OM617: Finding TDC on cylinders 2-5?
 

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?

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??:confused:

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

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:confused:

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.

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.

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...

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

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.

You can put marks 72-, 144-, 216- and 288-degrees on the crank pulley. Starting at #1 TDC, you'll hit some TDC every 144-degrees.

Sixto
93 300SD

Thanks! That's exactly the information I was looking for. I knew somebody here would have the answer:D

doesnt have to be exact. I just look at cam lobe positions.

............every 72 degrees........;)

Yes, but not TDC with both valves closed.

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

once you remove the rockers the valves will be closed.
John

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...

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

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

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.

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.

I should not have used the word ' shave' ....that is from the old Hot Rod days.. meaning to take off metal to increase compression...
I should have used the word ' surface' to indicate a necessary machining operation for making the head surface flat.
I expect that Brian's post is correct... but be sure to note which type of precombustion chamber tip/piston top you have and be sure the measurements coincide with what you have ( in case the are different for the two types )....

Okay, I've got the new valve seals installed. I ran out of daylight so I haven't bolted the valve cover back in yet, so I haven't test-run the engine, but I'll just assume that I did it right:rolleyes:

It's really easy to keep the valves from falling into the cylinder. With the cam's intake lobe in about the 11 o'clock position, you can actually press down on the valve spring and tap the valve on the piston! On two cylinders I even had to lower the piston a little to get enough clearance to remove the valve springs.

cool

Thanks for posting that... That is the method described in the FSM...the safe method...

That's really the only method that will work. If you use compressed air you won't be able to remove the valve springs. Compressed air may work well for a pushrod engine, but not when there's a camshaft in the way.

A note or two for people searching for info on this job:

A good tool for removing the old valve seals is a valve adjusting wrench. It fits snugly under the valve seal, and you can pry it out by tapping down on the wrench's handle with a rubber mallet.

Valve seals are apparently delicate, so be careful when tapping them in. I used a large flat blade screwdriver to tap gently around the edge, like tapping in a dust cap. Placing a box wrench flat on the seal and hammering on the shaft won't work...it'll deform the seal:o . After I let it sit overnight I'll know if my #1 exhaust valve seal leaks. If it hangs in there until I need to rebuild the engine I'll be satisfied:D

Cateaux, glad you got it done! I'm lucky and I have the luxury of doing the job with the head off the engine. By the way, I got my rebuilt head back last night from the machine shot. It really did almost bring a tear to my eye. They installed new exhaust guides and "surfaced" it and holy crap is it pretty.

Thanks for the help guys. I should have it ready to go on in the next couple of days and I'll measure the pre-chambers.

True. If it were a two-stroke. Otherwise, the cylinders are firing every 145 degrees of engine rotation.

In this case, however, it won't matter if he isn't opening any valves (since he is removing the rockers first).