Robs questions

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

Taper in a bore is when the top of the bore is slightly wider than the bottom. This is due to the piston rings wearing on the bore. Ovality is when the bore is slightly wider in one direction than in the other. This happens because the piston does more work across one way, than the other. This eventually causes the bore to take an oval shape.

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

Side clearance on a piston is the gap between the piston ring and the ring groove. It can be checked with a feeler gauge and should be about .03 to .065mm

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

To tell the difference between the exhaust and intake valves is easy. The intake valve is the bigger one so that as much air/fuel as possible can get into the cylinder.

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

The exhaust valve operates at about 700 degrees C.  Intake valves however only reach temperatures of about 200 Degrees C.

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

A piston will operate at about 300 Degrees C. It is at this temperature because of the cold air/fuel entering the cylinder. The speeds at which a piston has to cope with varies. From as little as idle speed of about 800 RPM (piston moves up and down 800 times a minute) up to about 20000RPM which formula 1 cars can rev to.

Why do we have piston ring end gap clearance ?

In this photo, we can see the end gap
in the piston ring.

We have this amount of clearance so that the piston ring can push put on the way up on the compression stroke. On the compression stroke, the air/fuel mixture gets in behind the ring and pushes it into the bore.

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

This animated picture shows how the ring functions.

If we dont have this clearance, ie carbon has built up in the groove and holds the piston ring against the bore, this will cause excessive wear on the bore and create a taper on the bore.

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

These steel shims or washers are there to protect the aluminium cylinder head. Because aluminium is a soft metal, the hard steel of the spring would wear the aluminium out. So we have the shims there to prevent the aluminium from being worn away.

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

S.I: Spark ignition: Uses petrol fuel, the fuel air mixture is ignited by a spark plug.

C.I: Compression Ignition:  Uses diesel fuel, the extreme pressure and heat ignites the fuel as it is injected into the cylinder in the compression stroke.

7/4/11 What is the purpose of the margin on the valve?

The valve margin on a valve is there to dissipate heat. If the margin is too small, the head of the valve will get too hot, and will ignite the fuel/air prematurelly.
What is the purpose of a core plug?

The core plug serves as two purposes. One is when the engine is cast. Excess sand from inside the water jacket gets out of these core plug holes after the engine block has set.  The second is to protect the block or head from cracking. If the water in the water jacket freezes, it expands and presses against the block. When the pressure inside the block gets too high, it pushes the core plugs out. If these core plugs/holes were not there, the expanding water will crack the block or head.

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

testing for crankshaft run out.

End float on the crankshaft is the movement of the crankshaft length-wise. This will become eccessive if the end thrust bearings are worn. To measure the end float, force the crankshaft endwise as far as it will go and measure the clearance at the end thrust bearing with a feeler gauge.



Testing for crankshaft end float.



Runout on the crankshaft is when the shaft does not run straight and true. This can be measued by setting up a dial test indicator on the end of the crankshaft (the round, not the flat, cross section part) and turning the shaft. The DTI will show any run out as the shaft is turned.

1.What is the firing order of a four stroke engine?
There are various firing orders for different engines with different layouts and number of cylinders. But for most inline four cylinder engines is 1,3,4,2.
2.What is the difference between a S.I and C.I engine?

A spark ignition engine (S.I) has a spark plug whereas a compression ignition engine (C.I) does not have a spark plug as it ignites under compression.

Diesel piston

3.Is there any difference between a S.I and C.I piston if so what?

Petrol Piston

The difference between petrol and diesel pistons is that while the head of petrol pistons are relatively flat, the head of diesel pistons have a cup in the top piston.

4.What year was the diesel engine invented and by whom?

The diesel engine was invented by Rudolf Diesel in 1897.

5.What is the purpose of the oil jet on the connectng rod, and where does it spray?

The purpose of the oil jets is to lubricate the bore. As the piston moves up the cylinder, the pressure inside the crankshaft forces oil up through the connecting rod, (the oil jet)and squirts onto the cylinder bore.

6.On the connecting rod where is the big end and little end?

Con rod. The  left side is the small end and
the right side is the big end. This particular
rod would be out of a Vee type engine because
the bearing isnt vertically aligned with the
verticle of the rod, it has been rotated.

The little end of the connecting rod is the end that connects to the piston via the gudgeon pin. The big end of the connecting rod is the end that connects to the crankshaft.

7.What is the purpose of the thrust washers on the crankshaft?
The thrust washer on the crankshaft is there to prevent the endwise movement of the crankshaft.

Direct Injection Diesel

Indirect injection Diesel

8.On a diesel engine whats the difference between direct injection and a indirect injection type?

On a direct injection engine, the fuel is injected directly into the combustion chamber. On a indirect injected engine, the fuel is injected into a pre-combustion chamber, just off the combustion chamber which is then ignited.

9.Explan the term valve timming?

In a piston engine, the valve timing is the precise timing of the valves opening and closing. The valves have to open and close at the right time to allow air and fuel to enter, and allow exhaust to exit. If the valves are open at the Wrong time, the engine wont run correctly and the piston could hit the valves, which will bend them.
10.Why do we have valve clearance?

Valve clearance is the gap between the cam shaft or lifters and the top of the valve stem. We have this small clearance to ensure that the valve completly closes. Engines with mechanical ajustments have to be adjusted about every 32000 km's.  If the clearance is too great, there will be a knock coming from the valve train.

Most modern engines have hydraulic lifters that automatically ajust to compensate for for wear.


What is the purpose of a glow plug on a diesel engine?
The purpose of the glow plug is to warm up the cylinder prior to starting the engine. It will often be on for 2 to 5 seconds on a cold morning/day.

Diesel cylinder with glow plug on the left of the cylinder.