What Do You Do When You Remove The Crank Pointer And Can't Find Top Dead Center? - PeachParts Mercedes-Benz Forum

Squiggle Dog · #1 11-01-2019, 10:47 PM

I removed the timing pointer for the crank on an OM603 engine because I wanted to derust and paint it. When I put it back on, I noticed that it is adjustable, so I think I have a big problem as the timing could be off by as much as 4 degrees. So, now I don't know exactly where top dead center is.

I installed a new timing chain, and when the notch on the camshaft is perfectly centered on the mark of the bearing cap, the crank pointer can be moved from top dead center to 4 degrees before top dead center. The chain is tight, so I think I can slide the pointer to top dead center as I doubt the piston would be anywhere before top dead center with the marks on the cam lined up.

It's a lesson in not loosening the bolt on the crank pointer, and also that you don't know if it's been messed with before, so I guess you can't necessarily trust the pointer to be accurate.

dude99 · #2 11-01-2019, 11:10 PM

On motorcycles you thread a dial gauge into the spark plug holes to verify top dead centre...... maybe you could do the same with the prechamber removed. Thats the only way I can think of to verify it as 100% accurate.

Actros617 · #3 11-01-2019, 11:29 PM

Since you have the camshaft aligned with 0 TDC prior to removal you can verify the timing by removing injector 1 and 6 and insert 2 equal length rods, rotate engine until the stick reach its maximum and mark on the stick right where cylinder 1 and 6 crest/before moving down, use the camshaft lobe on cylinder #1 to identify if its which stroke it is on ie Intake stroke, POWERSTROKE, Exhaust stroke. Then finally verify that the camshaft is at aligned with its marking and cylinder 1 & 6 stick is at its highest marked position and it should get you to at TDC aligned.

I've attached image of my OM603.971 that is 0|T mark the piston positions #1 and #6 should be at TDC, and Cylinder 1 Cam profile

Cylinder 1 cam profile at 0|T, both lobe are pointing up Exhaust at 10 o'clock and Intake at 2 o'clock position

Crank position

Diesel911 · #4 11-02-2019, 12:49 AM

Similar to the mentioned old motorcycle timing.
The manual has you pulling a pre-chamber out and using an long extension on a dial indicator that goes down to a flat spot on the piston head. You need to get the piston close to TDC because most dial indicators have a limited range of motion.

I had a device that screwed into the spark plug hole and it had a plunger and the plunger had markes on it around the plunger and you could also apply pencil marks and by trial you determined where TDC was. However, on the motorcycle it is not detrimental to rotate the engine in the opposite direction of rotation and then rotate it back in the normal direction of rotation which made it less time consuming to do. I did that when setting the Magneto Timing on my BSA A10. Don't remember all of the details.)
One aspect of the tool with the plunger that was not good was the spark plug hole was at an angle to the piston and that meant the tool was also at an angle. A dial indicator straight up and down would have been better but you would have needed to remove the Fuel Tank.

barry12345 · #5 11-02-2019, 12:58 PM

If you do not want to pull the pre chamber. Bringing the piston close to TDC. Fill cylinder with a fluid of your choice. Possibly motor oil . Rig a straw or clear small diameter tube. Figuring how to seal it at the injector hole. As you rotate back and forth a little. Your high point should be accurate. Should not take that much fluid.

Clear it out later by using the starter and some rags. Before installing the injector. I recommended this once before and they were happy with it.If I remember they wanted to check if their pointer was accurate.

Squiggle Dog · #6 11-02-2019, 04:45 PM

Thank you all for the excellent responses! I think that filling cylinder #1 with oil and using a straw as a gauge will be the most practical in my situation.

Diesel911 · #7 11-02-2019, 08:08 PM

Quote:

Originally Posted by Squiggle Dog

Thank you all for the excellent responses! I think that filling cylinder #1 with oil and using a straw as a gauge will be the most practical in my situation.

That won't work because if the piston ring gaps allow the oil to drain out.

Squiggle Dog · #8 11-02-2019, 09:57 PM

Quote:

Originally Posted by Diesel911

That won't work because if the piston ring gaps allow the oil to drain out.

Fortunately, it worked for me. I already had a compression tester fitting installed into the prechamber so I would feel it shoot out air at me so I would know if the piston was in its compression stroke. But, the crank turns once, while the cam rotates twice. It seems that the only thing that determines if the piston is in the compression or exhaust stroke is the cam, right? So, it really shouldn't matter, I wouldn't think--just set the crank to top dead center and then line up the cam marks.

Anyway, I fit a piece of clear tubing over the end of the compression tester fitting and pushed a small funnel into the end so I could pour in oil. I turned the crank to 10 degrees before top dead center and then slowly and carefully poured oil into the funnel. I found that it would burp itself and air bubbles would come up as I was rotating the crank. Eventually all the air bubbles were purged.

I very slowly turned the crank and could see the oil level rise up. Once the piston reached top dead center, the level would immediately fall. I turned the crank slowly in each direction until I found the exact point where the oil level was at its highest. I then adjusted the crank pointer accordingly and tightened it in place. I checked my work several times and am sure I got it within 1/8 of a degree of accuracy.

One big screw up is out of the way, and now I am sure that the crank pointer is accurate. I see that the cam is 1 degree advanced, however. Perhaps that is due to the height of the head gasket or having an old chain tensioner installed which has no oil pressure. But, that's probably a good thing as I plan to advance the injection timing from 15 degrees to 14 degrees. Apparently advancing both the cam and injection timing can yield higher performance (I think Doktor Bert ran the OM617 in his 300SD with the cam advanced 2 degrees and the injection pump advanced 4 degrees).

Maxbumpo · #9 11-03-2019, 07:33 AM

That is awesome, well done!