Misc Engine Questions

What is taper and ovality in the bore, how is it caused how do you check it?

Taper is the difference in measurement between the top and bottom.

It is caused by the piston rings wearing down the sides of the bore when they rub past it during the piston movement.

Ovality is the difference in diameter A to diameter B
Eg. creating an oval shape in the cylinder.

This is caused by the piston rings applying more pressure to one side of the cylinder than the other. This creates more wear on one side and less on the other making it slightly oval in shape

What is side clearance on a piston and how is it checked?

It is the gap between the piston and the bore.
Using dial bore gauge, measure cylinder bore. Measure piston diameter. Subtract piston diameter from cylinder bore diameter to get piston side clearance.

How can you tell the difference between an inlet and exhaust vavle and why?

The inlet valve is bigger because it is harder to draw in the right amount of air and fuel. The reason why it is harder is because it relies on a vaccuum created by the downwards motion of the piston.
The exhaust gasses are easier to expell so dont need such a large valve to get rid of them. This is because pushing them out rely on the piston physically pushing it out which is more efficient than using a vaccuum and the valve only needs to be small.



What sort of tempatures do the inlet and exhaust valves get upto?

Inlet valves: 250 degrees
Exhaust valves: 750 degrees

What temperatures and speeds must the piston be able to cope with?

300 degrees for the piston heads.
30 m/s for the spead they need to withstand.

Why do we have piston ring end gap clearance ?

To allow for expansion. When the piston ring gets hot it expands.

What could the result be if the piston ring end gap is too small?

If the gap is to small, when the piston ring expands both ends will touch each other then push out against the cylinder. This can result in the engine ceasing up and/or wear or major damage will occur to the cylinder.

Why do aluminum cylinder heads usually have a steel shim (washer) between the valve spring and the cylinder head surface ?

It is used to restore the spring tension. The reason is it adds material to the spring which has been lost through wear

It also stops the moving spring (which is a harder metal than the cylinder head surface) from wearing down the cylinder head surface by puting a barrier inbetween the two.

What is meant by the terms S.I and C.I in a four stroke engine?


S.I - Spark Ignition
C.I - Compression Ignition

What is the purpose of the margin on the valve?

It is to allow for expansion when the valve heats up. If there wasn't it would jam and cease up the engine, possibly bending or breaking the valve.

What is the purpose of a core plug?

The core or frost pluga are used to fill the core holes. (The core holes are created in the casting process to allow the casting material to be removed.)

What is end float and run out on a crankshaft, hows is measured?

The run out on a crank shaft is the difference, when the camshaft is turned, around the journal. Measured with a DTI guage.

The end float is how much the camshaft moves foreward when pushed foreward through the crank caps/bearings. Also measured with a DTI guage.