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Hey they click, they can be adjusted, they're somewhere in the head. What are they!? What do they look like and what do they do?
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Oooeer, someone may correct me here.....
Originally the camshaft sat at the bottom of the engine rather than the top (overhead cam). On top of this cam sat a small piece of metal (cam follower) and on top of this sat a rod of metal to transfer the movement of the cam to the valves at the top (push rod).
At the top of the push rod was a piece of metal pivoted in the middle designed to transfer the the upward motion of the push rod, as pushed by the cam, into downward pressure on the valve (the rocker) which pushed on the top of the valve and opened it.
In between the rocker and the valve stem there is a clearance to be set ensuring that the valve is able to close fully but also is able to be pushed right open. This adjuster ensures a gap remains causing a tapping noise between the valve stem and the rocker - this would be called the tappet.
Adjust it too small and the valve cannot close properly, adjust it too large and aside from a lot of noise, you restrict the ability of the valve to fully open.
The adjuster itself is frequently as simple as a bolt screwing into the rocker with a lock nut. One adjusts the gap, using feeler guage to measure the space, and then the lock nut locks (predictably)
Clear ? I read it back and it sounds convoluted even to me.
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Don't forget side valves where the adjustment was built into the tappet.
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Or, the old Ford E93A engines, and the side valve V eights, where there was no adjustment at all, except by grinding away some of the valve stem.
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Hmm, so overhead cams do not have tappets? (I know they don't have rockers).
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They (mostly) have little shims as a form of adjustement.
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Pedantically, there is no such thing as a tappet on an internal combustion engine. Tappets were screws that regulated (or "tapped") oil feed on some steam engines.
The term "tappets" is sometimes used on i/c engines to describe either the cam followers or the valve clearances and is the wrong terminology on both counts.
As Mark says, the use of the word "tappet" could have come from the tapping noise that valve clearances created - before the days of hydraulic followers (or "hydraulic tappets"!).
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To refine this a little further.
We are all referring to what we know as tappets rather than the accepted broader meaning in mechanical devices.
A tappet would be regarded as a follower, lever or pawl that rode on or engaged with a cam or toothed wheel...that in turn would convert the rotary motion to an angular, linear or intermittent one.
The term pre-dates the internal combustion engine and can be found in old engineering books associated with mechanical devices other than cars.
Strictly speaking, in the modern car engine, a tappet is the part of the valve train that actually follows the camshaft lobes. With overhead valves it will be in the engine block below the pushrod, with an overhead cam the tappet will usually be between the cam lobes and the top of the valve stem.
David W
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Spot on David!
Regards
John S
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Ha ha John...had to delve a little deeper than the Observers Book of Cars this time.
A handy copy of the 1901 Edition *Spons' Mechanics' Own Book* confirmed my thoughts.
Interesting Mark's understanding is that the tappet is the valve rocker adjusting screw itself....well that was exactly how some 1960s British motorcycle workshop manuals described them. Most odd.
David W
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G12 650cc Matchless
Norton Dominator SS 600cc
G8 Matchless
BSA triple & Trident
Norton Commando 750
Iused to read and work from those manuals !!
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G12 650cc Matchless Norton Dominator SS 600cc G8 Matchless BSA triple & Trident Norton Commando 750
Pushrod heaven!
For anyone who drives to France,
don't tell the garage that you're having trouble with your tapette...
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David
Can't go that far back, but the 'Manual of driving and maintenance for mechanical vehicles (wheeled) 1937' (the HMSO published Army manual) also defines tappets as what we'd call cam followers. Interestingly, the diagram shows them to be roller tappets!
Regards
John S
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Wasn't that great? I lifted the whole engine single handed out of my Prefect SV and did the job on the kitchen table 'cos it was freezing out in the garage! Mother was less than delighted.
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And, if you are doing that job again, I think that somewhere I still have the mushroom shaped tool which you needed to remove the split valve guides.
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So have I-you also had to grind the valve in or cut the seat back if the clearance was too big-Churchill made a special tool for holding the valve to grind off the end of the stem-there was also special dummy valve guide so you could recheck your clearances without having to refit the whole assembly everytime.
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Am I right that 'real' OHC cam engines have the camshaft directly over the valve stem, with a cam follower and shim, but that many so-called OHC engines have the cam on top but offset and still use rockers to transmit the motion?
Presumably the method of adjustment would be different?
BTW, On some engines, shims can be easily extracted and another, thinner or thicker, substituted for adjustment. Others. eg Triumph Stag, need the whole camshaft removing to extract the cam follower which has the shim inside it.
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Cliff
The term OHC is generally used for any engine which has the camshaft in the head and does not use push rods. However, some use the cam to actuate the valve directly with an cam follower (or tappet!) between the cam and valve. Valve clearance adjustment is usually by shims, but some other engines used screwed adjuster in the cam followers - the early vauxhall OHCs for example.
Other engines sit the cam in the head beside the valves, which are actuated via rockers. These don't usually have a cam follower; the camshaft actuates the rocker directly. Examples are the Ford 'Pinto' engines, and a number of Toyotas. Adjustment here is on the rocker rather than the cam - the Pintos, for example use an adjustable pivot for the rocker.
Regards
John S
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The "Pinto" has followers not rockers.
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jc
Depends how you define 'follower', but I reckon the 'pivoted arm' system it uses is more like a rocker than the conventional cylindrical cam follower.
Regards
John S
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I've read all the facinating replies, and here's my twopence worth...
Most cars these days (with any performance anyway) have 'hydraulic tappets'. As said in other replies, the 'tappet' forms a bridge between the camshaft and the valve.
A hydraulic tappet is like a cylinder within a cylinder, with a little oil hole (or two) to allow oil in and out and the inner cylinder (usually) being spring loaded. The tappet is full of oil - when the cam is fully open and the valve shut the spring opens the tappet up (and sucks oil in). During operation, the oil in the tappet, being (virtually) incompressible, drives open the valve according to the self-adjusted position. The hydraulic tappet is therefore self-adjusting and subsequently quieter and more reliable than a manually adjusted tappet.
Anyone want to shoot me down in flames?????
El Dingo (Martin).
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Sorry, I meant to add the following generalisms.
Valves are usually operated (in my experience):
- Direct via tappets from the cam (in DOHC engines).
- Via rocker arms acting about an independent pivot shaft (in SOHC and pushrod engines).
El Dingo (Martin).
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El Dingo
Then you get the 16v Dolomite engine which operated 8 valves direct from the cam and 8 via rockers......
Regards
John S
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Gahhhh! (Surrounded by flames) Is that right? Strange b***** engine...I must admit that when the Dolomite was around the most I could afford was a 850cc mini van (pushrod to all 8 valves) and a Honda CB72 (SOHC and rockers).
Thanks John S.
El Dingo (Martin).
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No, not just ONE pushrod to all eight valves... (oops)
El Dingo (Martin).
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ED(M)
Yes, the 16v Dolomite Sprint used this as a 'cheap' way of converting a single OHC engine to 16 valves. 8 operated direct by the cam, and the other 8 driven by rockers off the back (top) of the cam. I imagine it limited options for optimising inlet and exhaust timing independently, but it worked, albeit apparently not the most reliable of beasts.
Regards
John S
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