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I did the timing set on my 89 4.0 and noticed my timing marks were off 180 degrees (crank timing mark was at 12 o clock and so was camshaft timing mark) I didn't have enough time to straighten things out so i just put it back the way it was, I'm planning to set everything back to the way it should be and indexing the distributor since it's been bugging me, this is on my daily driver comanche, do any of you see any negative effects this may have on the engine meanwhile? do you guys recommend I do anything else while I'm in there straightening things out?

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This is funny........or I'm missing something????????

 

 

4 strokes will be 180 degrees off every other revolution.

 

One stroke is a compression stroke and one is an exhaust stroke.

 

Two turns of the crank to every one turn of the cam.

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This is funny........or I'm missing something????????

 

 

4 strokes will be 180 degrees off every other revolution.

 

One stroke is a compression stroke and one is an exhaust stroke.

 

Two turns of the crank to every one turn of the cam.

X2

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If cam was out 180 when you did timing,, you were at TDC cylinder 6, not TDC 1.

 

And no, it won't run if off between crank and cam. The flywheel windows for CPS will negate that possibility...unless you run plug wires to opposing cylinders as well as timing the distributor to match TDC 6.

 

 

You were simply off when you set it.

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I'm with Jeep Driver and Hornbrod on this. JeepcoMJ is overthinking this, and if you change the way it is, you will have issues.

 

The four strokes of a four-stroke engine are suck, squeeze, bang, and blow. During suck and bang, the piston moves downward. During squeeze and blow, it's moving upwards. This is two complete revolutions of the crank. The cam only goes through one complete revolution, though. All four strokes require different valve positions. The cam timing marks line up once for every two times the crank timing mark does.

When lining up timing, you're looking for tdc (top dead centre) on the compression stroke, i.e. between the squeeze and bang strokes. You're sitting at tdc on the exhaust stroke, between the blow and suck strokes. If you rotate the crank through another complete revolution (360°), I bet you'll find that suddenly the cam marks line up nicely.

When looking for tdc, it's recommended to pull #1 spark plug and stick your thumb over the hole while turning over the engine as it'll be pushed off under compression. I prefer pulling off the distributor cap and making sure the rotor is pointed at the #1 wire. As Hornbrod pointed out, since the dizzy is driven off the camshaft (and also does one rotation for every two of the crank, because you only need the spark every other time the piston's at tdc), the distributor will also be 180° out when your cam timing marks are out by 180°.

 

So in brief, the four strokes of your engine each correspond to half a rotation of the crankshaft. 4 x 1/2rev = 2 revolutions. Each stroke also corresponds with a quarter of a cam rotation. 4 x 1/4rev = 1 revolution. So two crank revolutions per cam revolution. If you turn the engine over with the timing cover off but timing chain on, you'll notice the small crank gear spins twice as fast as the large cam gear. #1 tdc occurs after the compression and exhaust strokes, and timing is set after the compression stroke; you're sitting at the exhaust stroke, so the cam timing is 180° out when compared to your marks. There is nothing wrong with your timing as long as you lined everything back up 180° "out", the way it you found it. If you find #1 tdc compression and look at the timing marks, they will line up.

 

If the engine runs normally, there's nothing wrong with your timing. Even being one tooth out would be noticeable. Pulling it back apart to fix something that isn't broke is wasted effort.

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I am under thinking it. If his timing looked 180 off when it was done, the cam was timed at top dead center cylinder 6.

 

The problem will come if the cam is set 180 degrees out from tdc1 and distributor is set at TDC 1.

 

So, very simply, you cannot run the engine 180 degrees out' you would have to reverse the plug order.

 

 

But his engine is NOT 180 degrees out. He simply rolled the crank to put the cam 180 degrees off. All this means is that the engine was not timed to cylinder one...but the entire engine was 180 degrees off at the time, and that puts it at correct timing as cylinder 6 compression is 180 degrees off from cylinder 1

 

 

He has no need to take it apart if it's running.

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#1 and #6 are companion cylinders meaning they are both at TDC or BDC at the same time just on different stroke. If you set the cam 180 out you just need to set the distributor 180 out to match. If you do this then turn the crank one revolution then everything is in the correct location again. No need to do the job over again everything is good.

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You don't have a problem. Simply rotate the crank 360°, and you'll see that you now have the cam at 6 and the crank at 12 again, the way we all agree that it should be. The crank turns past 12 each time the cam hits 12 and 6. Nothing's out 180°, nothing's amiss. And if you were to turn the crank another 360°, it would again line up crank and cam both at 12 o'clock. Relax, have a brew, and leave that distributor alone.

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  • 5 years later...

Sorry to bring back an old thread but I just changed out my timing chain and here’s what happened. Ran perfectly before the job, but I needed to change the crank seal so I figured to change the 200k old chain while I’m in there. 
 

Pan was off since I also did a RMS and knew for sure #1 was at TDC, crank and cam sprocket tick marks lined up perfectly but my distributor was 180* out. It wasn’t pointing at 1. What gives? Was I on exhaust stroke of TDC? Logically it doesn’t make sense to me that 2 crank rotations equate to one cam rotation.

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53 minutes ago, ARareBreed said:

Logically it doesn’t make sense to me that 2 crank rotations equate to one cam rotation.

That is, in fact, how it works. You'll notice that the cam sprocket is twice the size of the crank sprocket and has twice as many teeth. When connected together by the timing chain, the cam will rotate at half the speed of the crank.

 

When two sprockets are connected by a chain, the relative speeds of each sprocket can be found with the following formula:

 

S1 x T1 = S2 x T2

 

S being the rotational speed, and T being the tooth count of each sprocket. Rearrange some terms around and you can get:

 

S1 / S2 = T2 / T1

 

On a Jeep 4.0, the crankshaft sprocket has 18 teeth. We'll call that sprocket 1. The camshaft sprocket has 36. That'll be sprocket 2. Plugging in 18 for T1 and 36 for T2,

 

S1 / S2 = 36 / 18

or..

S1 / S2 = 2

or..

S1 = 2 x S2

 

In other words, sprocket 1 (crank) will rotate at twice the speed of sprocket 2 (cam). Since the sprockets are tightly connected to each other and cannot "slip", that means that the crankshaft will rotate twice for every rotation of the camshaft.

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1 hour ago, ARareBreed said:

 

Pan was off since I also did a RMS and knew for sure #1 was at TDC, crank and cam sprocket tick marks lined up perfectly but my distributor was 180* out. It wasn’t pointing at 1. What gives? Was I on exhaust stroke of TDC? Logically it doesn’t make sense to me that 2 crank rotations equate to one cam rotation.

 

Why doesn't that make sense? That is exactly how a 4-stroke engine works. They can't operate any other way. The crankshaft goes around twice for every one revolution of the cam shaft. The first time cylinder number 1 hits TDC it's on the compression stroke -- both valves closed. That's when it fires. Then the piston goes down on the power stroke. When the piston comes up again, the exhaust valve opens and the piston pushes the combustion residue out. When the pistol reaches TDC on the exhaust stroke, the exhaust valve closes and the intake valve opens. Then as the piston goes down it's on the intake stroke and it pulls in the fresh fuel-air mix for the next firing cycle. Then the intake valve closes and as the piston moves up it compresses the fresh fuel-air charge.

 

Rinse and repeat.

 

I'd say you had the #1 piston on TDC for the exhaust stroke, not the compression stroke.

 

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Alright, I understand now.

 

I still feel like the distributor is clocked 180* out. Since there are 2 turns of the cam for every 1 turn of the crank, if I rotate the crank 1 turn to be on the compression stroke.. the distributor would now be pointing at 1(that's what we want to achieve) but the tick marks would not be lined up between the two sprockets (the cam sprocket would now be 180* off).

 

How has my engine been running this way!

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3 hours ago, ARareBreed said:

Alright, I understand now.

 

I still feel like the distributor is clocked 180* out. Since there are 2 turns of the cam for every 1 turn of the crank, if I rotate the crank 1 turn to be on the compression stroke.. the distributor would now be pointing at 1(that's what we want to achieve) but the tick marks would not be lined up between the two sprockets (the cam sprocket would now be 180* off).

 

How has my engine been running this way!

NO! There is one turn of the CAM for two turns of the crank. 

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The timing marks on the timing gears really are only there to help you put it together quickly and easily.

 

There's nothing magical about the marks.  They are only there to make it easy for the installer.  You can ignore the marks altogether as long as you make sure the cam and crank are in the right position and everything bolts together.  Of course, some engines have specific alignment pins, keyways, or bolt patterns that force you to only put it on one way and that forces the marks to line up.  But it's not a huge deal as long as the cam and crank are in time with each other.

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