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Crankshaft Timing Belt Sprockets: The Truth


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73 replies to this topic

#26 WJM

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Posted 29 December 2006 - 11:43 AM

Yeah, I've seen alot of pullys and it is hit or miss as to if they have holes...and its hit or miss as to if the sprockets had pins. *shrug*

According to the parts catalog...electronic ones....

The front one is avalible with or with out the pin. The pin'd ones come on all turbo EA82s.

The rear one is the same partnumber from 85 to 94.

As for XT6: its different from the EA82.

This information comes from the Proquest EPC, and the SUBURU Fast-2 EPC.

I'm sure the paper book I have shows the same.

#27 hooziewhatsit

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Posted 29 December 2006 - 01:35 PM

hmm, I just looked in my '85 FSM, and it says the 'No. 2' sprocket, goes on first, closest to the engine. It also does not say how to specify the difference between the two.

Also, I have the two sprockets off a pre-87 MPFI turbo block. One sprocket has the dowel pin, and the other has the ID Dot. The hole the dowel pin is in, is slightly smaller than all the other holes.

Other than that, the two sprockets I have are identical to the ones Will posted earlier, and put in place with the dowel pin to the outside, looks like the 'wrong' picture in the first post.

The scan of the 89 FSM available online, on page 160, just says the sprocket with the dowel pin is the outermost one :rolleyes:

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#28 Marck

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Posted 29 December 2006 - 04:18 PM

I found something interesting to more confuse everyone. In my 87 FSM, look at figure 6 on page 12 in section 2-3. The picture shows that the inner sprocket (#2) has the dot, but the dot is at 12:00, instead of 5:30. The outer sprocket lacks this dot.

I found that according to WJM's first post, my sprockets are on backwards and lacks the dowel pin. I took both sprockets off and compared them. They look very different. I'll see if I can borrow a digi cam tonight to take a pic. I wonder if one of my sprockets is aftermarket? All I can say for now is that the one with the special dot hugs the crankshaft better when it is placed closest to the engine. I couldn't get a hold of a good quality camera, but here is some blurry pixs using a cheap old camera. First picture is of the front of the two sprockets and the second picture is of their back sides. The sprocket on the left of both pictures has the mark at 5:30 on the front side. The one on the right has a large notch at 6:00pm on the front side.

Attached Files



#29 WJM

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Posted 29 December 2006 - 07:35 PM

Holy geez....


Atleast by the time the thread ends, we all will know what goes where on any specific car right down to VIN specific. :clap:

#30 GeneralDisorder

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Posted 29 December 2006 - 08:27 PM

Pehaps it would be better to count the ribs between the cam and the pulley to determine which is which since there are inconsistencies with regard to dowel pins and markings?

Would that result in a single method with which to *always* determine the correctitude of the installation?

If it would, then MARK those suckers with a punch or a sharpie or something....

At the end of the day all we are looking for is a method to make sure they are in the right place - if counting the ribs proves to always work, then I would rather do that, and put out that information for future reference rather than muck with all 9 years of FSM's....

GD

#31 WJM

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Posted 30 December 2006 - 07:18 AM

Marck: Those are the same ones that were on my 88...which had an 87 engine in it.

I put the one with the 530 dot on the front, and the largenotched one on the back as the teeth positioning matched the other 'rear' ones that I had that lacked a dot at 530.

It ran better. *shrug*

#32 Marck

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Posted 30 December 2006 - 03:12 PM

Marck: Those are the same ones that were on my 88...which had an 87 engine in it.

I put the one with the 530 dot on the front, and the largenotched one on the back as the teeth positioning matched the other 'rear' ones that I had that lacked a dot at 530.

It ran better. *shrug*


I am going to switch them (put the "large notched one" closer to the engine) and then see if it runs better or worse.

#33 Marck

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Posted 30 December 2006 - 06:02 PM

After changing the sprockets so that the one with the dot at 5:30 is the farthest one from the engine, I found that the idle has dropped and the ignition timing had retarded by a couple of degrees. I increased the idle and set the timing to 20 degrees BTDC, which is what I set them to a month ago. At idle, I didn't notice the car running any smoother. When I drove it, it felt like it had a little more power. I don't like to rely on feeling alone because sometimes my brain can play tricks on me. Has anybody else swapped them and noticed a difference?

#34 joostvdw

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Posted 01 January 2007 - 11:49 AM

let me get this straight,
I assume the cam sprockets are the same diameter and have the same amount of teeth, they both have to turn at the same speed ratio to the crankshaft, so if the teeth on the camshafts are equal, the teeth on the crankshaft have to be equal too, then you can vary the length of the timing belts all you want, if the teeth ratio (crankshaft/camshaft) stays the same

so the sprockets on the crank should be the same.... and they're not.

the difference has to be the timing of the 2 camshafts, the cilinders fire after each other so the timing of the cams has to differ the time the rotor takes to reach the other contact point in the cap. that's not much, but to have your motor running perfect, you have to consider this in the camshaft timing,

it is very likely that subaru made the cams and sprockets the same and decided that they would create this timing difference by placing the keyway in the sprockets at a different place,

so question remains that we don't know which sprockets goes first, but atleast we know why they differ.

it would be possible for someone with the proper tools and a open block to measure and find out which sprockets goes where, but it would be a tedious job.

#35 WJM

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Posted 01 January 2007 - 11:57 AM

No, they are offset in their timing for CAMSHAFT timing...

I noticed a difference in how my RX ran before vs after.

If you alter the cam timing from the crankshaft...you alter the valve open/close vs stock. Obviously, the engineers had the cam timing-valveopen/close at exact times for optimal performance. They spent thousands of man hours working with this.

Even MORE important...the 2-4 cam drives the disty. The disty is the Crank Angle Sensor.

With the CAS not reading the ACTUAL crank position...the ignition timing will also be off.

*shrug*


The old guys at TechLine said the 530 dot is the front one. That seems to be most consistant.

I will order new ones and see how they come.

#36 joostvdw

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Posted 01 January 2007 - 12:17 PM

No, they are offset in their timing for CAMSHAFT timing...

I noticed a difference in how my RX ran before vs after.

If you alter the cam timing from the crankshaft...you alter the valve open/close vs stock. Obviously, the engineers had the cam timing-valveopen/close at exact times for optimal performance. They spent thousands of man hours working with this.

Even MORE important...the 2-4 cam drives the disty. The disty is the Crank Angle Sensor.

With the CAS not reading the ACTUAL crank position...the ignition timing will also be off.

*shrug*


The old guys at TechLine said the 530 dot is the front one. That seems to be most consistant.

I will order new ones and see how they come.


that's what I said right? the cams have to be timed different and they solved that by changing the cracksprockets. at least that's what I meant :-\

anyway, looking forward to your findings, I've been tracing a stumble for ages now and this could be it...

#37 mikeshoup

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Posted 01 January 2007 - 12:55 PM

I don't know if you're missing the point, or if I'm misinterpreting what you've said. The cams aren't timed differently. They are exactly 180 degrees apart. The offset in the cam sprockets allows that, since the length from one camshaft to the crank sprocket is different than the length from the other camshaft to the crank sprocket.

The valves still open at the same time on each side of the engine.

#38 hooziewhatsit

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Posted 01 January 2007 - 03:11 PM

I don't know if you're missing the point, or if I'm misinterpreting what you've said. The cams aren't timed differently. They are exactly 180 degrees apart. The offset in the cam sprockets allows that, since the length from one camshaft to the crank sprocket is different than the length from the other camshaft to the crank sprocket.

The valves still open at the same time on each side of the engine.


That makes sense. However, if they sprockets are on backwards, the cams would still be 180* from each other, but in relation to the crank, they would be slightly mis-aligned. Then, the disty would also be off from the true crank angle. That could cause issues I would think.

I think the only real way to settle this would be to count ribs on the belts, and see which sprocket orientation matches what the FSM says, just like GD mentioned a few posts ago.

I have an 87 SPFI engine buried in my shed I could try this on, unless someone else beats me to it.

-Dave

#39 joostvdw

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Posted 01 January 2007 - 03:35 PM

I don't know if you're missing the point, or if I'm misinterpreting what you've said. The cams aren't timed differently. They are exactly 180 degrees apart. The offset in the cam sprockets allows that, since the length from one camshaft to the crank sprocket is different than the length from the other camshaft to the crank sprocket.

The valves still open at the same time on each side of the engine.


My point is that the cams have to turn the same speed in correlation to the crankshaft, so it's all about speed and ratio's, length of the belts don't have anyhting to do with that. As long as the ribs on the pully's are the same (ratio's on both cams) they will spin the same speed.

but apparently the sprockets on the crank are slightly misaligned, so that has to have a reason, the only thing I could think of was that the cilinders don't fire batch but 1 at a time, so 1 cam has to be timed slightly different from the other to make sure the valves open at exactly the correct moment.

sorry if I don't make sense, it's still kinda foggy in my head :drunk: and english is of course not my native language.

#40 ccrinc

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Posted 01 January 2007 - 04:07 PM

Okay guys: the ONLY thing you have to remember is
The one with the smooth back goes in the front,
the one with the grooved back goes against the block. (The groove is clearance for the front main seal.)
It's that simple. No dots, no numbers, no pins.
They're all identical for all EA82 engines.

And yes, the engine won't run well if they're reversed.

Smooth = front
Grooved = rear
Got it? Good! :clap:

Emily
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#41 Subi81

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Posted 01 January 2007 - 04:18 PM

This might explain my funky idle and a distributor that just won't line up (car runs ok though). Thanks WJM for the research and attention to detail!!!

#42 TomRhere

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Posted 01 January 2007 - 04:32 PM

What Emily said!!!!

If you really look at the routing of the 2 belts you'll see why that offset is there. Some one made a boo boo in the placement of one of the items that the 2-4 belt drives. Well now, can't change cam placement that easily. Hmmm, make different size cam sprockets, lots of re-tooling work there, equal lots of money lost, same with making different belts.. Much easier and cheaper to just change the position of the key way broach in the crank sprocket, hence the offset.

#43 daeron

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Posted 01 January 2007 - 10:47 PM

how many teeth do the crankshaft sprockets have???

Because it would seem to me, if the sprockets had say, 23 teeth (or any other odd number) than NATURALLY, when you looked at the two on the nose of the crankshaft, one sprocket would have the teeth where the other sprocket has grooves, and vice versa.. take two five pointed stars, and rotate one 180 degrees from the other. One will have a point at the top, and two points at the bottom.. the other will have two points at the top (with a V at the 12 o clock position) and one point at the bottom (at the 6 o clock position.)

A question that others have asked in this thread and I will re state plainly.. if both crank sprockets have X teeth, and both Cam sprockets have Y teeth, then the relative length of each timing belt should make NO difference in the cam timing.. That makes sense to me, and to others, but we are trying to see if what makes sense to us is misconceived somehow. Can the timing belt length alter the relative "gear ratio" from one side to the other? that seems wrong, to me...

sometimes thinking about timing belts for too long on end gives me a migraine. im gonna stop reading this thread now :)

thanks for the info though, guys. Nothin like Data!! :banana:

#44 WAWalker

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Posted 02 January 2007 - 10:13 AM

Okay guys: the ONLY thing you have to remember is
The one with the smooth back goes in the front,
the one with the grooved back goes against the block. (The groove is clearance for the front main seal.)
It's that simple. No dots, no numbers, no pins.
They're all identical for all EA82 engines.

And yes, the engine won't run well if they're reversed.

Smooth = front
Grooved = rear
Got it? Good! :clap:

Emily
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Exactly!

And there is a picture to prove it.
http://www.ultimates...18&d=1167452940
Kind of fussy, but the difference is obvious.

#45 grossgary

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Posted 02 January 2007 - 12:13 PM

thanks emily for putting in words what i knew to be true. i can not understand how this is such an issue? i've never had a problem installing these things, they've always been obvious i just can't recall from memory what makes it obvious.

#46 GeneralDisorder

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Posted 02 January 2007 - 06:21 PM

A question that others have asked in this thread and I will re state plainly.. if both crank sprockets have X teeth, and both Cam sprockets have Y teeth, then the relative length of each timing belt should make NO difference in the cam timing.. That makes sense to me, and to others, but we are trying to see if what makes sense to us is misconceived somehow. Can the timing belt length alter the relative "gear ratio" from one side to the other? that seems wrong, to me...


You have failed to insert the variable for the size of the belt ribbing. By doing that, you have assumed it is of size 0. In which case you would be looking at a smooth belt - like a serpentine, or a v-groove. As it is though, the belt ribbing is not of 0 size. Thus you cannot pick an arbitrary rotation of the sprockets. Each sprocket MUST turn about 5(?) degrees before it will again line up with the belt ribbing. The situation is that the distance between the cam's, and the drive sprockets is different - for the eqaution to be satisfied for both sides, a different number of belt ribs must be used.

(NBR * BRS) = BL

Where:

NBR = Number of Belt Ribs
BRS = Belt Rib size
BL = Belt Length (between the sprockets)

By assuming the rib size is 0 (smooth belt), you invalidate this equation.

Furthermore, since the rib size, and the belt length are determined by the design of the belt, and the routing on the engine block respectively, those two variables ARE constant. This leaves only the number of ribs that can change for the two sides. And it does, and it is NOT a whole number. In the case of one side it's a whole number (purely by chance I'm sure), and on the other it's a whole number of ribs PLUS another half of one. That extra 1/2 rib requires that the two sprockets not be aligned with one another.

Make sense now?

Basically, BDG, and anyone else that thinks they *should* line up is not working with the correct mathematics. You cannot use whole number math for this. Fractional belt ribs ARE possible, and in the case of the EA82 they are being used. Thus the difference. It is possible to design it in a way that all the rib counts are whole numbers, but it would require a change to the tensioner location (mathematically, NBR and NBS held constant, and the BL changed to compensate). For whatever reason, they did not do that. Conversely - from an engineering standpoint - there is no reason to do it either. Except to help the grease monkeys not screw it up - but that's all-too-often not often a concern for the engineers.

GD

#47 pappy52

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Posted 02 January 2007 - 06:57 PM

You guys made me go look at my old parts. The set I have from an origonal motor shows, as Emily said, that the one with the groove was on the back. Also these had been on so long that there is a definate "shadow", or wear marks on the back of the non-grooved one where the sides of the teeth lived and worked 'lo these many years. The wear marks can also be seen in the pictures htat have been posted prior.

Just what I saw.

Jon :)

#48 daeron

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Posted 03 January 2007 - 12:55 AM

You have failed to insert the variable for the size of the belt ribbing. By doing that, you have assumed it is of size 0. In which case you would be looking at a smooth belt - like a serpentine, or a v-groove. As it is though, the belt ribbing is not of 0 size. Thus you cannot pick an arbitrary rotation of the sprockets. Each sprocket MUST turn about 5(?) degrees before it will again line up with the belt ribbing. The situation is that the distance between the cam's, and the drive sprockets is different - for the eqaution to be satisfied for both sides, a different number of belt ribs must be used.

(NBR * BRS) = BL

Where:

NBR = Number of Belt Ribs
BRS = Belt Rib size
BL = Belt Length (between the sprockets)

By assuming the rib size is 0 (smooth belt), you invalidate this equation.

Furthermore, since the rib size, and the belt length are determined by the design of the belt, and the routing on the engine block respectively, those two variables ARE constant. This leaves only the number of ribs that can change for the two sides. And it does, and it is NOT a whole number. In the case of one side it's a whole number (purely by chance I'm sure), and on the other it's a whole number of ribs PLUS another half of one. That extra 1/2 rib requires that the two sprockets not be aligned with one another.

Make sense now?

GD


Not yet. I haven't given up on reading it through and trying to grok.. but in my mind a timing belt is a chain. All my conception of chains and gearing in a chain-based (not straight gear) system goes back to bicycles.

I understand totally why the sprockets do not line up.. but what I am trying to grasp is how the length of the belt can alter the gear ratio of a system.. each cam sprocket has X teeth, and each crank sprocket has Y teeth. Correct? (I presume so for the purposes of this post)

So, it shouldnt matter if one side has to go through sixteen tensioners and idler pulleys, both pairs of sprockets (cam/crank = "pair) have the same gear ratio. As for relative POSITIONING, i never had a problem with that, that bit makes perfect sense and did as soon as I saw the front of my first multiple cam engine.

Obviously, if the sprockets are each supposed to have a given attitude relative to the crankshaft, and they are different, swapping them would cause improper cam timing. (ignoring the camshaft driven disty) Essentially the point of this thread is to ensure that everyone puts the right one in the right place. Rib counting only comes into play to make sure that your 1/2 rib is on the correct side, then.. right?

May as well mention that I am perfectly clear on the fact that you are counting ribs only on the "tense" side of the belt... i don't know better terminology for it, though.

I no longer even know if I am confused. I never THOUGHT that I was..

On a side note, im working at a BBQ place now, and all this talking about ribs makes me feel like im at work.

I may, or may not, have covered the point i initially wanted to cover when i started typing this post. I don't even really remember my point on my last post. I am gonna go eat, and unwind from work. Mods, if this post was gibberish, feel free to delete it, just send me a copy so I can try to re-gain the train of thought that has now so thoroughly derailed. :lol:

#49 GeneralDisorder

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Posted 03 January 2007 - 01:12 AM

I understand totally why the sprockets do not line up.. but what I am trying to grasp is how the length of the belt can alter the gear ratio of a system.. each cam sprocket has X teeth, and each crank sprocket has Y teeth. Correct? (I presume so for the purposes of this post)

So, it shouldnt matter if one side has to go through sixteen tensioners and idler pulleys, both pairs of sprockets (cam/crank = "pair) have the same gear ratio. As for relative POSITIONING, i never had a problem with that, that bit makes perfect sense and did as soon as I saw the front of my first multiple cam engine.


Oh - if you are refering to the gear ratio then you are correct. The ratio does not change. The cams rotate at exactly 1/2 crank speed.

Obviously, if the sprockets are each supposed to have a given attitude relative to the crankshaft, and they are different, swapping them would cause improper cam timing. (ignoring the camshaft driven disty) Essentially the point of this thread is to ensure that everyone puts the right one in the right place. Rib counting only comes into play to make sure that your 1/2 rib is on the correct side, then.. right?


Correct. But I wasn't advocating counting the ribs - I was merely describing mathematically *why* the two crank sprockets are not aligned.

GD

#50 daeron

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Posted 03 January 2007 - 01:33 AM

okay, okay, okay.

Like I said, I didn't THINK I was confused... :-p

That's why I just went ahead and posted it, even though I totally lost track of what I was trying to say. Sorry, I kept getting interrupted. Good to know I wasn't as far off-base as you thought. Thanks for the explanation, though.. I knew all of it, but i didn't really "understand" in that kind of detail.

I had a math teacher once who was fond of the analogy of a toilet.. everyone knows how to flush a toilet, but do you know how/why the toilet flushes? I now know how this toilet flushes. The awesome part about that metaphor is the odd context you find yourself using that phrase in sometimes.




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