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Old 03-24-2015, 11:30 AM   #1
zkhennings
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Join Date: Jun 2014
Chapter/Region: NESIC
Location: Boston Massachusetts
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2002 Impreza WRX
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Default FAQ: Valves during Tbelt change

This is a FAQ with pictures about valve position during timing belt change and how to easily 100% avoid bent valves.

I got sick of the stupid FSM and internet giving the direction of rotation for the different cams with no explanation . Both driver's side cams have one set of valves fully open when the timing marks are lined up. Well no matter how you rotate the cams, if one cam has open valves already while the other cam is getting rotated 360* then the fully open valves will hit the other valves at some point in the rotation.

When the timing marks are lined up the pistons are all at a halfway-ish point in the bores so no worry about hitting those against valves.

When the timing belt is lined up for install:

First understand that every cylinder is ALWAYS in a different stroke in the four stroke cycle with a four cylinder engine. At any point in time, one cylinder must be in the intake stroke with intake valves open, one cylinder must be in the compression stroke with both valves closed, one cylinder must be in the combustion stroke with both valves closed, and one cylinder must be in the exhaust stroke with the exhaust valves open. When valves are open, the cam is pushing them open and fighting against the valve springs which close the valves. Open valves means the cam is at risk of being spun by the valve springs pushing on the lobes. If the valves are closed then the cam can rotate freely a number of degrees without any valve springs pushing on it.

The passenger side valves are both in the closed position because one cylinder is in its compression stroke and the other is in the combustion stroke. This means you can move both cams by hand freely a decent amount until you feel resistance. This resistance is the cam starting to open the valve. So no worries about spinning the passenger side cams too much during alignment for the new belt.

The drivers side cams are trickier because both cams had to be in a lift phase since the cylinders needed to be in the intake and exhaust strokes yet Subaru needed the cams to stay in place.

Therefore both cams have one set of valves in the full lift position, because this allows them to stay in place when the timing belt is removed. Because they are positioned so precariously, if you bump the cams they will try to spin to an unloaded position, and if they spin the wrong way then the valves will crash.

The driver's side cams have the firewall side cylinder in the exhaust phase while the radiator side cylinder is in the intake phase.

This was the best way Subaru could set the timing marks so there is no risk of piston contact while all 4 cams simultaneously will stay in place, at the compromise of needing to take care with the drivers side cams since they are preloaded to spin on their own if you mess up and bump/spin them.


Here are some pictures with the Valve Covers off so the cam and valve positions can be observed.


PASSENGER SIDE: ALL VALVES CLOSED







DRIVERS SIDE CAMS NEXT TWO PICS:










As you can see I put a wedge in between the two gears to keep them from spinning.

The cylinder closest to the radiator has the intake valves in the full open position while the cylinder closest to the firewall has the exhaust valves in the full open position. Hopefully this can be seen easily from the pictures.

Now please read this carefully because I am in no way responsible if you can't read and mess this up.

HERE IS WHAT I RECOMMEND

If you want to unload those driver's side cams to make sure they don't crash by accident, turn the top pulley counter clockwise. This will put it into a no lift phase.

Turn the bottom pulley clockwise and then it will be in the no lift phase.


This would be the ideal direction for the cams to spin on their own as well. If your cams spin and the top one went clockwise, the valves may hit. If this happens I would rotate the cam back counter clockwise otherwise you will do more damage.

For the exhaust (bottom) cam, if it spins counter clockwise then there is a chance it will crash the valves. I would spin it clockwise to put it back or you will do more damage.

If you do not do what I do with a wedge, I would probably unload the cams first to prevent them from spinning on their own.

Then, when you want to install the timing belt, IF you have put your cams in the no lift phase and now need to line them back up:

THESE DIRECTIONS APPLY IF AND ONLY IF YOU HAVE THE CAMS NICE AND UNLOADED

Turn the top pulley clockwise to line it up, and the bottom pulley counter clockwise. There will be no risk of hitting any valves together unless you turn them too far and they jump past the position you wanted. Even then however, the valve springs would be fighting each other and the valves would be half open, I am not sure if this will make them crash or not, though I would guess yes.

USE A BREAKER BAR, NOT A RATCHET TO SPIN THE PULLEY, otherwise it can jump on it's own if you spun it a little past where it is lined up.

It is pretty simple stuff and it is nice to know what is actually happening. I spent quite a while looking for this information and even posted about it in a few places and got 0 replies so I figured I would just figure it out myself and share .

Please don't attempt any of this unless you understand what I am showing, follow the FSM. I tried to give ample warning, LET ME KNOW IF I SHOULD ADD MORE INFORMATION TO IMPROVE THE HELPFULNESS OF THIS, I am not responsible for you messing up your car.

This can also be useful if your timing belt breaks and you want to do a leakdown test or w.e. to check if the valves are bent, as you can put all the cams to where all the valves are closed and go through and test one by one.

Zach
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Last edited by zkhennings; 03-24-2015 at 03:29 PM.
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Old 04-13-2016, 06:27 PM   #2
SubLGT
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Join Date: Apr 2009
Location: Idaho
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2005 Legacy GT

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Thanks for the write up. Good info to know.
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Old 05-12-2016, 06:08 PM   #3
kevinoes327
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Location: Shirley, NY
Vehicle:
2002 WRX Wagon
Midnight Black Pearl

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Hey, I need help in this department, all the things I read or watched said to turn the upper clockwise and lower counter, I did and I hit interference I think because I cannot turn them more. They already jumped at one part of the disassembly which the video I was watching said would happen anyway. So turned top cam gear right and bottom left and both wont move anymore I thought they'd be free since crank id at top dead center
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