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Episode 17: Valve Adjustments

CB750_SOHC_Valve_Adjustment

Episode 17: Valve Adjustments

Let’s talk about valves. Well, listen to the start of the episode…not valves exactly, but valve adjustments! This is a critical maintenance item that often gets overlooked or put off, because you don’t know how to do it. To a newer rider/home mechanic, valves and valve adjustments can often seem like black magic.  However, it just takes a little bit of understanding how the system works, and why they need adjustment to get up the courage to attack this job yourself.  Make sure to listen all the way through, and refer to this post for the examples we mention.  I’ll post all our notes from the episode, so you can easily follow along.

Why adjust valves?
As metal heats and cools, it expands and contracts.  The specified gap is to allow for the metal to expand.

Tight valves: too little clearance between the valve stem/shim/bucket and the cam/rocker
-Valves won’t fully seat on the valve seat in head
-Valves expel most of it’s heat from the valve seat, if the valve is not fully seated to the head it could cause a burnt or melted valve

Loose valves: too much of a clearance between the valve stem/shim/bucket and the cam/rocker
-The valve train components will hammer on the valve every time it opens
-Ticking sound/noise

Getting to the adjustment:
-Remove gas tank
-Remove spark plugs (optional, but recommended)
-Remove tappet covers/valve cover/rocker boxes
-Rotate crankshaft or flywheel to Top Dead Center (TDC, T, 1-4 T) for that cylinder on the compression stroke
-Check if component is loose

Rocker Arm Type – Nut and Tappet
-Check clearance between screw adjuster and tip of the valve stem w/ feeler gauge, record gap
To adjust:
-Loosen locknut
-Slide in appropriate feeler gauge
-Adjust screw adjuster til the feeler is snug
-Tighten locknut
-Double-check the clearance – the adjuster can tighten gap as you tighten the locknut

Shim Type
If you need .10mm and you have .05mm clearance with a 270 (2.70mm) shim, you will need to replace that 270 shim with a 265 (2.65mm) shim. The 265 shim is thinner so that would add .05mm to your current valve clearance to give you .10mm clearance.
-When removing the valve cover, be careful not to tear the paper gasket.
-While on TDC on the compression stroke, measure the clearance of each valve between the cam lobe and the shim/bucket. Intake and exhaust.
-Some shim type systems have what looks like a rocker arm, in this case they are called cam followers.
-Write down these measurements down in relation to each valve per cylinder.

Removing Shim Over Bucket Type
Rotate the bucket til the notch is facing outwards
Rotate crank til the cam lobe presses down on shim and shim bucket
Use shim tool to hold the edge of the shim bucket in place
Rotate crank again til the cam lobe is out of the way
With a thin flathead screwdriver, pop the shim out of the shim bucket
Write down the shim size & the corresponding valve (ie: cylinder 1, Intake)
Shim number will be on the bottom. If not legible measure with calipers: 2.70mm = 270 shim

Be careful not to force anything. If it feels like it’s binding, it is.

If it doesn’t need to be adjusted, don’t remove it.
After all shims are removed, do the math to replace them with the appropriate shim number.

Example:
Clearance Specified: .10mm
Clearance Measured: .05mm
Current Shim: 270 (2.70mm)
Difference: .05mm
Replacement Shim: 265 (2.65mm)

Formula to calculate Shim Needed:
(all numbers are in mm, except for Shim Needed, which is in shim number. ex: 270)

Shim # Needed = [ [Current Shim (mm)] – [Clearance Spec (mm)] – [Clearance Measured (mm)] ] x 100

Example decreasing shim with numbers:
Clearance Specified: .10mm
Clearance Measured: .05mm
Current Shim: 270 (2.70mm)
Difference: .05mm
Replacement Shim: 265 (2.65mm)

=[2.70 – .10 – .05] x 100
=[2.65] x 100
= 265 Shim Needed

Example increasing shim with numbers:
Clearance Specified: .10mm
Clearance Measured: .15mm
Current Shim: 270 (2.70mm)
Difference: -.05mm
Replacement Shim: 275 (2.75mm)

=[2.70 – .10 – (-.15)] x 100
=[2.70 -.10 + .15] x 100
=[2.75] x 100
= 275 Shim Needed

Shim Under Bucket Style
Same process, only cams have to be removed to remove shims

Where to get shims
Where to get shims
Unused shims
Mechanic shops/ swapping shims
Buy new at various retailers

ALWAYS DOUBLE CHECK YOUR MEASUREMENTS & MATH!

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