CNC Turned Brass Carburetor Throttle Shaft
Throttle shafts turned and ground from C36000 brass, with journals held to h6 and round to 0.005 mm at Ra 0.4 µm, straightness at 0.02 mm over the full length, the throttle flat milled square to 0.02 mm and shoulder spacing within 0.02 mm to control end float.
- Journals Ground to h6
- Throttle Flat Within 0.03 mm
- End Float 0.02 mm
- Straight to 0.02 mm Over 80 mm
*Send the carburettor body bore and the throttle plate drawing. The flat depth sets the plate angle at idle, and 0.05 mm too deep turns a smooth idle into a 1,400 rpm hang.
Brass Throttle Shaft Overview
A throttle shaft is a small, precise spindle: two journals run in the body, a flat carries the throttle plate, and the ends take a lever and a return spring. If the flat is not square to the axis the plate binds in the bore and the throttle will not close fully.
The shaft is turned between centres with the journals left 0.05 mm oversize, then ground to h6, and the throttle flat is milled from the journals as the datum so the plate sits at the right angle.
C36000 Brass
C36000 is used because it is stable in fuel, machines to a fine journal finish and does not seize in a brass or zinc carburettor body the way an aluminium shaft does when it corrodes.
303 Stainless or 2011 Aluminium
303 is chosen for ethanol-blended fuel and marine engines. 2011 aluminium is used in modern throttle bodies where weight matters and the body is also aluminium, hard anodized to reduce wear.
A brass shaft in an aluminium body needs a dry lubricant; without it the two metals corrode together and the throttle sticks after a season of storage.
Brass Throttle Shaft Specifications
Turning and grinding data for throttle spindles, with the journal, flat and end float values that decide whether the throttle closes cleanly.
| Specification | Machining Data | Specification | Machining Data |
|---|---|---|---|
| Product | Throttle Shaft / Carburettor Spindle | Machining Process | CNC Turning, Cylindrical Grinding, Milling |
| Material | C36000 Brass, 303 Stainless on Request | Envelope | Ø 3 – 14 mm, Length 30 – 140 mm |
| Journals | h6, Roundness 0.005 mm | Journal Finish | Ra 0.4 µm Ground |
| Throttle Flat | Milled to 0.03 mm, Square to the Axis 0.02 mm | Flat Length | ±0.10 mm |
| Straightness | 0.02 mm Over the Full Length | End Float | 0.02 mm Between Shoulders |
| Shaft Ends | Slotted, Threaded or D-Shaped | Slot Width | ±0.05 mm |
| Shoulder Spacing | ±0.02 mm | Runout | 0.01 mm at the Journal |
| Customization | Lever Arm, Return Spring Hook, Cross Hole, Anti-Tamper Slot | Lead Time | 4 – 6 Days Sample, 10 – 14 Days Batch |
| Inspection | Micrometer Journals, Runout Between Centres, Flat Depth Gauge | Packing | Tube per Shaft, Oiled Paper |
Key Features of Our Throttle Shafts
Ground journals, flat cut from the journal datum, end float controlled.
Journals Ground, Not Just Turned
Grinding removes the 0.008 mm lobing a turning insert leaves, so the shaft runs in its bush without a high spot that shows as a sticky first few degrees of throttle.
Flat Cut From the Journal Datum
The throttle flat is milled with the journals as the datum, square to 0.02 mm, so the plate sits at the correct angle and seals against the bore at idle.
Shoulder Spacing Held to 0.02 mm
End float is set by the distance between the shoulders, not by a washer, so the shaft cannot walk and wear the body bore on one side.
Runout Checked Between Centres
Every shaft is indicated between centres after grinding, and anything above 0.01 mm at the journal is straightened or scrapped rather than shipped.
Shaft that has to let the throttle close?
Surface Finishes for Throttle Shafts
The journals are the bearing surface and must stay bare; the rest can be coated for appearance and wear.
| Finish | Main Purpose | Finish | Main Purpose |
|---|---|---|---|
| Natural Brass | Bearing surface as ground | Nickel Plating | Wear resistance on the flat |
| Clear Lacquer | Stops tarnish on shelf | Tin Plating | Fuel resistance on brass |
| Bead Blasting | Matte on the non-bearing length | Passivation | Cleaning before coating |
| Plating Build | 3 – 5 µm, Journals Masked | Ultrasonic Clean | Removes grinding swarf |
| Journal Protection | Caps During Coating | Assembly Oil | Light Film, Fuel Compatible |
| Salt Spray | 72 h on plated shafts | Straightness Check | After Coating |
*Plating on a journal closes an h6 fit by 6 to 10 µm and the shaft will not enter its bush. Journals are masked with a tape sleeve and checked again after coating.
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Machining Process for Throttle Shafts
The journals are ground between centres after turning, so the shaft runs in its bushes without the 0.01 mm high spot that a turned finish leaves.
01. Body and Plate Review
Check the body bore, the plate thickness and the lever fixing, then set the journal size and the flat depth from the drawing.
02. Turn Between Centres
Turn the shaft between centres, leaving 0.05 mm on the journals and holding the shoulder spacing to ±0.02 mm.
03. Grind the Journals
Grind to h6 at Ra 0.4 µm, checking diameter and roundness on a micrometer stand before the shaft leaves the grinder.
04. Mill the Throttle Flat
Mill the flat from the journal datum, check depth and squareness, then cut the end slot or thread if the drawing calls for one.
05. Straighten, Check, Oil
Indicate runout between centres, straighten if needed, apply a fuel-compatible oil film and pack each shaft in its own tube.
Send the carburettor body and plate drawing, and we will quote a shaft with ground journals and the flat cut from the same datum.
Brass Throttle Shaft FAQs
A turned journal has 0.008 mm of lobing from the insert radius, which is enough to feel as a tight spot when the throttle opens from idle. Grinding removes it and holds roundness at 0.005 mm.
It follows the plate thickness and the bore angle, usually 0.5 to 0.8 mm. The depth is held within 0.03 mm because it sets the plate angle and therefore the idle bypass.
Yes, from the old shaft or a drawing. Worn journals are measured and the new shaft is made to nominal, not to the wear, since the body bore has usually worn too.
C36000 is stable in E10 and E85 but can dezincify in the presence of water and chlorides. For marine or long-storage engines we recommend 303 stainless instead.
Yes, a lever arm and spring hook can be cut from the same bar, which keeps the lever square to the flat and removes the need for a separate pinned joint.
0.02 mm between the shoulders as supplied. The body bore usually adds 0.02 to 0.05 mm, so the assembled float lands between 0.04 and 0.07 mm, which is the normal range for a butterfly throttle.
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