4.94 / .1945 Valve Guide Reamer Engine

Valve Guide Reamer 4.94 mm (.1945") – Engine Valve Guide Finishing Tool

Valve guide reamer Ø4.94 mm for precision finishing of valve guides in engine cylinder heads.
Used after drilling or rough reaming to bring the guide to final size with low stock removal.

Application:

Designed for sizing valve guides in automotive, motorcycle and industrial engines.
At Ø4.94 mm, this tool is suited to small guide bores where a slight size change directly affects valve stem clearance, oil control and stem support.

What it actually does:

Single-pass finishing operation with minimal material removal.
Because the tool removes only a light allowance, it improves surface finish and stabilizes final bore size rather than correcting major bore error.

Diameter 4.94 mm (.1945"):

• Sets the final guide bore size for this exact stem and clearance range
• Tight size control matters because small-diameter guides react strongly to slight oversize
• No geometry correction — the reamer follows the existing hole axis

Cutting design:

• Spiral flutes move chips smoothly and reduce scoring of the finished guide surface
• Light finishing geometry lowers cutting force, so the reamer stays stable in narrow valve guide bores
• Smoother entry reduces chatter, which helps keep bore finish consistent

Tool geometry:

The geometry is optimized for valve guide finishing, where rigidity and alignment are more important than high stock removal.

A front guide pilot keeps the tool aligned with the existing bore during entry and cut. Because the pilot supports the tool before the flutes fully engage, radial runout is reduced, tool deflection is limited and the finished guide stays closer to size.

Material & hardness:

Made from HCS (high carbon steel) with hardness ~HRC 62-64.
The fine carbide structure allows a sharp edge, so the tool cuts cleanly and leaves a smooth guide surface in finishing work.

• Sharp cutting edge → cleaner sizing and lower surface roughness
• Low cutting force → less deflection in small valve guide bores
• HCS has low heat resistance → excessive RPM quickly overheats the edge
• Overheating above ~200°C causes rapid dulling, so cutting oil is mandatory
• Lower wear resistance than HSS → use only for light finishing stock

In practice:

Finishing tool only.
Recommended sequence: drilling → rough reaming → finish reaming.
Best results come from leaving a small, even allowance because the tool sizes the bore but does not straighten a misaligned guide.

Specification:

• Tool type: Valve guide reamer
• Diameter: 4.94 mm (.1945")
• Flute type: Spiral
• Guide type: Front pilot
• Material: HCS
• Hardness: ~HRC 62-64
• Application: Finishing / sizing
• Total Length: 145 mm
• Flute Length: 45 mm

Cutting performance:

• Low stock removal
• High size accuracy
• Smooth bore surface
• Requires continuous lubrication because heat dulls HCS quickly

Recommended cutting parameters:

• Cutting speed (Vc): 3-8 m/min
• RPM (Ø4.94 mm): 193-516 rpm
• Feed: 0.03-0.10 mm/rev
• Infeed: single-pass finishing
• Coolant: cutting oil (mandatory)

• Use the lower end of the RPM range because lower speed reduces heat build-up in HCS
• Flood lubrication helps flush chips, so the guide surface is less likely to score
• Avoid dry cutting because heat softens the cutting edge quickly
• Do not stop rotation inside the guide because chip packing can mark the bore
• Keep spindle and guide aligned because the tool follows the existing axis

Use for:

• Valve guide finishing
• Cylinder head repair
• Automotive, motorcycle and industrial engines

Limitations:

• Not for drilling
• Not for heavy stock removal
• Will not correct guide misalignment or major bellmouth