60 Tiger Cub Motorcycle

FRS 106, Michael Littman – Spring 2019

Top End

1. Head

Purpose of the Head:

The head is the upper half of the top end of the engine that contains the inlet valve, exhaust valve, inlet rocker, and exhaust rocker. The arrangement of valves is known as an Overhead Valve (OHV) because the valves are located above the cylinder. These components work together to facilitate the intake of air and fuel into the cylinder of the engine and the letting out of exhaust.

Cleaning :

The first step we took was cleaning up and preparing the different parts of the head for assembly. We covered up precision parts and threads with painter’s tape before sandblasting the other surfaces of the head. The image on the right shows the surface after sandblasting.

Grinding the Valves:

Two new valves that fit perfectly into the head were selected. We could tell the difference between the intake and exhaust valve because the exhaust valve is smaller than the intake. Exhaust valves are naturally smaller than the intake because pressure from within the cylinder is already forcing exhaust out of the cylinder, so a large opening for the exhaust to flow out is not necessary. The other smaller components within the head including the rockers, spindles, valve covers, studs, and assortment of washers and bolts were cleaned out with a solvent and wire brush in the parts cleaner.

The next thing we did to prepare the head was grind and lap the valves. This process is done to make sure that the valves fit snugly into its seat allowing no air to escape. To grind the valves properly, we had to ensure that the top, seat, and throat were at precisely the proper angle. The grinder tool has a conical shaped end which has sanding blocks attached to it. Different tips can be attached to have a conical end with 30, 45, or 60 degrees. We used these to grind the metal of the head down, so that the seat was at 45 degrees, the top at 30 degrees, and the throat at 60 degrees. Below is a diagram of the different grinding angles on the valve seat


After grinding the seats, we lapped the valve together with its seat. This process is applying an abrasive (in the form of the rough/fine lapping compound) between two surfaces and rubbing it together. This polishes the surfaces to make them fit more seamlessly together. We repeatedly covered the surfaces with blue ink during both the grinding and lapping process and spun the valve in the seat to see where the blue ink would spread. This represented the parts that made contact, so we could see the parts which needed to be ground or lapped more. With this done, all the parts were prepared and ready for assembly.


First, we assembled the valve springs. We had to use a special tool that allowed us to compress the valve springs in their place, so we could lock in the top collar with the split cotter. It is important for the valve springs to be locked in because these have a high spring constant and could potentially hurt somebody if they were loose and could come flying out due to an unstable setting.

We also assembled the inlet and exhaust rockers. This was difficult because the rocker spindle had grooves and lips which would catch on various parts and not sit at the exact center. There were also small holes for oil which had to be aligned for proper flow. What made this assembly all the more difficult was the involvement of springs which had to be pressed and held for long periods of time while assembling this. Its tension also made it difficult to press everything in together.

The rocker arms are connected to the pushrods and these work together to open and close the exhaust and inlet valves. When the pushrod of the camshaft pushes up, this makes the rocker arm push down and apply pressure on the different valves to keep them closed. It is important the valve opens and closes properly because this controls the breathing of the engine, and several negative effects can result when the timing of inlet and exhaust is offset.

Below is a diagram showing the assembly of the inner components of the head.

2. Cylinder

Purpose of the Cylinder:

The cylinder is the middle part of the top end of the engine. This is essentially the space which the piston moves as it compresses air and fuel.

Parts Cleaning/Sandblasting:

We sandblasted the cylinder barrel after ensuring  that all the openings were closed. We covered the open surfaces to stop the sand from corroding the threads in the holes as well as on the protruding surfaces.

We aptly cleaned the cylinder to get rid of the sand trapped in the holes, using water, and sprayed it with WD-40 to ensure it doesn’t rust. The fins were the most difficult part to sand blast and we had to hold the sand blasting gun at an angle in order to clean the fins.

We sprayed painted our cylinder using a shiny silver paint giving it a glossy look.


The boring process:

The boring process was a delicate process that required ultimate precision. It involves removing a thin layer of the inner metal of the cylinder. We used a dial to measure the evenness through the barrel and we were lucky to work with a completely even cylinder barrel, apparently, this wasn’t the case in the previous years. Since we intended to use the 40mm piston, we had to ensure that we remove 1/5,000 inches of metal to ensure that we remove the rust while keeping the cylinder barrel airtight.

We mounted the cylinder barrel onto a lathe, and using a cutting tool, we were able to precisely cut 1/5000 inches of the steel.

The honing process

We used the honing tool and WD40 to clean the cylinder barrel giving it a smooth finish. The honing tool was made of coarse stones which helped in cleaning the cylinder barrel.

Compression Ratios:

Different pistons have different compression ratios. This is heavily dependent on the shape of the top of the piston. Below is an image showing different pistons with differently shaped domes.

A higher compression ratio is desirable because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature.

However, higher compression ratios increases likelihood of an engine knock which reduces efficiency or even end up damaging the engine.

Compression Ratio(CR) = (Displacement Volume + Clearance Volume ) ÷ Clearance Volume

For the pistons shown above they have compression ratios of 7:1, 9:1 and 10.5:1.

Phase diagram of the 4 stroke process in the engine top head:


3. Carburetor – The Amal Monobloc 376

Purpose of the carburetor:

The purpose of a carburetor is to mix air and gasoline to achieve a stoichiometrically efficient mix for combustion. The main parts of a carburetor are the body, the throttle and the pilot outlet.

Parts of the Carburetor:


The Amal Monobloc is, as the name suggests, composed of a one-piece (mono-block) aluminium cast block as the body. Some of its important parts:

15 – Main jet: controls fuel intake to the mixing chamber when the throttle is open

14 – Needle Jet: controls fuel intake when opening and closing the throttle

19 – Pilot Jet: controls fuel intake when the engine is idling

22 – Pilot stop screw: almost perpendicular to the Pilot Jet, blocks it partially depending on how deep it is screwed in and therefore controls fuel-air ratio when engine is idling

24 – Throttle stop screw: determines how deep the throttle needle sits in the main jet therefore determining how much fuel should enter the mixture when throttle position is changed

27 – Float: floats on top of the fuel level and pushes up the plastic Needle (part 33) to prevent more fuel intake when the chamber is full. Moves down when the level of fuel in the chamber decreases to allow more fuel into the chamber.


Fuel calculations:

Professor Littman put up some fantastic combustion calculations on our library, here is the link:

Fuel – Rich/Lean, causes and fixes

The fuel-air mixture’s perfect ratio can get skewed due to certain circumstances. Sometimes, there may be too much fuel in the mixture – that’s when the engine runs rich – and sometimes there may be too less fuel in the mixture and the engine runs lean. Causes for this could range from incorrect adjustment of the pilot jet, to wrong sized main and needle jets to reduction of atmospheric air pressure at higher altitudes (as pointer out by Pirsig in Zen and the Art of Motorcycle Maintenance).

Below are some common causes of the same, taken from Amal’s handbook:

But how does one detect whether their engine is being fed a rich or lean mixture? Here are some things to look out for:

Heavy lumpy running with, usually, black smoke (not the blue of too much oil) from the exhaust, indicates richness. When the mixture is weak the running is erratic, and may be accompanied by spitting back through the carburetor and knocking of the engine. Another indication is firing in the silencer with the throttle closed and the engine on the over-run.

Fixing these issues is deceptively simple, below are some solutions to the same :

Engine running rich Engine running lean
Needle jet worn— Replace needle jet Needle jet damaged — Replace jet
Choke plunger leaking – Replace choke plunger Choke plunger body worn (MKII) — Replace carburetor body
Air bleed too small (MKII) — Remove or install larger air bleed Air bleed too large (MKII) — Install smaller air bleed
Float bowl vent blocked — Clear blockage by cleaning with wire-brush Float Bowl gasket surface warped — straighten or replace bowl
Float needle leaking fuel — Replace needle and/or seat Float needle seat too small — Check cycle’s specifications
Slide too tight in body ­­— Slide clearance should be .0035 (Amal 367) Air Leak at float bowl gasket — Replace gasket to .004″ clearance in bore
Air leak at worn slide or body — Replace or re-sleeve
Air leak at flange or spigot — Replace body
Air leak at choke cable hole — Install cable or plug
Pilot Jet blocked – Clean out with #78 Dril

More information on tuning a carburetor can be found here:

Bernoulli’s Principle and the Venturi Effect:

Bernoulli’s principle, in a nutshell, says that if a fluid’s flow velocity is increased, its pressure decreases. This is modelled by Bernoulli’s incompressible flow equation:

P1 + dh1g + dv12/2 = P2 + dh2g + dv22/2


P1 = Pressure from the intake

d = density of fluid

g = gravitational acceleration

h1 = height of intake

h2= height of output

P2= Pressure of output fluid

and v1, v2 are velocities of the fluid at the intake and output manifolds


Another principle, derived from Bernoulli’s equation, is the Venturi effect:

P1 + dv12/2 = P2 + dv22/2


A1v1 = A2v2

This is the same as Bernoulli’s equation except an assumption that both the intake and output valves are at the same height. This may not be true when riding on a slope, but for the sake of understanding the carburetor’s working, we can make this assumption but the horizontal distance between the intake and output manifold of the carburetor is so small that it ceases to matter.

In the carburetor, there is a narrowing, a ‘venturi’, where a vacuum is created due to the venturi effect. There is a small hole in the narrowing called a jet which is fed fuel via the float chamber. So the high velocity filtered air (due to suction from the vacuum generated from the piston) causes the fuel in the float chamber to rise. The faster the filtered air comes in through the carburetor throat, the lower the pressure in the venturi. This leads to a higher pressure difference between the venturi and the float chamber, and thus more fuel flows out of the jet and mixes with the airstream. This means that the more the throttle is closed, the richer the fuel mixture is and the engine runs slightly rich. This mixture is then combusted in the cylinder to provide power.

Here are some calculations done with our specific carburetor, the Amal Monobloc 376, the variable parameters can be changed here to result in a changed fuel-to-air ratio: https://docs.google.com/spreadsheets/d/1NaLf1fZrY1uG5mZOw995alVADTYU5nIPq7BErYh2jss/edit?usp=sharing

An excel sheet of the same has been uploaded below.


Ultrasonic Cleaning:

We had to clean our carburetor before reassembling it. To do so, we used Ultrasonic cleaning instead of simply parts cleaning it. Ultrasonic cleaning, as the name suggests, uses ultrasonic (above 20MHz) sound waves to clean parts. One may think that it works by resonating with the natural frequencies of the small grime particles and thus remove them, but this would be a poor design for a cleaner as differently sized particles of different materials would have different natural frequencies – the machine would have to send in a spectrum of ultrasonic waves which could cause interference and not clean anything. Instead it cleans by agitating the cleaning solvent to create small vapour bubble in it. This is known as cavitation and is caused by rapid, non-homogenous changes in pressure in the fluid. These cavitation bubbles then exert tremendous pressure, when they burst, on the impurities which then detach from the body. This force does not do anything to the part due to its high mass, strong lattice alloy structure and a net force of null due to an uniform force from all sides of the part immersed in the fluid. The solvent used is customized to the part to be cleaned and the kind of impurities on it. We wanted to clean the body of our carburetor which is made up of aluminium and had grime and rust on it so we used parts-cleaning fluid (oil based organic solvent) to do the trick.

Here is our presentation: