Tappets - what are they?!

[hornmeister2000]

OK, this is off topic although still a "technical" question but I thought I'd post it here as I don't touch these on my Maestro. Basically, until about July I knew very little about cars (although I could manage a service). In July I bought a Morris Minor and decided I'd have to learn a LOT about how cars, engines, etc work. I think I've learnt quite a lot and recently changed the whole ignition system on the Minor and dismantled the power steering at the weekend on the Maestro. The one thing I can find no information about, though, is tappets. I can adjust them on the Minor but I don't have a clue what they actually do. Can anyone enlighten me please?

[Dave]

The tappets are small rocking metal 'levers' that push the valves open. Imagine a see-saw hinged pivoted in the middle, one side resting ontop of the valve, the other side being pushed upward by the con-rod (the rod connecting the up/down motion of the cam-shaft, to the bottom edge of this see-saw). As the con-rod pushes up, the tappet moves, and inturn pushed the valve down (open)...

check out: http://auto.howstuffworks.com/engine.htm
The tappets are shown in orange, note, the engine shown is overhead cam (OHC) so there are no conrods.... on your minor, and your Maesty if it is a 1300, the cam shaft (blue), is down inside the block with the motion of the cams linked to the tappets via con-rods.

Hope this helps,
Dave.

[Dave]

Just noticed, you dont have a 1300 Maesty... the 2.0EFi is overhead cam as shown in the animation.

Also, to clear it up a bit, when you talk about 'adjusting' the tappets, there is a screw in the end that pushes down on the valve, the valve is actually only in contact with the screw. So you can alter the gap between the top of the valve and the bottom of the screw by winding it in and out. The correct gap will be specified in the Haynes manual.

Dave

[F690OTF(RIP)]

You mean push-rods, not con-rods. Con-rod being a contraction of 'connecting rod', which is what connection the piston to the crankshaft.

Not trying to be needlessly pedantic (although I am good at that); it just seems best not to confuse people more than necessary :)

I always thought that 'tappet' was a fancy name for 'cam-follower'. On an OHC engine (such as the one on HowStuffWorks), these also operate the valves. On the A-Series engine, those things to which you refer as tappets are on the 'rocker shaft', so I thought they were called 'rockers'. Do the push-rods follow the cams directly, or is there something in-between (which would be a tappet by my understanding)? Of course many modern engines have hydraulic tappets. I'm now not quite sure what this would mean...

Sorry to confuse the issue! :rolleyes: I'm sure someone will be along soon who knows for sure.

[MaestroMatt]

I'm fairly sure Dave is right - the tappets are the levers and to adjust the tappets is to adjust the screw that governs the distance between lever and valve.

[H48HPE]

on the A series engine (morris minor engine??) the rocking metal levers dave talks about are called rockers, they fit on the rocker shaft and are covered by the rocker cover(with the oil filler in) one side of the rocker pushes the valve open and the other side gets pushed up by a push rod which is pushed up and down by a cam on the cam shaft. between the cam and the pushrod there is a cam follower i think.

by adjusting the tappets on the a series engine I think you mean adjusting the valve clearances. this is the gap between the valve and the rocker, it has to be set correctly or the rocker will rattle against it.

at least thats the way I see it.

andy

[Dave]

Yep, spot on... the Rocking part is "the Rocker", and the tappet is the screw assembly on the end of the rocker that pushes on the valve.

Of course i mean push-rod!

Dave

[threelitre]

Well, the A-series does have tappets, sitting on top of the cam-shaft and driving the push-rods. But on the A-series they cannot be adjusted, since the adjustment takes place on the adjusting screw at the valve-end of the rocker. On engines like the R- or O-series there are no adjustment screws. So the tappets (acting directly on the valves) have to be 'adjusted' - this is done by inserting small shims between tappet and valve, so that the given free play of the valve is reached. You can imagine a tappet as a small cup sitting upside down over the valve, the adjusting shims are inserted into the tappet before placing it over the valve. Engines with hydraulic tappets need no adjustment for the valves, since the tappet contains a small piston which is adjusted automatically via oil pressure from the engine.

Regards,
Alexander

[J199 HHG]

But the reason for requiring a tappet adjustment in the first place is as follows.

Basically, if the distance between the camshaft and the rocker was precisely known, one could make the pushrods the correct length.

But the engine height will change if you skim the block or head, or as your head gasket compresses over time. Also, the height of the engine changes as it heats up, and as the block / head materials are different to that of the pushrod, then the relative lengths change.

So your adjustment should be carried out at the temperature specified in the manual.

And in an ideal world, when the engine is at operating temperature, the valves are allowed to fully close, with minimum play in the valvetrain.

If you make the gap too small, your valves will not close, and you will loose compression and burn the valves.

If you make the gap too big, the engine will rattle.

[Landcrab1800]

Got to have my ten penneth,


The whatsitcalled pushes the oojiwhat which rocks the thingamybob causing the whatyoucallit open.

Hope this clears things up.:laugh: :laugh:

But seriously

the cam has a cam follower, this operates the push rods, these strike the tappet plate or threaded ball, the tappet is attached to the rocker arm which has a striker plate that pushes the valve stem.

On OHC there are no rockers, pushrods, tappets or striker plate just shims and buckets.
The exception to this is the Triumph 2ltr 'sprint engine' which does have rockers and its bits.

Todays homework is......:laugh:

[Ricky]

Originally posted by Landcrab1800

The whatsitcalled pushes the oojiwhat which rocks the thingamybob causing the whatyoucallit open.


I was watching an old Alistair McGowan on UK Gold the other day, and you've just reminded me of the moira stewart sketch he did, where she had to announce the sad death of "Oh What's He Been In"..... :laugh: :laugh: :laugh: