk of the screw bottoming out at the bottom of the thread.

Can I also use the washer set with original Piaggio screws?

Yes. The set is designed for the corresponding original M7 shaft screw mounting with an M9 washer mount. The correct spacing and insertion depth must be ensured during installation.

Is the set also suitable for older engines with stud bolts?

Yes, provided the shaft and mounting surface geometries are compatible, it can also be used with studs and nuts. The BGM2255 shaft screws also allow for a corresponding conversion of the mounting system.