Top Dead Center - why?

[frag]

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!

 

Lets get the confusion to its max:

1) Haynes 89100 page 2A-4: "Top dead center (TDC) is the highest point in the cylinder that each piston reaches as it travels up the cylinder bore. Each piston reaches TDC on the compression stroke and on the exhaust stroke, but TDC generally refers to the piston position on the compression stroke."

2) Chilton's 64302 glossary, page 10-77. "Top Dead Center : the point at which the piston reaches the top of its travel on the compression stroke."

[frag]

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.

 

Not absolutely sure about what I'm gonna say Wawalker, but i followed Haynes in the first part of my cam belt replacement and put piston #1 at TDC (compression stroke) while setting the crankshaft mark and IIRC the three sets of marks then lined up. For what it's worth.

[tcspeer]

Surely the marks on the belt will line up again sometime, Where are the engineers that can figure out how many rotations it would take?

[WAWalker]

Not absolutely sure about what I'm gonna say Wawalker, but i followed Haynes in the first part of my cam belt replacement and put piston #1 at TDC (compression stroke) while setting the crankshaft mark and IIRC the three sets of marks then lined up. For what it's worth.

 

Did you pull #1 plug to put it an TDC compression? Or are you assuming that #1 is at TDC when you align the marks.

 

I work on Subarus 5-6 days a week. Have done many many timing belts, head gaskets..................I know where the pistons are at when the timing marks are lined up.

[Legacy777]

Man.....you guys are freaking beating a dead horse, then kicking it, and then shooting it.

 

The bottom line to the question asked is.....you really don't need to be concerned with TDC when doing the timing belt. Just rotate the engine until the crank & cam marks are aligned, take the belt off, put the new one on, and be done with it.

[Legacy777]

Surely the marks on the belt will line up again sometime, Where are the engineers that can figure out how many rotations it would take?

 

Sure, the marks will line back up after the belt has made one complete revolution of the entire cam/crank gear/pulley train. As to how many rotations did the crank or cam make.....don't have a clue.

[WAWalker]

Man.....you guys are freaking beating a dead horse, then kicking it, and then shooting it.

 

 

No, not beating a dead horse. Just trieing to clear up some of the bad information floating around here. There is a rash of it. Sadly some of it is comeing from repair manuals that people are paying money for.:-\

[tomson1355]

Sure, the marks will line back up after the belt has made one complete revolution of the entire cam/crank gear/pulley train. As to how many rotations did the crank or cam make.....don't have a clue.

 

The marks on the belt will line up, but the mark on the crank sprocket will only line up if the # of teeth on the belt is divisible by the # of teeth on the crank sprocket. The cam sprocket marks will line up also only if the crank has made an even number of revolutions.

 

If the # of teeth on the belt is not divisible by the # of teeth on the crank sprocket, say 500 and 22, it will take many more than one turn until everything (timing belt marks and crank/cam marks) lines up again. Eleven turns of the belt in this case.

 

Unless, of course, I'm wrong.

[Setright]

Had in look in Gregorys "Automotive Mechanics" book. TDC is only the top dead center, they specifically state "on the compression stroke" if that's what they mean :-)

[Dr. RX]

Sure, the marks will line back up after the belt has made one complete revolution of the entire cam/crank gear/pulley train. As to how many rotations did the crank or cam make.....don't have a clue.

Crank once, cams 1/2 rotation. There are 720 degrees of ration in a 4 stroke engine, the crank rotates twice for even single turn of the cam. So, if the cams have only rotated a half a turn, do you really think the lines will line up with the timing marks after one turn?

[Legacy777]

The marks on the belt will line up, but the mark on the crank sprocket will only line up if the # of teeth on the belt is divisible by the # of teeth on the crank sprocket. The cam sprocket marks will line up also only if the crank has made an even number of revolutions.

 

If the # of teeth on the belt is not divisible by the # of teeth on the crank sprocket, say 500 and 22, it will take many more than one turn until everything (timing belt marks and crank/cam marks) lines up again. Eleven turns of the belt in this case.

 

Unless, of course, I'm wrong.

 

There are marks on the back side of the belt. When I do timing belts, I align those marks on the belt with the marks on the cam sprockets & crank gear. The belt is a complete circle. Those marks on the back side of the belt will come around to the exact same spot. If you mark a spot on the belt with a marker. That spot will show up at the same spot after the belt (not the engine, the belt!) has made one complete revolution through the various timing belt components.

 

If you made another mark on the other side of the belt, and watched for that initial mark you made, and rotated the engine so that the BELT made one revolution, the second mark you made will be in the exact same location as when you initially marked it.

 

Will the marks on the cam pullies & crank gear line up with the initial marks on the belt? I don't know. I'll be putting a timing belt on the motor I'm building this weekend, and will play with this. I'm pretty sure the marks will line back up.

[Legacy777]

Crank once, cams 1/2 rotation. There are 720 degrees of ration in a 4 stroke engine, the crank rotates twice for even single turn of the cam. So, if the cams have only rotated a half a turn, do you really think the lines will line up with the timing marks after one turn?

 

I said nothing regarding the rotation of the engine, I said the marks on the belt will line back up after one complete rotation of the TIMING BELT.

[frag]

Did you pull #1 plug to put it an TDC compression? Or are you assuming that #1 is at TDC when you align the marks.

 

Answer: Yes, I was using a compression gauge on #1 cylinder.

 

I work on Subarus 5-6 days a week. Have done many many timing belts, head gaskets..................I know where the pistons are at when the timing marks are lined up.

 

If I had to bet on this right now, I would rather put my money on your experience than on the exactness of my recollections . Dont take it personnal though if I'm not absolutely convinced and will be waiting for more proofs.

[WAWalker]

Answer: Yes, I was using a compression gauge on #1 cylinder.

 

 

 

If I had to bet on this right now, I would rather put my money on your experience than on the exactness of my recollections . Dont take it personnal though if I'm not absolutely convinced and will be waiting for more proofs.

 

I can provide pictures.

[frag]

Concerning the Haynes recommendation to put piston #1 at TDC (compression stroke) before removing the covers when replacing the cam belt, Motor Magazine, seems to give the same advice. Am I right in thinking that putting the crankshaft pulley (not sprocket) timing mark at zero is equivalent to putting #1 piston at TDC (compression stroke), if of course the old belt is still properly installed ?

 

 

 

"The easiest way to do it on a 2.2 is to

bring the timing mark on the harmonic

balancer up to 0 on the engine’s timing

tab mounted on the front cover. Then

remove the outer cam belt covers and

check the timing marks on the front

face of each cam sprocket. These indentations,

which we’ll discuss in greater

detail next time, face the front of the car

and must both be pointing straight up.

If both cam sprocket marks aren’t exactly

at 12 o’clock high, the t-belt was installed

incorrectly."

[frag]

I can provide pictures.

 

Not that I doubt your word, but that would help settle that question once and for all.

TIA.

[cookie]

say to bring the engine to TDC on the compression stroke is tradition. Many engines have set up this way for years. Since many mechanics work on quite a number of engine types during thier career I think folks tend to fall into a pattern in thier thinking.

When one really had to worry about TDC on the compression stroke was in installing a distributor when you could easily get it backwards if you got TDC on the wrong stroke.

On the new Subies I would go with the factory instructions out of concern for bending a valve. I would also make cam locks so they could not turn during installation.

A real pro like Wwalker knows ways to take shortcuts, but if you don't work on a type of vehicle every day I say take no chances.

[WAWalker]

Concerning the Haynes recommendation to put piston #1 at TDC (compression stroke) before removing the covers when replacing the cam belt, Motor Magazine, seems to give the same advice. Am I right in thinking that putting the crankshaft pulley (not sprocket) timing mark at zero is equivalent to putting #1 piston at TDC (compression stroke), if of course the old belt is still properly installed ?

 

 

 

"The easiest way to do it on a 2.2 is to

bring the timing mark on the harmonic

balancer up to 0 on the engine’s timing

tab mounted on the front cover. Then

remove the outer cam belt covers and

check the timing marks on the front

face of each cam sprocket. These indentations,

which we’ll discuss in greater

detail next time, face the front of the car

and must both be pointing straight up.

If both cam sprocket marks aren’t exactly

at 12 o’clock high, the t-belt was installed

incorrectly."

 

All your reference materials are generalized, and mostly biased toward American made cars. You are reading these and taking the info as gospel, big mistake. I take no personal offense, but I feel for the people who are relying on this fourm for correct info to repair their cars.

 

Try this. http://endwrench.com/main.php?smPID=PHP::article_detail.php&&RECID=171

 

And this. http://endwrench.com/main.php?smPID=PHP::article_detail.php&&RECID=184

[cookie]

That would make me feel a lot more confident that the average tear down manual. Looks like the engine I read about (sohc) would not need cam locks.

[Dr. RX]

I said nothing regarding the rotation of the engine, I said the marks on the belt will line back up after one complete rotation of the TIMING BELT.

OK, if there are 255 teeth on the belt, and there are only 20 teeth on the crank gear, when the belt has made one rotation, the crank will have rotated 12 times, plus 15 teeth. I can't remember the exact number of teeth at the moment, but that is the reason why the line don't line up after they are used to set up the engine.

[WAWalker]

Not that I doubt your word, but that would help settle that question once and for all.

TIA.

 

http://www.ultimatesubaru.org/photos/data/500/3702_2L_005-thumb.jpg

[frag]

All your reference materials are generalized, and mostly biased toward American made cars. You are reading these and taking the info as gospel, big mistake. I take no personal offense, but I feel for the people who are relying on this fourm for correct info to repair their cars.

 

Try this. http://endwrench.com/main.php?smPID=PHP::article_detail.php&&RECID=171

 

And this. http://endwrench.com/main.php?smPID=PHP::article_detail.php&&RECID=184

 

1) I know both these enwrench references and have used them - with other sources - when I replaced my belt.

2) Refering to my preceding post, you're way off the mark when you say that this material is "generalized" and "biaised toward American made cars". The article I quoted was writen by Dan Marrinucci, of the "Froreing Service" section of Motor Magazine and deals specificaly with cam belt replacement on the 2.2 L Subaru engine.

3) I take nothing as Gospel, be it something written in a book or posted on this forum or elswhere. I just report it as info or opinion open to discussion (read again my posts in this thread, specialy my second answer to DR. RX). There are probably people here that think their word is Gospel or a quote from Gospel. I'm not one of them.

4) I take exception to your last remark. I think this Board is open to all (experts, pseudo expert and amateurs like myself) and I often make a special effort to make clear in my posts that I'm only stating an opinion open to discussion. I think members and visitors are intelligent enough not to blindly follow anything anyone writes here in answer to their questions (be it comming from a "pro" or an amateur). I have many times, in other threads and in this one, when it was the case, specified that it's only my opinion or my experience and that the poster would be well advised to wait for other opinions to take a decision.

Examples taken from my posts in this thread:

- (end of post no 8) : " I would be interested to know what other members think of this."

- (post 27) " : Not absolutely sure about what I'm gonna say Wawalker, but..."

- (post 38) : " If I had to bet on this right now, I would rather put my money on your experience than on the exactness of my recollections..."

- I dont see anything resembling those precautions in Dr. RX. or in your posts.

5) Finally, going back on all that I've written in this thread, I dare you to find anything that is false.

(a) I've stated that the acronym TDC is often understood in the automotive world (and not by ignorants) as generaly meaning the upper most position of the piston on the compression stroke and the fact that two of the most read automotive manuals, Haynes and Chilton's, (and there are others) side with me on this cannot be just brushed aside in an authoritarian and know-it-all gesture.

I did'nt say that it was incorrect to use it as meaning the uppermost position of the piston on any stroke, I just said there seemed to be two (who's reading the Gospel here...) conflicting and equally legetimate usages of the acronym TDC. Anyone pretending that TDC has only one meaning or one usage in the automotive world is ill informed and it's a good thing that I could set that straight.

( I've also stated that there are two independant sources that advise - to make things easier - to put the crankpulley timing mark at zero while checking (that only in Haynes) that #1 piston is at TDC (compression stroke) before removing the cam covers, advising that, while unecessary (like 777 made clear), it could make aligning the three sets of marks easier.

© The rest of it was only my trying to explain to Dr. RX. that our disagreement was not the consequence of conflicting conceptions on how the internal of an engine works, but just a misunderstanding based on words. This was apparently unsuccessful.

 

In conclusion,

(1)I dont see how anyone reading my posts in this thread could come out of it with false informations that could jeopardize their attempt at replacing their cam belt.

"I feel for the people who are relying on this fourm for correct info to repair their cars" is an abusive and insulting statement.

(2) I dislike very much the way Dr. Rx and now yourself are talking down to people on this forum.

[frag]

http://www.ultimatesubaru.org/photos/data/500/3702_2L_005-thumb.jpg

 

I dont know if I've got a problem on my end but all I get is a thumbnail.

[Dr. RX]

I dont know if I've got a problem on my end but all I get is a thumbnail.

I don't mean to talk down to you, but you only see a thumbnail because that is what it is. As for the rest of what you said, maybe the problem is that you are not willing to listen what others are telling you. Quoting from Haynes or Chilton's means nothing in my book, on any given day, I can find errors with either book.

 

These are my last words in the post, flame away.

[WAWalker]

Frag,

 

My replies had nothing to do with the TDC argument. It is not relavant to the question asked. Question was.

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

Question was awnsered.

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.

When replaceing a SUBARU timing belt TDC is irrelavant.

 

There is a reason why Subaru did what they did. Any Subaru repair info that you read, be it a FSM or info published in The End Wrench, will tell you, and provide you with pictures of which timing marks are meant to be used when replacing the timing belt. They are not the marks that line up when #1 is at top dead center. That is FACT and not my opinion. Other sources, i.e. Haynse and Chilton, and any others you have read that say to set #1 at TDC when replacing a SUBARU timing belt are wrong simply because that is not the way that SUBARU desinged it to be done. SUBARU engines were designed so that when the proper timing marks are lined up when replacing the timing belt all pistons will be midway down the cylinder. So............ if the cams need to be rotated there will be no chance of valve to pistion contact.

It would seem that you were questioning where the pistions are when the proper marks are alignend. There are two sets of marks so I can understand the missunderstanding there. But for a first timers doing there first timing belt it could be critical for them to have the proper marks aligned to avoid unwanted and expensive engine damage. This is why I do not post my opinion in responses to technical questions. If someone is asking a question about replacing there timing belt, they need facts not opinions.

 

Finally, I did not mean to talk down to anyone, and if it seemed that way I apologize.