Unloading cams safely for timing belt replacement.

[mrfixiter]

I have a 2.5l dohc 1997 Legacy GT wagon. Basically I'd like to know the proper procedure for unloading the driver side cams so the valves don't get damaged. There seems to be conflicting advice out there in the land of cyberspace. The service manual shows that the intake cam should only be rotated clockwise and the exhaust cam should only be rotated counterclockwise. But it seems to me that this only applies while you are trying to install the timing belt.

 

In this thread, http://legacygt.com/forums/showthread.php/2-5-dohc-timing-belt-help-120380.html?p=2616725 from another forum, I found this advice:

"If you haven't taken the t-belt off yet, with the crank gear lined up the left bank (right side as you look at front of engine) cams will be loaded (valves opened), the right bank will not be. You need to be careful because when you take the belt off, and the gears unload in the wrong direction you can bend the valves. You want to turn the top left gear counterclockwise, and the bottom left gear clockwise, to unload them. Then lock the cam gears to remove the bolts."

 

Then is this article, http://legacygt.com/forums/showthread.php/replacing-cam-crank-seals-w-full-timing-belt-replacement-126988.html

 

The procedure is given as follows, "The driver side cams are loaded, which means the cams are pushing against the springs to open the valves inside the cylinders. If they cams are turned the wrong way they cam close [*doesn't he mean open at the same time?*] both the intake and exhaust at the same time which will cause internal valve damage. The intake cam on the driver’s side can only be turned in the clockwise direction. The exhaust cam can only be turned in the counter clockwise direction."

 

Can both of these explanations be correct? From my calculations, with the crank positioned with the timing marks lined up (the timing belt mark not the

TDC arrow mark) Cylinder #2 is in the middle of the intake stroke and the intake valves should be open. Cylinder #4 is in the middle of the exhaust stroke and the exhaust valves should be open. If the exhaust cam is held in place and the intake cam is rotated clockwise, the next set of intake valves to be opened would be in cylinder #4 which already has its exhaust valves open. This doesn't sound like a good thing. If however the intake cam is rotated counterclockwise, neither cylinder #2 or #4 will have a conflict.

 

Now if the intake cam is locked instead and the exhaust cam is allowed to rotate counterclockwise, then the #2 cylinder which already has the intake valves open, will now have its exhaust valves opened too and again this is bad news.

 

Your comments to help clear up this mess are most welcome.

[idosubaru]

it's really simple - line up all the timing marks and remove belt that way. no problems.

 

the cams that are prone to "snap" with the belt off do not snap in such a way to cause any problems (IF you remove the belt with all the sprocket timing marks lined up). so if it does snap, just put the thing back.

 

it's really simple. not necessarily easy, but it is simple.

[johnceggleston]

if the crank sprocket is in the correct position, on the timing mark, the pistons are at the half way position and there is no possibility of pistons bending valves.

 

turning cams by hand, you would have to be very forceful to do some damage. valves bending valves is possible, i guess, but not likely, i don't think. (please correct me if i'm wrong.) and since the nature of the cams is to "close" the valves, the tendency is to "avoid" damage.

 

having said this, is it possible that you could force the cams around and cause a problem, probably, but if you set it up as it should be before you remove the belt and do not mess around with the cams too much while hanging the new one you should be ok. lots of first timers have done this successfully. just make sure you are using the correct timing marks and you should be ok.

 

there are links below in my signature to some great pictures and articles for a timing belt change. read up and have fun. if you have more questions , ask.

Edited August 17, 2010 by johnceggleston

[GeneralDisorder]

Yeah - line it up and take the belt off. If you don't touch the cams they should sit right where you put them. If one does snap to one side you can just move it back.

 

You aren't going to bend a valve with valve spring tension . The pistons are nowhere near the valves with the crank at the timing mark, and the force of the valve springs is not going to be enough to bend a valve with another valve just from the cam falling over to the side. This just isn't going to happen - if the valve's stem was that weak they would just snap off from smacking closed against the seat .

 

The problem with interferance engines comes when the belt breaks while the engine is running - then you have momentum and when things with momentum collide - stuff gets broken. Without the engine running you would have to be really jerking stuff around with a breaker bar to harm anything even with a piston strike on a valve face - it is common practice on interferance designs to roll the engine over by hand after timing it just to double-check that it doesn't stop somewhere due to a collision.

 

Trust us - you aren't going to break anything. Valves are not toothpicks.

 

GD

[subaru360]

As long as you line the crankshaft up on the timing mark before you take the belt off and don't move the crank, you can turn the cams 100 times in either direction with no ill effects. With the crank on the mark, the pistons are all down.

[mrfixiter]

As long as you line the crankshaft up on the timing mark before you take the belt off and don't move the crank, you can turn the cams 100 times in either direction with no ill effects. With the crank on the mark, the pistons are all down.

 

I thought that in addition to the danger of having the pistons hit the valves, which is eliminated by positioning the crank in the correct position, that opening both intake and exhaust valves at the same time would cause damage as well. I mean the service manual puts a big "X" on the idea of having the cams rotate in opposite direction of which it is indicated. I would think there would be some reasoning behind this warning as seen here: http://members.dslextreme.com/users/farberbear/Fix_it_stuff/Subaru/belt.jpg

[GeneralDisorder]

The valves can interfere with each other - but the forces availible to rotate the cams under valve tensioner are insufficient to cause damage.

 

FSM's are for parts hanging monkeys and are written in a very rigid manner. They want no possibility of someone screwing something up due to misinterpretation of the procedure. They are overly cautious with stuff like this.

 

GD

[vasy]

An EndWrench article recommends using a special tool to hold both left-side cams before removing timing belt. This tool ensures that the cams don't move at all. But it doesn't do any good if one needs to remove the cam sprocket to replace cam seal, am I right? You can't use this tool if the cam sprocket is not there! EndWrench did not address this left-side rotating-cam issue if the seal needs to be replaced.

 

A related question here - how to remove cam sprocket to replace the seal. I know you can remove valve cover and use an open-end wrench to hold a particular spot on the cam while loosening the sprocket bolt. My question is: can this be done to both intake and exhaust cams (is there room to put your hand and wrench to hold the lower cam)? I see more room around the upper cam, especially if you need to hold the cam and remove the bolt by yourself. Welcome any comments.

[johnceggleston]

You can't use this tool if the cam sprocket is not there! EndWrench did not address this left-side rotating-cam issue if the seal needs to be replaced.

 

A related question here - how to remove cam sprocket to replace the seal.

 

if your belt is in tact leave it on until you have cracked loose the cam sprocket bolts. you can use a breaker bar on the crank pulley bolt and a ratchet on the cam bolt. the crank pulley should be lots tighter than the cam bolts. break all 4 cams loose then remove the belt. installing and torquing the cam bolts is the challenge. i used a chain wrench on my ej22 but you have to be careful to protect the cam sprocket. i wrapped it with a piece of the old timing belt. some? / one of the sprockets are plastic? so be careful.

 

if the driver side intake valves are open then the exhaust valves are closed. they aren't open at the same time. with the pistons in the correct position it doesn't matter how the intake cam ''snaps'' there is nothing to hit.

[Ej20dohc]

I for got to rotate the timing belt once I removed the pulley but I still aligned all the marks and when I removed the belt the pulleys moved but I just moved them back in to its mark is that fine 

[GeneralDisorder]

It was "fine" back in 2010. Unfortunately physics and reality have warped significantly in 12 years and now it is possible to annihilate the visible universe if you do this. It's actually quite dangerous - an intern at my shop accidentally destroyed a good part of the city of Portland a while back when the cam slipped about half a tooth and he put it back...... he was summarily dismissed and things got better after a few days. I'm told he still has a severe hangnail that's stubbornly remained untreatable despite visits to numerous specialists in Austria. 

GD

[jonathan909]

2 hours ago, GeneralDisorder said:

It's actually quite dangerous - an intern at my shop accidentally destroyed a good part of the city of Portland a while back when the cam slipped about half a tooth and he put it back...... he was summarily dismissed and things got better after a few days.

So Portland has been restored?

[GeneralDisorder]

It was already fairly wrecked after the riots and homeless camps.... no one really noticed it missing 

GD