find TDC on 1989 W124 with M103

[dka-66]

I have a 1989 300TE with the M103 in it, I recently torqued down my head and had already set the creak to TDC..... My Brother in Law came over to "help" and now I have an engine that is no longer set at top dead center, the cam is obviously not an issue but the timing chain was not on which is what is making this so difficult.

I can put the damper on but it does not look like there is a mark on the block or the lower timing cover that I can see and the TDC is obviously off (and it is adjustable anyway).

I am hope someone here can be of help because I am in need of it.

 

[BlackC55]

There is a mark on the Crank pully from memory it is marked with OT.

Put it on OT (TDC) then set the cams up then put the chain on then set the tensioner up.

haynes do a manual if that is any help.

 

[dka-66]

BlackC55 thanks for the reply.... the Damper has the OT on it, the problem lies in that I removed the lower timing cover and in doing so the TDC Sensor which has the mark that you line up the OT to is also adjustable so I have no "real" set mark to line it up to.

 

[grober]

Normally you should be able to mount a dial gauge thro the spark plug hole and check TDC with that but the angle of the spark plug hole on the M103 engine may prevent that. This is another indirect method which I can't vouch for but might well be OK.



How to Find Your Engine's Top Dead Center (TDC)

If you have a car with no timing marks on the crank pulley, here is a really neat, accurate and cheap way of finding the engine's TDC. This is done with the engine in the car and the head torqued down.
1. Make an indicator from some clear plastic/ silicon tubing, a jar of light oil, and an old sparkplug.
2. Break up an old sparkplug and attach a length of clear plastic tubing to it (make it airtight). and the narrower bore the more accurate--within reason.
3. Remove all the spark plugs.
4. Stick your thumb OVER the #1 cylinder spark plug hole. Rotate the engine with a socket on the crankpulley bolt observing the correct direction of rotation until you feel pressure on your thumb. That's the compression stroke. TDC is at the top of this stroke.
5. Screw in the sparkplug with plastic tubing attached and insert the other end of the tube into a jar of light oil. Continue rotating the engine. Bubbles will appear until the piston reaches the top of its travel. When it starts down on the next stroke, the bubbles will stop and oil will begin traveling up the tube. Stop at a convenient point and mark the tube. Then mark the crank pulley and the engine body at a convenient spot.
6. Rotate the engine backwards and watch the oil recede into the jar. Continue rotating. As the piston continues past tdc and downward it will again suck oil into the tube. Rotate the engine till the oil again reaches the mark. STOP! Mark the crankshaft pulley where it lines up with the mark you made previously on the engine. You should now have two marks on the crankshaft pulley. The midpoint of these two marks lined up with the mark on the engine is tdc.

 

[dka-66]

Grober, thank you for the help..... I also found this one.

Take an old spark plug and remove the guts (porcelain and middle electrode), then create an internal thread through the middle of the hollow spark plug or weld a nut (about 5/16 or 8mm) to the spark plug shell so you can thread a long bolt through it. Now install this new TDC tool into No. 1 cylinder (with bolt backed off).

This procedure works best if the chains are off. Loosen the rocker arm adjusters fully (you don't want to introduce valve head to piston), as you will rotate the engine around without the valves in synch.

Install the crank pulley and rotate the engine (CC or CCW) so the crank pulley TDC notch is about 1/2" or so from the TDC line on the crankcase (for No. 1 cylinder). Thread the bolt in by hand until the bolt stops against the piston. A nut snugged on the bolt helps hold this position. Place a temporary mark on the pulley opposite the TDC mark on the crankcase. Now rotate the engine (by hand) again in the opposite direction. As you approach TDC, slow down, then gently continue until the piston stops against the TDC bolt. You'll feel the resistance. STOP and again mark the pulley opposite the TDC mark on the crankcase. You now have two marks on the pulley. True, absolute TDC is exactly halfway between the two marks. Repeat as often as you like to validate. If halfway between marks happens to land on the factory notch, great. If not, create a new notch as this is your new TDC reference point for accurate cam and ignition timing. Paint it with white paint for visibility and don't forget to remove your TDC tool.

I always try to keep the parts I pulled out until the job is done and also keep some parts for my 66 Mustang and 65 Beetle in the tool box just in case. This is one time that I threw all the things out that I removed because my bench was getting cluttered.

 

[grober]

I should perhaps add that the walls of the pipe-plastic or silicon tubing should be sufficiently robust to resist any slight negative pressure since this might effect accuracy. In theory should be the same both sides of TDC but best eliminated from the equation. The purely mechanical method you detail would certainly be more useful in a circumstance where the production of cylinder vacuum was poor or impossible --e.g. worn piston rings/ valves. It would be interesting to hear from anyone who has tried this successfully.

 

[Farmbenz]

The best method by a country mile is the "positive stop" screwed into the spark plug hole as detailed above. The tool is dead easy to make - it doesn't need to be pretty and if you're clever enough to put it somewhere safe you will use it again and again. For years at a specialist engine tuner we had many cases where the timing marks were not accurate, we checked them as a matter of course - and I still do this now.
Cheers, Farmbenz