Honda C90 - finding TDC on the compression stroke

8th Jan 2021
Tags: Engine

As explained in this extract from a Honda manual, valve adjustment measurements must be taken at top dead centre (TDC) on the compression stroke.  This short article explains how to check your engine is in the right spot to measure the valve gap.

valve adjustment instructions from the 1978 C90 Z2 owners manual

To understand Honda’s instructions we need to recall the basic operation of a 4 stroke engine: the piston rises and falls twice in order to complete a full cycle where fuel and air is drawn into the cylinder, compressed, ignited and the resulting gases expelled from the engine:

The second illustration shows the point in the engine cycle when the piston is ascending on the compression stroke

On the first stroke the intake valve opens and the piston descends creating an area of low pressure that draws in fuel/air mixture from the carburettor into the cylinder.  As the piston ascends the intake valve closes, sealing the cylinder and causing the mixture to be compressed.   The mixture is then ignited by a spark from the spark plug and the expanding gases force the piston downwards.
Finally, on the fourth stroke, as the piston ascends the exhaust valve opens and the gases are expelled.

This is the reason these engines are called “4 strokes”.  The mnemonic suck, squeeze, bang, blow is a handy way to remember the sequence.

This excellent film of an engine converted to use a perspex cylinder lets you see this process in action:

It is now easier to understand Honda’s instructions: as discussed in an earlier post the point of this adjustment is to create a small amount of play between the tappets and the rocker arms when the valves are closed.   We now know that both valves are closed when the piston is at the top of the cylinder on the compression stroke so we want to make sure the engine is in this position before making any adjustment.  We can’t see the position of the piston in the cylinder of course, but the ‘T’ marked on the alternator rotor by the factory, when aligned with the pointer on the crankcase, shows that the piston is at the top of its stroke (the “T” stands for top dead centre).

But as we have seen above the piston is at the top of the cylinder twice for each cycle of the engine, so another step is needed to confirm that we are on the right stroke. Honda’s suggestion is to wiggle both the tappets and this works because, when the valves are closed, the pressure on the tappet is relieved so you can feel a tiny amount of play between the rocker arm and the valve stem.

It is just an indication you are at the right spot though. What do you do if someone has already over tightened or loosed the tappets so that this test is no longer reliable?

One way about it is this: while recalling the suck/squeeze/bang/blow cycle, rotate the engine (anticlockwise as per the directional arrow on the rotor) and watch the intake valve until it moves down and then comes back up, closing the valve.   You have just watched the engine suck in the fuel/air mixture from the carburettor and then begin the compression stroke. This means that continuing to rotate the engine until the next top dead centre (TDC) will get you to the right spot.

So the rule is quite simple: before adjusting the tappet valve gap put the engine on the next TDC after the inlet valve closes.

One benefit of doing it this way is that, if you forget the rule, you can still work it out for yourself so long as you can remember the suck/squeeze/bang/blow mnemonic.

Now we know how to set up the engine before adjusting the valves – keeping in mind this must be done when the engine is cold – we can look at how to do the actual measurement in the next post.



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