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Valve clearances


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So you fancy having a crack at valve clearances? It makes no odds as to whether its a 4 stroke outboard or diesel inboard, principals are the same...... It is always better to have clearances set at the top of their clearance

First of all there are 3 general types of tappet arrangement;

Hydraulic These are great as no scheduled maintenance. Should the lifter fail then its a replacement job. This style relies on oil pressure to operate, so if you start getting rattly tappets, turn it off, you are short of oil.... Personally, I've never had one fail, but I have had issues when they've all been replaced. Ended up towing vehicle about in gear to get enough oil pressure to expand them!!!

Bucket and shim These come in two styles, shim under bucket and shim over bucket. Over buckets are easier to do, as normally there is a valve tool to create enough gap between cam and bucket to remove shim. On some engines these have been known to spit shims out at extreme performance conditions. The under buckets have no such issues, but generally these are a cam out job! So this will mean new belt kit for the modern engines.

The trick is to measure all the gaps before removing shims, then hopefully the sizes are still readble, otherwise its micrometer time, or the very least, a good set of verniers. Normally shims come in 0.05mm increments.

Once measured you can then workout what size shim you need to replace with. If you cant find a shimming chart then basic maths is required.....

Locknut and Screw Ultimately the easiest type to deal with. Some screws have a square drive on the end and there are adjuster tools to make the job easier.....


Soooo, now we know about the different types, what is it we are trying to achieve? The basic idea is that as the engine warms up, parts expand, including the valves. So to get a nice seal when the valve is closed we need a gap between the valve and cam or rocker. If too tight, then when the engine warms, you will hear it drop a cylinder or more out... This is why it's important for engine to be stone cold

So next we need to know when to adjust each valve in turn. This is where some understanding of how the engine works comes in. You need the cylinder on firing stroke at TDC. So if you put the engine on its TDC marks, then either cylinder 1 or 4 will be on firing stroke. For an overhead cam engine, the cylinder with the cam lobes pointing away from the valve is on firing stroke. So once that cylinder is adjusted, turn the motor through 360 degrees and the other cylinder will be on firing stroke. If you turn the crank through 180 degrees, the cylinders 2/3 will come in to play, repeat process, turn through 360 again for the 2nd cylinder of that pairing.

For engines with rockers and pushrods, as you aproach TDC, watch the valves, you will have one cyinder where the exhaust is closing and the inlet is opening, this cylinder is "on the rocks" (rocking between valve open/valve closed), so the other cylinder is on firing stroke.


If you know the firing order, life gets a bit simpler to understand what the engine is doing. Cylinders 1 and 4 go up and down together and are 180 degrees to 2 and 3, so firing will either be 1,3,4,2 or 1,2,4,3. For all 4 cylinders to go through a firing stroke, the crank will turn through 720 degrees. With a 4 stroke, a piston goes up on compression stroke, down on power stroke, up on exhaust stroke  then down on induction stroke.... Easily remembr as Suck, squeeze, bang, blow..... Lust like a 2 bit hooker!!

You will also need to know the spec for clearances, and if a shim engine, a shim chart is useful....

Tools required Am going to assume you have access to the valves already, so you will need:

A socket/ratchet/spanner to fit the crank pulley to turn the crank

Feeler gauges

Spanner and driver/pliers or adjuster tool for locknut and screws

verniers or micrometer for bucket and shim engines

Chalk for marking pullies (makes it easier to see)

Head Torch

Obviously the list is not exhaustive as different engines an have skightly different requirements

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1 hour ago, Saintly Fish said:

Nice article that Steve. I have no clue what your talking about, but it impressed me all the same! 
Maybe a little section or key page if you like would be good so we know what the terminologies mean ? 

Will add pictures etc at a later point


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