Task – Adjust valve clearances on my old 1925 350cc sidevalve

Why do I need to do this?

On elderly ladies like Genevieve we need to take into account valve seat recession caused by normal use and the fact that this can accelerate though the use of modern lead-free fuels. (Although I do use a fuel additive to help with this)

What tools do I need?

Firstly, what you don’t want to use is adjustable crescent spanners.

I picked up a small selection of arcane spanners from trawling around automotive swap meets (Autojumbles for our International viewers) that fit the bill perfectly. ( Thanks to McSnotty for pointing out these devices as being thoroughly useful tools)

For this task I needed 2 times 5/16 BSF spanners, a set of imperial feeler gauges, a piece of wire (Bronze welding rod) and a large adjustable spanner. I needed this only to loosen the Spark plug. Don’t think you can get away with using adjustable spanners on anything else , all you’ll do is round off your nuts( which according to McSnotty is mighty painful)

Tools of the trade
The Huge spanner in the photo isnt required but I have this to remove the valve covers if required (and for killing spiders).

Down to action

The manual says to adjust the valve clearances while the motor is Hot. (Again thanks McSnotty for the info)

Always check the manual for your Make and Model of motorcycle for its Valve clearance gaps and if it should be done Hot or Cold. If in doubt ask an expert or Join Barnstormers and go to our Contact page and we may be able to assist.

Put the bike up on the rear stand, start the old girl and let her heat up while popping inside for a cuppa (she runs pretty cool so this take a wee while)

Check that she’s heated up (a good indication is that the crankcases are getting toasty)

We need to adjust the Valve clearances when the bike is at Top Dead Centre (TDC to the common man) on the compression stroke. There’s no timing marks but is easily achieved by doing the following.

  1. Kill the motor and remove the spark plug.
  2. Put the bike in 2nd gear
  3. Turn the motor over by turning the rear wheel.
  4. Watch the valves and when the Exhaust opens and closes followed by the Inlet opening and closing then you know you are on a compression stroke.
  5. Insert the wire rod into the spark plug hole and keep turning the motor over, gently,  watching the rod rise out of the hole when the rod starts to go back down the hole stop and wind the motor backwards a touch. This should be Top Dead Centre (TDC , That is when the piston is at the top of its stroke). This doesn’t have to be 100% accurate but this ensures that both valves are fully closed.
Watch out its still hot in here....
And there it is...
And there it is...TDC

The valve clearances are measured at the bottom of the valve between the valve stem and the adjuster. Measure the current clearances with the feeler gauges. Don’t push your feeler gauges into the gap as this can cause them to bend at the tips. Pull the gauge in from the side if you can as this won’t damage your gauges.

Start off with what they should be (0.004)
The mighty feeler gauges

So my valve clearance was 4 thou (0.004) for the exhaust and 12 thou (0.012) for the inlet (thou being a thousandth of an inch)

The exhaust is spot on and wont be touched but the Inlet is way too big, it should be 4 thou (0.004). This cavernous gap could be costing me upwards of 1/8 of a horsepower and causing me to needlessly downshift on the hills.

To adjust the valve clearance loosen the locknut (using both 5/6th BSF spanners) and adjust either up or down to get the correct gap. When you think its about right then tighten up the locknut and re-measure. Repeat if necessary until its adjusted correctly.

When checking valve clearances use the feeler gauge one either side of the actual specified gap. In my case the gap for both inlet and exhaust should be 4 thou (0.004) so I use the 3 thou (0.003) and the 5 thou (0.005) gauges. The 3 thou gauge slides in easily but the 5 thou gauge binds a little when being pulled into the gap. The 4 thou gauge is a nice fit.

Using the BSF spanners
Using those BSF spanners

So we are pretty much all done.

All that remains is to restore the spark plug to its original position. While its out check its condition, give it a clean and as you have your feeler gauges out check the gap ( I’m running mine at 18 thou or 0.018 ).

Don’t forget to take the bike out of gear and you are all done and ready to ride.

Its the open road for me.....
Its the open road for me.....

It so easy even a grown up could do it 😉