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.

vstech · #8 11-07-2006, 10:02 AM

Quote:

Originally Posted by Cateaux

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.

if you can't figure out the 72degrees part, you should feel pressure push your finger off the GP hole as the piston rises in the sleeve.

John

leathermang · #9 11-07-2006, 03:00 PM

Nowhere in the FSM does it suggest using air pressure to TRY to hold the valves up during this procedure.

Why ? Two reasons... if you do not have PERFECT seal at the valve lip to head then you risk dropping the valve into the bore... one tiny piece of carbon, a slightly warped valve... BINGO... your worst self made nightmare... unless of course you were just looking for an excuse to take the engine apart.. Bonus mental image... if you really lose air pressure fast you will drop BOTH valves .... ( if you are unlucky enough to undo the bad one second ).

Reason Two... with 21 to 1 compression ration your piston is so close to the head that if you machine it at all you have to install thicker shims under the precombustion chambers to raise them so that the piston won't hit them..

Extra bonus reason.... if you put 100 psi into the cylinder you have 1200 lbs of force on the top of that piston... and you have no legit way of clamping that kind of force at the crank...
Actually at half stroke it is more than that due to the throw of the crank...

sixto · #10 11-07-2006, 03:24 PM

I would think 20 psi is plenty to keep the valves in place even with the constant leak past the rings.

A ring gear clamp such as used to hold the crank to torque the crank damper bolt should work.

Do you need compressed air at TDC? Won't the valve rest on the piston after it falls 1mm?

Sixto
93 300SD

leathermang · #11 11-07-2006, 05:06 PM

Forget using air for this procedure...
The FSM calls for using the piston at TDC for holding the valve UP...

sailor15015 · #12 11-07-2006, 07:07 PM

I have the head off my project engine right now and it is amazing how little clearance there is between the head and the pistons. At tdc, the pistons are nearly even with the block and there is hardly any recess of the head above the pistons to speak of. The firing order of the 617 is 1-2-4-5-3, correct? Set #1 at tdc using the pulley markers and do its seals. Rotate engine so both lobes of #2 are facing up(shouldn't take too much rotation, about a fifth of a turn) and do the same thing. Repeat in the order the pistons fire and you should always find tdc. That seems logical to me. Anything I'm overlooking?

A little OT-Leathermang, I'm having some head work done on my project engine right now and I get it back tomorrow. I think they're shaving the head to make sure its flat but I'm not sure. Should I install extra shims underneath the prechambers just to be safe? I've got the tools to pull them so its no big deal.

Brian Carlton · #13 11-07-2006, 07:11 PM

Quote:

Originally Posted by sailor15015

I think they're shaving the head to make sure its flat but I'm not sure. Should I install extra shims underneath the prechambers just to be safe? I've got the tools to pull them so its no big deal.

Install the prechamber and measure the protrusion beneath the surface of the head. The specification is .307" to 331". If they protrude beyond .331" then you'll need the shims.