What's a tappet?
Discussion
I'm no numpty (I think
) when it comes to understanding car engines, be it valve timing, spark timing, fuel injection, I understand the principle of all of these things but the one thing I have never understood is what exactly is a tappet and why do they need adjusted every once in a while.
As I understand it thses little things live somewhere in close proximity to valves on overhead cam (DOHC) style engines and need sorting at about 25000 miles give or take a bit.
So what exactly are these little things and why do they need sorted.
Regular (old school) style pushrod type engines will go pretty much forever without needing the valves to be sorted as I understand.
I know that TVR AJP engines have all got tappets can a Tiv owner exlain? please?
Feel free to get all technical, I study engineering and understand most things mechanical. I tried to search howstuffworks but came up with nothing.
It's not important but it's annoying me!
Cheers
Andy
) when it comes to understanding car engines, be it valve timing, spark timing, fuel injection, I understand the principle of all of these things but the one thing I have never understood is what exactly is a tappet and why do they need adjusted every once in a while. As I understand it thses little things live somewhere in close proximity to valves on overhead cam (DOHC) style engines and need sorting at about 25000 miles give or take a bit.
So what exactly are these little things and why do they need sorted.
Regular (old school) style pushrod type engines will go pretty much forever without needing the valves to be sorted as I understand.
I know that TVR AJP engines have all got tappets can a Tiv owner exlain? please?
Feel free to get all technical, I study engineering and understand most things mechanical. I tried to search howstuffworks but came up with nothing.
It's not important but it's annoying me!
Cheers
Andy
I'm no tech-head but my simple explanation is that the tappet is the bit between the camshaft lobe and the valve. On an OHV it is at the end of the rocker which is 'rocked' by the push-rod. On an OHC there is no need for push-rods as the camshaft is in close proximity to the valves. Adjustment is necessary to allow for wear. Hydraulic tappets adjust automatically but have their limitations in high-performance applications (this is why, for example, the Escort RS1600i was homologated for saloon car racing - it did away with the XR3i's hydraulic tappets).
>> Edited by reardrive on Thursday 21st February 21:36
>> Edited by reardrive on Thursday 21st February 21:42
>> Edited by reardrive on Thursday 21st February 21:36
>> Edited by reardrive on Thursday 21st February 21:42
Its a bit like a see saw 
the cam lobe lifts one end, and the other end 'taps' down on the top of the valve, opening an exhaust or inlet valve. After a few K miles of 'tapping' things get worn and gap grows between tappet and valve top. So this is where the adjustment comes in. Rover lump has hydraulic tappets & a clever use of oil pressure to adjust the gap.
the cam lobe lifts one end, and the other end 'taps' down on the top of the valve, opening an exhaust or inlet valve. After a few K miles of 'tapping' things get worn and gap grows between tappet and valve top. So this is where the adjustment comes in. Rover lump has hydraulic tappets & a clever use of oil pressure to adjust the gap. Just to confuse you further, old pushrod engines can have tappets too, under the pushrod. The tappet sits between the pushrod and the cam lobe.
And the Yanks use the term interchangeably with rocker arms!
And on bike engines the "tappets" under the cam lobes are referred to (probably more correctly) as shims.
That better? Thought not.
And the Yanks use the term interchangeably with rocker arms!
And on bike engines the "tappets" under the cam lobes are referred to (probably more correctly) as shims.
That better? Thought not.
quote:
Just to confuse you further, old pushrod engines can have tappets too, under the pushrod. The tappet sits between the pushrod and the cam lobe.
And the Yanks use the term interchangeably with rocker arms!
And on bike engines the "tappets" under the cam lobes are referred to (probably more correctly) as shims.
That better? Thought not.
nice one Phil ! explains it pretty good.
hmmmm
I wished I hadn't asked now
I have always considered the see saw thingy to be a rocker, I didn't know some people also called them tappets. I stand corrected.
I think (to my mind anyway) that a tappet is the shim which is between the valve lobe and the top of the valve. This applies to engines which have the valve actuating the valve directly (ie. not via a rocker arm)
I know that Merc favour the rocker arm activated by overhead cams but other firms (like Honda) have the cam lobes acting directly on the valves themselves.
I could be completely wrong about that though.
ah sod it soon we'll all have soledoid/pnuematic actuated valves anyway....
and it's friday afternoon so finish lecture and then
student life isn't all bad
Cheers for all the replies
Andy
I wished I hadn't asked now
I have always considered the see saw thingy to be a rocker, I didn't know some people also called them tappets. I stand corrected.
I think (to my mind anyway) that a tappet is the shim which is between the valve lobe and the top of the valve. This applies to engines which have the valve actuating the valve directly (ie. not via a rocker arm)
I know that Merc favour the rocker arm activated by overhead cams but other firms (like Honda) have the cam lobes acting directly on the valves themselves.
I could be completely wrong about that though.
ah sod it soon we'll all have soledoid/pnuematic actuated valves anyway....
and it's friday afternoon so finish lecture and then
student life isn't all bad
Cheers for all the replies
Andy
Well my old MGAs had 4 cyl single cam push-rod engines and I used to adjust the tappets on them (often 
), no shims just a threaded adjuster on the end of the rocker arm with a lock nut. Mind you the Twin Cam had shims under the cam followers and these were a right pain to adjust! Rich...
), no shims just a threaded adjuster on the end of the rocker arm with a lock nut. Mind you the Twin Cam had shims under the cam followers and these were a right pain to adjust! Rich...quote:
And on bike engines the "tappets" under the cam lobes are referred to (probably more correctly) as shims.
I always thought that a shim was a shim and as such came in different thicknesses to suit, surely the "buckets" that they sit in and which contact with the cams would be a better comparison to a tappet. Although having said that they are actually nothing like tappets and are purely a different way of doing the same job.
quote:
Indeed, so in an OHC engine the shim sits in the bucket adjusts the valve clearance and with a push-rod engine either the rocker arms or the push-rods have a threaded screw which adjusts the valve clearance so it is a good question isn't it! Where is the "tappet"
Hasn't got one, it has a cam follower instead!
Peter Humphries (and a green V8S)
The 'tappet' is a bit of a misnomer in a modern OHC engine but typically what most people refer to as a tappet is usually the cam follower. This follower is between the camshaft and either the end of the valve in a modern OHC design (speed six engine) or the pushrod in an OHV design (Rover V8). When the 'tappets' are 'adjusted' this refers to re-setting valvegear clearences, in the case of the rover it has a hydraulic follower where the pushrod pushes on a piston within the follower and the reaction against it (valve clearence) is determined by oil level in a cavity below that piston. On some OHC designs there is very little room for this extra piston, which in any case makes the valvegear heavier and reduces how adventurous you can be with the cam profile. So on these engines the valve clearence is set with a 'shim' between the end of the valve and the follower. As bits wear out this clearence needs adjustment, hence 'adjusting the tappets'.
Phew
Matt
Phew
Matt
While were on the subject of Tappet adjustment, If they are not adjusted, can it cause the engine to effectively burn oil on starting?? My mate has an escort which spews enormous clouds of blue "oil" smoke when he starts it cold, he thinks it could be the Piston Rings but has been advised to check the tappets!!! Are tappets easy to do? roughly how much would it cost for them to be done at a garage??
Cheers
Stuart!
Cheers
Stuart!
the reference to tappets in that case will probably be the stem seals on the valves.....the bits that get opened and closed by the tappets, if these are shagged oil drains past em (when the engine is off) and collects in the bores, when you start up, grey smoke...tell him to sell it
>> Edited by apache on Saturday 23 February 00:43
>> Edited by apache on Saturday 23 February 00:43
Your all a bunch of Tappets
This is to tvradict
Does your friends escort spue out the smoke on start up or is it all of the time??
If it is only on start up, then it is the Valve Oil Seals that have whorron out and they will nead replacing and to do that you will have to take the head off to drop out the valves.
Is you friends Escort an old XR3I by any chance with over 100,000 miles on the clock, if so it would be best if your mate puts in a new cam in aswell, cause the CVH engine is famouse for cam wear and with a new cam it will bring back lost power
Good luck
This is to tvradict
Does your friends escort spue out the smoke on start up or is it all of the time??
If it is only on start up, then it is the Valve Oil Seals that have whorron out and they will nead replacing and to do that you will have to take the head off to drop out the valves.
Is you friends Escort an old XR3I by any chance with over 100,000 miles on the clock, if so it would be best if your mate puts in a new cam in aswell, cause the CVH engine is famouse for cam wear and with a new cam it will bring back lost power
Good luck
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