Rocker arm shimming clearance question

I think the valves will turn a little, this helps cut the carbon, i:e if your seat width is uneven, carbon would build either side of the narrow seat, so if the valves turn a little either back and forth as mentioned or round and around, this keeps the seat clean , and as said rotators, also some keepers or collets touch together and allows the valve to turn. Springs have natural frequency and do lots of crazy things at certain rpm s.

marshg246 said:

How Vintage British Engines Achieved Valve Rotation​

Below is a clean, subsystem‑level breakdown of how the major marques handled it.

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Norton (Singles & Twins)​

Norton relied almost entirely on passive rotation caused by:

• Off‑center rocker pad contact
• Asymmetric rocker geometry
• Valve spring wind‑up and release
• Slight guide clearance
• Seat impact forces

Norton did not use rotocaps or mechanical rotators on any of the classic singles or twins (ES2, Model 7, Dominator, Atlas, Commando).

Why Norton didn’t need rotators​

Norton’s rocker arms — especially on the Dominator and Commando — have a naturally offset pad sweep. This offset is baked into the rocker spindle geometry and produces consistent slow rotation without extra hardware.
Norton’s exhaust valves typically show:

• A slow, steady rotation pattern
• Even seat wear
• No swirl marks indicating excessive rotation

This is exactly what you want in an air‑cooled engine with hot exhaust valves.

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Triumph (Pre‑Unit & Unit Twins)​

Triumph also relied on passive rotation, but their rocker geometry is different from Norton’s.
Key features:

• Rocker arms are narrower
• Pad sweep is more centered
• Valve stems are longer and more flexible
• Spring wind‑up plays a bigger role

Triumph’s rotation is driven more by:

• Spring torsion
• Valve bounce at higher RPM
• Slight rocker offset (less than Norton)

Triumph’s exhaust valves rotate more reliably than intake valves, which is typical for engines with hotter exhaust seats.

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BSA (A‑Series, B‑Series, C‑Series)​

BSA’s approach is closer to Triumph than Norton.
Characteristics:

• Moderate rocker offset
• High spring wind‑up due to long stems
• More guide clearance than Triumph
• Heavier valve gear → more bounce → more rotation

BSA singles (like the Gold Star) show very clear evidence of rotation on exhaust valves, especially at high RPM.

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Why None of These Engines Used Rotocaps​

Rotocaps (spring‑loaded mechanical rotators) were used in:

• Heavy‑duty diesels
• Some aircraft engines
• Later automotive engines with high exhaust temps

But British motorcycle engines avoided them because:

• They add mass to the valvetrain
• They reduce RPM capability
• They complicate rocker geometry
• They weren’t needed — passive rotation was enough
• Air‑cooled engines already had generous guide clearances

The British philosophy was:
“If the geometry already rotates the valve, don’t add parts.”

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Summary Table: How Each Brand Achieved Rotation​

Brand	Rotation Method	Notes
Norton	Strong passive rotation from rocker offset	Most consistent rotation of the group
Triumph	Moderate passive rotation from spring torsion + slight offset	Exhaust valves rotate more than intakes
BSA	Moderate–strong passive rotation from bounce + guide clearance	High‑RPM singles rotate aggressively
AJS/Matchless	Similar to Norton	Rocker geometry naturally off‑center
Velocette	Spring torsion dominant	Very smooth rotation, low wear

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Why This Matters for Restoration​

When you rebuild these engines today:

• If you center the rocker tip perfectly, you may stop valve rotation.
• If you use modern tight guides, you may reduce rotation.
• If you use roller rockers, you may eliminate rotation entirely.
• If you use modern high‑rate springs, rotation may increase.

Vintage engines were designed around passive rotation, so maintaining the original geometry is critical.

Click to expand...

I don't know where you got this from, but regarding Norton twins , it is not correct.

Let's see what AI has to say:

Quote:
"In standard factory configuration, the valves on a Norton Commando do not rotate.
Unlike some modern or specialized engines that use mechanical rotators (such as Roto-Caps) or offset rocker geometry to force rotation, the Norton valve train is a static design.

Design Characteristics

• Locked Mechanism: The valves are clamped tightly between the valve keepers (collets), which prevents them from spinning freely within the retainer assembly.
• Evidence of Non-Rotation: Used Commando valves typically show linear wear marks (single straight lines) on the valve tips from the rocker adjusters, rather than circular wear patterns that would indicate rotation.
• Limited High-RPM Movement: Some incidental rotation may occur only at extremely high RPMs if the valve springs begin to lose control (valve float), but this is not a designed function and can actually lead to damage.
• Why Don't They Rotate?
  • Simplicity: The Norton Commando's overhead valve (OHV) design follows a traditional, "lawnmower-style" approach where the rocker arm is generally centered on the valve stem.
  • Fuel Requirements: In the 1960s and 70s, valve rotation was often used to scrub lead deposits off valve seats. However, Norton engineers did not incorporate the complex bearing or ramped rotator mechanisms required to achieve this.
• While some performance builders may attempt to modify the head to promote rotation—such as using bronze spacers or slightly offsetting the rocker arms—the stock assembly is intentionally stationary.
• . "

"The Norton Commando engine does not feature designed-in valve rotation, and the standard valve train is not engineered to promote consistent valve rotation during operation. While some valve rotation can occur at high engine speeds due to minor torque imparted by the compressing valve spring, this is not sufficient to provide the benefits of controlled rotation, such as improved heat distribution and cleaning of combustion deposits. The primary reason for the lack of intentional valve rotation in the Norton Commando is the design of the valve spring retainer system, which typically uses a keyhole configuration that restricts rotation by maintaining constant contact between the retainer and the valve stem.

Attempts to promote valve rotation, such as using a .005" shim under the spring retainer or lapping the spring and retainers to reduce friction, are not standard practices and may not yield reliable results. Some sources suggest that special components like rotator caps with radial needle bearings or collets designed to allow free rotation are required for consistent valve rotation, which are not part of the original Norton Commando design. In fact, the use of such systems was historically more common in older engines using leaded fuel, where rotation helped scrub lead deposits from the valve and seat; however, with unleaded fuel, the hard deposits formed can accelerate wear, leading manufacturers to discontinue rotating valves in many applications.

There is no evidence that the Norton Commando was ever designed with rotating valves, and modifications to achieve rotation would require significant re-engineering of the valve assembly and rocker arm system. Some forum discussions have referenced videos or anecdotal observations of valve movement, but these are likely due to high-RPM conditions causing temporary separation of the spring and retainer, not sustained, controlled rotation. Therefore, while valve rotation can theoretically occur under specific conditions, it is not a feature of the standard Norton Commando engine and is not recommended without proper engineering modifications."

Continuing to post unsubstantiated info from old threads proves nothing no matter how many times the subject comes up.

You cannot show where Norton made any effort to center the rocker adjuster over the valve stem center.

Since Norton made no such effort it is true that they are not designed to be centered and you'll not find any from the factory that were unless a fluke. The same is true of Triumph. I've also never found them centered on BSA.

I suspect that if Norton had wanted them centered they would have done away with the spring washer and said to use the needed number of thin washers to center them - would likely have cost less!

Unwatching - I'm tired - argue amongst yourselves!

marshg246 said:

Continuing to post unsubstantiated info from old threads proves nothing no matter how many times the subject comes up.

Click to expand...

If comnoz says they don't rotate then I tend to believe him over you no matter how old the information.

Post in thread 'Promoting Valve Rotation?' https://www.accessnorton.com/NortonCommando/promoting-valve-rotation.7491/post-94033
"There is no valve rotation in a Commando."

marshg246 said:

Unwatching - I'm tired - argue amongst yourselves!

Click to expand...

As usual. I don't think we are arguing amongst ourselves.

Thanks everyone for your input. My takeaway from all this is that I'm just going to put it together according to manual procedure and not worry about end play or if the valves rotate or not . Seems like it will work fine either way.

I did look at the OEM Exhaust valve tips on a Stock OEM MK3 head a while back that had 17 k miles on all original parts , It had one straight line wear mark. I did mark and look at one exhaust valve on my Street bike a MK3 and it did not move after about a 10 mile ride. It did not move.

I did look at videos of slow motion valve springs working in action and I can see how a spring and valve could move with the harmonics of the spring and how the spring "vibrated ".

Oddly, My friend has a T 140 Triumph and his spring and valve did turn after he had a new valve job done and he did a clearance check after his test drives. But more checks need to be done on his and my bikes..

I would have to see more examples of marked valve springs and valves to draw conclusions as to what happens. I could also have to do with the surfaces of the parts and how the spring "grips" the top retainer and the bottom spring seat and head.

But a good valve job as well as engine rework can last a long time even under racing conditions on a well tuned bike using stock parts with a well prepared bike, in my experience, no matter what the parts are doing. The Norton engine is a one tough cookie.