The Big Three

Kody's and Josh's lawnmower engine will turn over but it won't start... What could be wrong?

Three fundamental things can happen:

1. a bad fuel mix,

2. lack of compression

3. lack of spark.

They'd already checked the fuel and the spark plug. Lack of compression? Time for a valve job.

Part of any good engine reconditioning is a valve job. If the valves leak, the engine will just not run properly. So we want good valve seats.

The purpose of grinding the valves is to make a closed valve perfectly air-tight by grinding the valve edge against the seat.

Worn and carbon covered valves prevent valves from sealing properly.


Doing a valve job:

You will need a valve lapping tool, some grinding

paste and some lubricating oil...

A valve lapping tool
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Some grinding paste


Engine lubricating oil

Here's what we did...

First we removed the cylinder head from the engine so the valves, guides and seats could be checked for wear. If valves are worn and pitted, they can be resurfaced by grinding them to make perfect seals.

This restores compression and oil control.

The photos below show Kody and Josh doing a valve grind. Photos of some of the details are missing but hopefully the procedure will still be clear...


Turning the crankshaft to seat the intake valve ready

for measuring the gap beneath the valve stem.

This is a useful measurement needed for checking the

proper assembly of the valves when they are reassembled.


Using a guage to measure the valve stem clearance

beneath the intake valve stem.

Pointing to the exhaust outlet.Exhaust valves run much hotter than intake valvesand are more likely to be damaged and covered incarbon.
Using a cloth to clean the valves and ports, readyfor inspection for damageThe springs and spring retainers have already been removed.The grinding paste is ready on the right...

The valve grinder lapper - the tool used to lap the valve

head and seat. A small amount of grinding compound

is put on the edge of the valve face and in the valve seat

in the cylinder block.

The tool is designed to grab securely when it is pressed

down firmly on the valve, ready to rotate.

You can see a spring retainer next to the valve.

When the edge of the valve is ground smooth itshould look like this - a nice matte finish with no linesor pits in it.

Kody's grinding technique - you have to grind and grind

and grind, back and forth, back and forth.... by rolling

the shaft between your hands.

The tool must be perfectly straight otherwise the angle

of the seat will be damaged and won't seal properly.

Both the seat and the valve should be ground perfectly smooth.


Putting some valve lubricant on the valve stem

before replacing it. Lubricating the surfaces

reduces friction between the parts during operation.

Less friction means less wear and tear.


Reseating the springs and spring retainers can be fiddly.

If they are not seated properly the pressure on the valve

will be uneven and cause it to leak pressure and oil


We didn't have a spring compressor so the retainer

was finally nudged in place using two screwdrivers.

The valve height was checked again with the guage...

grinding the valves can alter their height.

Valve height is important because it affects valvetrain

geometry and guide wear.


Reassembled and ready to test...


Hooray! It started first time!