OM617: Finding TDC on cylinders 2-5? - PeachParts Mercedes-Benz Forum

Cateaux · #1 11-06-2006, 08:28 PM

On Wednesday I plan to tackle my valve seals. My question is, how do I know when cylinders two through five are at top dead center? If I just point the corresponding lobes on the camshaft away from the rocker arms will that be close enough, or will I end up dropping a valve into the cylinder?

vstech · #2 11-06-2006, 08:38 PM

first off, you want bottom dead center, not top. if you do top, you will have to hold the crank still while you change the spring retainers. I would rotate the engine until the exhaust starts to open, that should be pretty close to bdc.
then of course pressurize the cylinder and remove the keepers.
John

Brian Carlton · #3 11-06-2006, 08:41 PM

Quote:

Originally Posted by vstech

first off, you want bottom dead center, not top. if you do top, you will have to hold the crank still while you change the spring retainers. I would rotate the engine until the exhaust starts to open, that should be pretty close to bdc.
then of course pressurize the cylinder and remove the keepers.
John

How do you pressurize the cylinder with the piston at BDC? The valve(s) are open??

vstech · #4 11-06-2006, 08:42 PM

if you don't have air compressor fitting, perhaps tdc would keep the valves from falling all the way out, while the spring is removed. I do not know if it will work though. in that case, rotate the engine until the pressure blows out the gp hole... that way the cylinder is fully at the top of it's travel.
John

Cateaux · #5 11-06-2006, 09:09 PM

The rocker arms will be removed for access to the spring, etc., so the valves won't open. I'm sorry I referred to them earlier, as that was misleading. According to the mb.braingears.com online manual:

Step 3: Turn piston of wanted cylinder to TDC. Using tool combination, turn crankshaft for this purpose.

I have a deep 27mm socket, and a remote start switch if I can get away with using it. But how do I know when the cylider has reached TDC

Cateaux · #6 11-06-2006, 09:15 PM

Vstech,

I've seen TDC whistles for gassers, but I've heard that they're not particularly accurate. I've never used one myself. With an opening as large as an open glowplug hole, would I be able to feel air blowing through the hole? If I'm turning the engine over using the crank bolt it won't be rotating very quickly.

bgkast · #7 11-06-2006, 09:27 PM

I thought you could put the piston at TDC and avoid pressurizing the cylinder. The valve can’t fall in with the piston at TDC.

Too bad you can't use the old dowel in the spark plug hole like on a gasser.

Cateaux · #8 11-06-2006, 09:33 PM

Quote:

Originally Posted by bgkast

I thought you could put the piston at TDC and avoid pressurizing the cylinder. The valve can’t fall in with the piston at TDC

That's the impression that the manual gives. It just doesn't say how to tell where TDC is. If all else fails, I may set the #1 cylinder to TDC then note the position of the camshaft. Then I can get close to TDC on the other cylinders by camshaft position. If only all the cylinders were marked on the crankshaft pulley like #1...

bgkast · #9 11-06-2006, 09:49 PM

you can't look into the glow plug hole and see the piston can you? I've never tried, but I imagine that the pre-chamber is in the way.

Brian Carlton · #10 11-06-2006, 09:51 PM

Quote:

Originally Posted by bgkast

you can't look into the glow plug hole and see the piston can you? I've never tried, but I imagine that the pre-chamber is in the way.

You'd need to have a mirror to bend your vision

by 60 degrees or so, and then you'd need to see though five tiny holes (.5mm) at the bottom of the prechamber.........

Mustang_man298 · #11 11-06-2006, 10:13 PM

In all theory, the piston should be not totally dead at, but just off of dead top center for this operation. Why you ask? The only reason for placing the piston here is for an emergency backup should the valve get pushed hard enough so as to break it's seal and allow it to fall in the cylinder, resulting in the need to tear open the engine to fish it out which the piston now prevents. Now, why pressurize the cylinder? This is done as a means of holding the valve in place while the spring is removed, the force of the air in the cylinder wants to push the valve out, holding it in place, think of it as a cork in a champagne bottle, shove the cork in too hard and the cork will go in the bottle and the champagne will spray out around it, hence having the piston near TDC as backup. Now, why near and not at TDC? As was mentioned above, the crank will have a tendency to want to roll to BDC when under pressure, so you place the piston just off so as to control which way it will want to roll, then using a block and ratchet attached to the crank bolt, you can set the ratchet to hold itself in the direction the crank wants to turn, set a block down on a jack or in the frame below the crank, and brace the ratchet against the block, so as to hold the crank and know it wont suddenly jump in the other direction on you. I have done seals before this way and it certainly beats tearing a head off over such a simple repair.