TDC Why?

[The Dude]

I'm about to do my first timing belt replacement. The new timing belt is going into my non-turbo 2006 Forester. The 89101 Haynes manual says to position the engine at TDC for cyclinder 1 before attempting to remove the crankshaft pulley. Why, it doesn't make any sense to me?

If I try the screwdriver through the bell housing trick to lock up the engine it might not be possible to be exactly at TCD anyway.

 

So, is TDC necessary or just busy work? Thanks.

[subaru360]

You need to line up all 3 timing marks before you pull the belt. 2 on the cam pulleys go straight up and one on the tabs on the back of the crank pulley to the notch in the oil pump.

 

This is not tdc. With all the marks lined up the pistons are all halfway up, which is what you want.

[MilesFox]

because if you turn the crank independent of the cams, you can collide the valves. best to have it all lined up before starting, or you will be stuck!

[johnceggleston]

on the ej22, if you line up all the timing marks you will not be lined up for the screwdriver through the bell housing trick. but if you turn the crank about 30* counter clockwise you will be able to use the ''through the bell housing'' trick. this will move the pistons but you would have to rotate the crank 90* to bring one to the top of the block.

 

i don't know if '06 engines are the same.

 

hth

Edited April 11, 2011 by johnceggleston

[Olnick]

So, is TDC necessary or just busy work?

 

I remember this question years ago. TDC is absolutely unnecessary and no one could figure out why Haynes said it!

 

Good luck.

[The Dude]

You need to line up all 3 timing marks before you pull the belt. 2 on the cam pulleys go straight up and one on the tabs on the back of the crank pulley to the notch in the oil pump.

 

This is not tdc. With all the marks lined up the pistons are all halfway up, which is what you want.

 

 

I understand the crankshaft aligment thing. The Haynes manual reads "Position the engine at TDC for cyclinder 1 ". This is the first step for removing the drivebelts, cooling fans and shroud, and the crankshaht pulley. Positioning cylinder on at TDC before performing these tasks seems completely unnecessary to me.

[987687]

The haynes has some mis-information on engine stuff. Important engine stuff. Torque specs, sequences, etc. I use the FSM because that way I know it'll be done right.

 

My only use of the haynes is for my GL because that is a lot if different cars mashed together, and it covers most of them in one book...

[Fairtax4me]

Yeah just take whatever the Haynes says with a grain of salt. Setting at TDC is useful in certain situations, but removing the belts is not one of them.

Set the cams at their proper timing marks and make note of the position of the marks relative to the notches in the covers before removing the belt. (this is especially important on DOHC engines to prevent cam "snap" which happens when the loaded cam slips position and the valve spring pressure forces it to turn)

Sometimes the marks don't line up exactly with the covers. The passenger side on my wagon is nearly a full tooth width off from the notch in the cover. You can't see it when looking at the hash mark, but if the engine is set at TDC (the arrows mark TDC) it's plainly evident the passenger cam is not lined up properly.

[logic23]

when the timing marks are lined up all pistons are 1/2 way down/up. This keeps the valves from hitting the pistons if the cam jumps while the belt is off. So you are setting the engine 90 deg after TDC.

[987687]

when the timing marks are lined up all pistons are 1/2 way down/up. This keeps the valves from hitting the pistons if the cam jumps while the belt is off. So you are setting the engine 90 deg after TDC.

 

Correct, it's 90 deg past. If you look at the crank gear the arrow is TDC and the mark is 90 off. You can use all the arrows if you really wanted to.... BUT DON'T! heh. Everyone has a WTF moment their first timing belt.