Rocker arm shimming clearance question

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fid04

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Nov 12, 2022
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Just got my head serviced with new valves, guides and springs. All rockers had the spring/shim. One had 3 washer shims, the rest had one. Doesn't look right to me. What are thoughts on using, vs eliminating the spring/shims and just using just solid shim washers? What should the end clearance be, and should the rocker be centered over the valve stem? Any other information about installing the rockers is welcome.
 
The reason I like the stock Thackeray spring set up is that it makes it easier to assemble the head especially when it's 200 or more degrees. I assemble it cold with an undersize spindle.
Fiddling with spacers and shims gives the opportunity to align the rocker tip to the valve stem but it would be a lot of work for maybe a minute gain. Count me out.
 
I am running solid shim washers on my Commando. I used to have them on my Dominator, but I have had the head apart a few times in the last few years, and like seattle##gs says, they are a bit of a pain to align and install, so I decided to go back to Thackery washers. Initial install takes time and patience, each shim has to be ground down to size. I probably spent an hour or more on each one, grinding them to size by hand with a high quality fine grinding stone.

I do not notice any difference is noise. I think there is probably a slight reduction in friction with the solid shims. If you have the time and patience I don't think there is any overarching reason not to use them?
 
Last edited:
fid04 said:
Just got my head serviced with new valves, guides and springs. All rockers had the spring/shim. One had 3 washer shims, the rest had one. Doesn't look right to me. What are thoughts on using, vs eliminating the spring/shims and just using just solid shim washers? What should the end clearance be, and should the rocker be centered over the valve stem? Any other information about installing the rockers is welcome.
Click to expand...
I wish you were a Premier Member so I could PM you as I really don't need the feedback that will likely come from this:

The thin flat washer goes on the outside of each rocker. Unless someone has modified the rockers, there must be only one.
The spring goes on the inside of the rocker by itself.
The adjuster is NOT supposed to be centered on the valve stem. If very far off, suspect that someone before you has "fixed" the rocker.


A very famous race engine builder likes the adjuster dead center on the valve stem and no spring washer. He grinds the rockers and adds flat washers as needed to make that happen. I obviously disagree.

Some here also don't like the spring washers and have other solutions. I obviously disagree.
 
General end float, for crankshafts, camshafts in cars . 002"-.006". I'd say 4-5 thou would be fine, if you go that way. I had to file some oil grooves into mine, to help lube my exhaust valve, but that was an old BSA b33, not many original parts on the inside 🤣🤣. Cheers
 
marshg246 said:
I wish you were a Premier Member so I could PM you as I really don't need the feedback that will likely come from this:

The thin flat washer goes on the outside of each rocker. Unless someone has modified the rockers, there must be only one.
The spring goes on the inside of the rocker by itself.
The adjuster is NOT supposed to be centered on the valve stem. If very far off, suspect that someone before you has "fixed" the rocker.


A very famous race engine builder likes the adjuster dead center on the valve stem and no spring washer. He grinds the rockers and adds flat washers as needed to make that happen. I obviously disagree.

Some here also don't like the spring washers and have other solutions. I obviously disagree.
Click to expand...
Seems to be different schools of thought on the subject. Curious if you have ever put any kind of reference mark on the side of the valve stem to test out your theory and see if they do indeed turn.
 
Way back, in Comnoz’ fabulous ‘spintron’ thread, he suggested that the Thackeray washers probably provide a small damping effect to the rocker which would be beneficial for valve control, especially at higher rpms.
 
fid04 said:
Seems to be different schools of thought on the subject. Curious if you have ever put any kind of reference mark on the side of the valve stem to test out your theory and see if they do indeed turn.
Click to expand...
You can look at the wear pattern on the valve stem and see that they do.
 
C

Post in thread 'Thrust washers on rocker shaft'

marshg246 said:
BTW, IMHO, centering the rocker is bad - the valve is supposed to turn.
Click to expand...
I have only seen the top of used valve stems with one straight line scratch marks caused by the valve adjuster contact. There is no way for a valve to turn and there is no evidence that I have seen that they do. I like to see nicely centered valve stem to adjuster contact. And use bronze spacers instead of the Spring "washer" as they call it in the parts book. And different thickness "thrust" washers to get minimum end play. They can be had in 15 and 30 thou.
  • Like


https://www.accessnorton.com/NortonCommando/thrust-washers-on-rocker-shaft.40994/post-700245
"I just looked at maybe 20 old valves, and they all have linear wear marks.
They did not rotate."
 
I don’t believe replacing spring washers with shims, on a rocker shaft make a Commando go faster. It just seems like a “racing” thing to do.

I’m not clever enough to pass a judgment on centre or off-centre valve tip contact.
 
L.A.B. said:
C

Post in thread 'Thrust washers on rocker shaft'

marshg246 said:
BTW, IMHO, centering the rocker is bad - the valve is supposed to turn.
Click to expand...
I have only seen the top of used valve stems with one straight line scratch marks caused by the valve adjuster contact. There is no way for a valve to turn and there is no evidence that I have seen that they do. I like to see nicely centered valve stem to adjuster contact. And use bronze spacers instead of the Spring "washer" as they call it in the parts book. And different thickness "thrust" washers to get minimum end play. They can be had in 15 and 30 thou.
  • Like


https://www.accessnorton.com/NortonCommando/thrust-washers-on-rocker-shaft.40994/post-700245
"I just looked at maybe 20 old valves, and they all have linear wear marks.
They did not rotate."
Click to expand...
An interesting point in the two videos that @baz posted in that thread, is that the valves springs and caps, (so presumably the valves as well), appear to be rotating even when the valves are closed, in which case it makes no difference where the rocker contacts the valve tip.

baz

Post in thread 'Thrust washers on rocker shaft'


not a Norton but interesting
  • Like
 
Setting the rockers with a precise amount of offset is time consuming. It is unlikely they did it at the factory, and IF they did, and felt it was important, it would have been mentioned in the workshop manual.
Surely some must have left the factory off center, but it certainly wasn't intentional.
Greg seems to think it's important, so we must assume he adjusts each rocker individually. Shimming only works one way. The other direction requires machining the rocker.
So Greg, enlighten us how you machine the rockers, or do you just bend them ? 😏
 
Last edited:
marshg246 said:
AFAIK, no one is talking about valve rotators here - that's completely different tech as are valve springs designed to turn valves.
Click to expand...
Exactly.

"A valve will turn a tiny bit each time the valve opens due to the twist imparted by the compressing of the spring. If there is sufficient friction [ie no rotator provisions] then the valve will turn back to it's original position as the spring returns to the seated position."
 

How Vintage British Engines Achieved Valve Rotation​

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

🔧 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.

🔧 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.

🔧 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.

🔧 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.”

🔧 Summary Table: How Each Brand Achieved Rotation​

BrandRotation MethodNotes
NortonStrong passive rotation from rocker offsetMost consistent rotation of the group
TriumphModerate passive rotation from spring torsion + slight offsetExhaust valves rotate more than intakes
BSAModerate–strong passive rotation from bounce + guide clearanceHigh‑RPM singles rotate aggressively
AJS/MatchlessSimilar to NortonRocker geometry naturally off‑center
VelocetteSpring torsion dominantVery smooth rotation, low wear

🧠 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.
 

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