Top Dead Center - why?

[Dickensheets]

My Haynes book says find TDC for #1 piston prior to replacing timing belt. I can't figure out why this is important. What did I miss?

 

Ryan

[Legacy777]

The haynes book is stupid. There's absolutely no reason to do that. Just align the timing marks that are on the pullies to the marks that are on the cam covers. Make note of the mark that's on the crankshaft gear. The mark is on the front of one of the teeth that is read by the crank sensor. It's not the arrow on the front of the gear.

[Dickensheets]

Thanks Josh.

[Dr. RX]

When the crank shaft timing mark is set, #1 is near TDC.

[frag]

When the crank shaft timing mark is set, #1 is near TDC.

 

Is that not only once every two revolutions ? On the other one, when the crankshaft timing mark is set, #1 is at the top of the exhaust stroke.

[Setright]

I think what Haynes wants to avoid is a cam spring pushing a camshaft when the belt is off. At TDC the internals will stay put.

[Dr. RX]

Is that not only once every two revolutions ? On the other one, when the crankshaft timing mark is set, #1 is at the top of the exhaust stroke.

When the crank mark and the cam marks are aligned, #1 will be TDC. Since these are four stroke engines, #1 is TDC every other stroke, one stroke down, next stroke up, next down then up, four strokes, it is TDC at the top of ever up stroke, it is in the proper place when the cams are aligned too. This is basic Gas Engine 101 stuff, if you are going to work on engines, you need to understand these simple principles.

[frag]

When the crank mark and the cam marks are aligned, #1 will be TDC. Since these are four stroke engines, #1 is TDC every other stroke, one stroke down, next stroke up, next down then up, four strokes, it is TDC at the top of ever up stroke, it is in the proper place when the cams are aligned too. This is basic Gas Engine 101 stuff, if you are going to work on engines, you need to understand these simple principles.

 

Dr. RX, thanks for answering, but with all due respect, I think that what you write is open to discussion on at least one count.

"Since these are four stroke engines, #1 is TDC every other stroke, one stroke down, next stroke up, next down then up, four strokes, it is TDC at the top of ever up stroke"

(1) In lots of automotive glossary on the web, TDC is explicitely referred to not as any topmost position of the piston but as the piston's top position on the compression stroke.

In the post of mine to which you are answering, TDC is used to mean "the piston's top position on the compression stroke"

Have a look here: http://autorepair.about.com/cs/generalinfo/l/bldef_762.htm : "Definition of TDC: The point at which the piston reaches the top of its travel on the compression stroke."

and this: "Top Dead Center -- when a piston is at the top most position during the compression stroke. TDC can also refer to the top most position of the piston in the exhaust stoke, but "true" TDC is on the compression stroke." This is from here: http://www.auditmypc.com/acronym/TDC.asp

 

Also on this site where a thorough explanation of the workings of a four stroke engine is given TDC is only used to describe the top position of the piston on the compression stroke. This is here: http://techni.tachemie.uni-leipzig.de/otto/otto_g0_eng.html

 

And finaly this: "True top dead center occurs when both the intake and exhaust valve are closed when the piston is at TDC. This is technically the end of the compression stroke and the beginning of the power stroke." This is from: http://www.hotcamsinc.com/instructions/Hot%20Cams%20YZ426%20Camshaft%20Install%20Intructions.pdf

 

I think that maybe one of the first chapter of "basic four stroke Gas Engine 101 stuff" should point out that the exact meaning of TDC does'nt seem to be unanimous and that if some people use it to describe the top most position of the piston on any stroke, others prefer to use it only to describe the top most position of a piston on the compression stroke.

 

I would be interested to know what other members think of this.

[Dr. RX]

Dr. RX, thanks for answering, but with all due respect, I think that what you write is open to discussion on at least one count.

"Since these are four stroke engines, #1 is TDC every other stroke, one stroke down, next stroke up, next down then up, four strokes, it is TDC at the top of ever up stroke"

(1) In lots of automotive glossary on the web, TDC is explicitely referred to not as any topmost position of the piston but as the piston's top position on the compression stroke.

In the post of mine to which you are answering, TDC is used to mean "the piston's top position on the compression stroke"

Have a look here: http://autorepair.about.com/cs/generalinfo/l/bldef_762.htm : "Definition of TDC: The point at which the piston reaches the top of its travel on the compression stroke."

and this: "Top Dead Center -- when a piston is at the top most position during the compression stroke. TDC can also refer to the top most position of the piston in the exhaust stoke, but "true" TDC is on the compression stroke." This is from here: http://www.auditmypc.com/acronym/TDC.asp

 

Also on this site where a thorough explanation of the workings of a four stroke engine is given TDC is only used to describe the top position of the piston on the compression stroke. This is here: http://techni.tachemie.uni-leipzig.de/otto/otto_g0_eng.html

 

And finaly this: "True top dead center occurs when both the intake and exhaust valve are closed when the piston is at TDC. This is technically the end of the compression stroke and the beginning of the power stroke." This is from: http://www.hotcamsinc.com/instructions/Hot%20Cams%20YZ426%20Camshaft%20Install%20Intructions.pdf

 

I think that maybe one of the first chapter of "basic four stroke Gas Engine 101 stuff" should point out that the exact meaning of TDC does'nt seem to be unanimous and that if some people use it to describe the top most position of the piston on any stroke, others prefer to use it only to describe the top most position of a piston on the compression stroke.

 

I would be interested to know what other members think of this.

Well that is true, but if you rotate the engine until the #1 piston is at the top of the stroke, and then set the cams, #1 will be at the top of the compression stroke. Now if you crank it through until the #1 pistoin is again at the top of the stroke, and then reset the cams again, the #1 piston will again be at the top of the compression stroke. So you see, it both cases that #1 piston was at the top of it's stroke, aka top dead center of the stroke, but until the cams were set to match the position of #1 piston, there was no compression stroke. Only after the cams (valve timing) is set does it determine if it is the compression stroke or not. Is this too advanced for you???

[NorthWet]

All of the manuals and references that I have bothered to read (not too much of that in the last 25 years of wrenching) have qualified TDC with "on the compression stroke" when they needed something having to do with the compression stroke. Otherwise, TDC refered to piston position, not piston position relative to the rest of the engine. The timing marks of most engines refer to TDC purely by piston position.

 

From a purest, mechanical point of view, TDC is just that... not qualified by anything outside of crank, rod, and piston.

[frag]

Well that is true, but if you rotate the engine until the #1 piston is at the top of the stroke, and then set the cams, #1 will be at the top of the compression stroke. Now if you crank it through until the #1 pistoin is again at the top of the stroke, and then reset the cams again, the #1 piston will again be at the top of the compression stroke. So you see, it both cases that #1 piston was at the top of it's stroke, aka top dead center of the stroke, but until the cams were set to match the position of #1 piston, there was no compression stroke. Only after the cams (valve timing) is set does it determine if it is the compression stroke or not. Is this too advanced for you???

 

Not at all DR. Even if lots of other things often appear this way to me (too advanced for me to comprehend). That's exactly how I am and was seeing things. But I would like you to see that this implies that you first post ("When the crank shaft timing mark is set, #1 is near TDC.") is true only if we give the acronym "TDC" the general meaning that you give to it (any topmost position of the piston) and if the old belt is not on. If the old belt is on (like it is in most cases of belt replacement) and if we give TDC the more restrictive meaning that I gave it (top most position of piston #1 on the compression stroke), the sentence becomes false: one could have the crankshaft timing mark set and #1 piston at the top of the exhaust stroke. In that case of course the right camshaft pulley mark would not be aligned.

 

Understand that I'm just trying to explain how I read your post, and how it could legitimately be read that way since it was devoid of any contextual informations.

I'm sorry to detect traces of agressiveness in the last sentence of your last post. I would like to believe that this is not necessary.

[frag]

All of the manuals and references that I have bothered to read (not too much of that in the last 25 years of wrenching) have qualified TDC with "on the compression stroke" when they needed something having to do with the compression stroke. Otherwise, TDC refered to piston position, not piston position relative to the rest of the engine. The timing marks of most engines refer to TDC purely by piston position.

 

From a purest, mechanical point of view, TDC is just that... not qualified by anything outside of crank, rod, and piston.

 

I'm ready to believe that I've probably read less reference manuals than you have, but all of the references that I have read concerning engine timing refer without exception to "degrees before TDC" withoug qualifying it.

 

"The timing marks of most engines refer to TDC purely by piston position". True. But timing marks are useful only if they refer to the piston #1 at some degrees BTDC on the compression stroke. I dont see how setting spark on the exhaust stroke would be useful. The timing marks mechanicaly refer only to the #1 piston position but are useful only when the camshaft position is such that this piston position coincide with a precise position of the valves, in that case, shut.

[tcspeer]

Frag, I was on your side of this debate last year and went round and round with O.B.W. but he finnally proved his point to me. If the belt is on, their is only one top dead center. With the belt off it dont matter the crank makes two turns to the cams one. So you can have T.D.C. at what used to be the exhaust stroke. Iam thanksful that someone made the job eaisly for me.

[frag]

Frag, I was on your side of this debate last year and went round and round with O.B.W. but he finnally proved his point to me. If the belt is on, their is only one top dead center. With the belt off it dont matter the crank makes two turns to the cams one. So you can have T.D.C. at what used to be the exhaust stroke. Iam thanksful that someone made the job eaisly for me.

Hi tcpeer! Dont ask me to explain cause I'm too tired and should already be in bed, but I absolutely aggree with the description you just gave. I think maybe my last answer To DR. RX should make clear what the crux of the debate with him was. I hope.

Good night!

[NorthWet]

Ask a hotrodder "degreeing in a cam" and they are likely to tell you that the most useless use of TDC is on the pistons compression stroke. Nothing significant is happening then, just the spark event(s), and that(they) can be considerably off without major effects.

 

I was not trying to impress anyone with how much (or how little ) I have read, just that what might be "fashionable" today is not representative of what has been considered standard terminology for a much longer period of time.

[Dr. RX]

I'm ready to believe that I've probably read less reference manuals than you have, but all of the references that I have read concerning engine timing refer without exception to "degrees before TDC" withoug qualifying it.

 

"The timing marks of most engines refer to TDC purely by piston position". True. But timing marks are useful only if they refer to the piston #1 at some degrees BTDC on the compression stroke. I dont see how setting spark on the exhaust stroke would be useful. The timing marks mechanicaly refer only to the #1 piston position but are useful only when the camshaft position is such that this piston position coincide with a precise position of the valves, in that case, shut.

Sorry Frag, but I really don't think you understand. TDC happens every time the piston reaches the upper most point in the stroke. Now, if you include the valve timing in this, then there is a TDC on the compression stroke and a TDC on the exhaust stroke. When setting the engine timing you must use the TDC on the compression stroke, which is what the article was talking about. As for timing marks, there is one to tell the position of the crank, and one (or more) to tell the position of the cam(s), it takes both sets of marks to be 'timed' together to make the engine run. When you are talking about setting timing BTDC, then you are talking about setting the ignition spark, not the alignment of the engine components. Is that clearer now?

[frag]

Sorry Frag, but I really don't think you understand. TDC happens every time the piston reaches the upper most point in the stroke. Now, if you include the valve timing in this, then there is a TDC on the compression stroke and a TDC on the exhaust stroke. When setting the engine timing you must use the TDC on the compression stroke, which is what the article was talking about. As for timing marks, there is one to tell the position of the crank, and one (or more) to tell the position of the cam(s), it takes both sets of marks to be 'timed' together to make the engine run. When you are talking about setting timing BTDC, then you are talking about setting the ignition spark, not the alignment of the engine components. Is that clearer now?

 

I understand all of that Dr. RX. The problem is one of language (the most important of which is definition of TDC which, once again, does'nt seem to be as unanimous as you seem to think - i gave a few examples) and communication. Your first post was open to at least two interpretations and in the absence of context I chose one which was not the meaning you intended. That's all. And we should leave it at that.

Have a good day.

[Legacy777]

We're venturing down the road of semantics........

[frag]

We're venturing down the road of semantics........

 

Yea. Maybe we should have a special forum for that.

As soon as a moderator detects it is happening, zooooom!

[cookie]

years ago when mixing parts on a Norton I had blown up I found that the crank I used had the position for the rotor in a different position. I got a time consuming course in basic engine timing punctuated by many kicks on a heavy starter pedal.

[Dr. RX]

Well, it not a matter of sematics or context. In each case you stated, TDC is the same, when the piston is at the highest point in the upward stroke. Where the examples vary is additional information they provide, like 'compression stroke'. If you are taking compression stroke, then it must be assumed that the valve timing is to be taken into consideration in which case it would be the TDC when all the valves are closed.

 

I think most of your confusion is coming from the fact that you are trying to set the engine before you remove that old belt. Forget about that, remove the belt, then set up the engine to add the new one. I was told by a Subaru mechanic that once the timing is set and the engine is rotated, those marks never line up as they should again. You do need to know if you engine is a clearance engine or not, as that will necessitate which direction you need to rotate the cams to align the marks. The way that I do it is the make sure the #1 is TDC on the compression stroke, then I remove the old belt. I align the cams to the timing marks, then install the new belt using the line on the belt to match with the timing marks. I've done 3 DOHC and 4 SOHC EJ series engines that way, and everyone of them fired up on the first crank. I hope this is what you were looking for.

[frag]

Well, it not a matter of sematics or context. In each case you stated, TDC is the same, when the piston is at the highest point in the upward stroke. Where the examples vary is additional information they provide, like 'compression stroke'. If you are taking compression stroke, then it must be assumed that the valve timing is to be taken into consideration in which case it would be the TDC when all the valves are closed.

 

I think most of your confusion is coming from the fact that you are trying to set the engine before you remove that old belt. Forget about that, remove the belt, then set up the engine to add the new one. I was told by a Subaru mechanic that once the timing is set and the engine is rotated, those marks never line up as they should again. You do need to know if you engine is a clearance engine or not, as that will necessitate which direction you need to rotate the cams to align the marks. The way that I do it is the make sure the #1 is TDC on the compression stroke, then I remove the old belt. I align the cams to the timing marks, then install the new belt using the line on the belt to match with the timing marks. I've done 3 DOHC and 4 SOHC EJ series engines that way, and everyone of them fired up on the first crank. I hope this is what you were looking for.

 

Corky - If I can call you by this name - I think this is becoming hopeless:banghead: and that we should let it drop.I get the feeling that you've have'nt read my posts with enough attention or are confusing me with another poster. I'm not in the process of replacing my belt, I've done it 2 years ago and very successfully. My only mistake was to have commented (out of nowhere, all my excuses) on your first post in this thread presuming that by TDC you meant TDC on the compression stroke, wich in the context was a legetimate but erroneous interpretation. All of what followed was only a consequence of this misunderstanding. That's all. Let's forget about this please, or Josh will zoom us to the new "semantic" forum.

[cookie]

That would easily solve the problem as I think we all agree except for the minor semantics anyway.

[Setright]

I would like to add to the confusion, simply because I also believe that it is important for us to speak the same language and use the same terms for the the same things:

 

 

Top Dead Center refers to a CRANKSHAFT position. Pistons only recipcrocate. TDC only gives meaning in conjunction with a rotational motion

 

Yes, of course this affects piston position, but it would only be called "Top" if it were the piston itself we were referring to!

[WAWalker]

Let me throw a monkey wrench into this discussion.

 

Since this is a Subaru fourm and I assume this was started with a Subaru related question................You don't want to find TDC of anything when replacing a Subaru timing belt. When all the timing marks are lined up (in the 12 o'clock possition on an EJ engine) all four pistons at are in the middle of their stroke.

 

You should NEVER just pull an old belt off and then rotate the engine and or cams to line up the timing marks. The timing marks should be lined up before removing the belt.

 

The marks on a timing belt will not line up again after rotating the engine. But the timing marks on the crank sprocket and cam always will. You should rotate the engine by hand after a timing belt install to make sure these marks line up before starting the engine.