Finding TDC at the end of the compression stroke

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lsquaredb

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Hi Everyone,


I'm trying to install the 123 distributor.

Is there a simple way to find TDC at the end of the compression stroke? I can rotate the crank so that the OT mark on the vibration damper on the point, but the spark plug hole is so recessed that I cant put my thumb on it to feel the compression.

I looked on the internet and there was some discussion of removing the valve cover and lining up marks on the timing gears.

Thanks.

Leonard
 
Remove valve cover, make sure number 1 valve lobes point down with no lift on the valves, cam sprocket will line up with center of the cam tower. Sorry I don't have a pic. Damper on OT is only correct every two turns of the crank.
 
get a screwdiver and set it in the spark plug hole
turn over motor by hand till the driver comes to top and starts down again
go back and forth to get the center
thats at the top of the cylinder.
 
Timing mark on damper lined up with cover mark with o/t ball in timing window will confirm tdc. If ball not in window then go around once more. Distributor rotor should point to about 8 o'clock or so. Not advisable to go counterclockwise when searching for tdc. Remove plugs for easy turning.
 
I ended up turning the crank until the OT mark was lined up and the rotor was pointed to the wire to cylinder no. 1. The odd thing is that is at 12 o'clock.
 
lsquaredb said:
I ended up turning the crank until the OT mark was lined up and the rotor was pointed to the wire to cylinder no. 1. The odd thing is that is at 12 o'clock.
Click to expand...
You can be one tooth off when you insert the distributor. Just pull it up and move the rotor a little farther and then drop in again.
With the 123 ignition the position is not as critical as you just adjust the difference out because it is electronic.
As long as you can do their setup procedure with the green light you will be good to go.

Since these engines are not a true hemi combustion chamber (meaning the screwdriver would go straight down on top of the piston) I would not recommend putting a screw driver in the spark plug hole. You could damage a piston.

Just to clear up something. When setting the timing you always want to keep turning the engine the way it runs. Clock wise for M30 motors. This takes up for any slack in the chain and clearance in the rods. You can turn the engine CCW and it will not hurt anything as long the tensioner is installed it's just that you need to go back CW at least one revolution to get the slack back out before setting any timing.
There are a number of ways to find true top dead center of an engine. I did mine when the head was off. Sometimes engines will not be marked very carefully and you could find the factory timing marks are quite a few degrees off. Most M30's are pretty darn close to TDC. This is only critical on extreme high performance engines.
 
I get the idea and process of finding TDC by turning the engine the way it runs. I do it that way myself. I read that in any event the timing marks are pretty close on M30s.

But, ever the engineer, I wonder if manually turning the engine (presumably with a wrench or socket on the Jesus Nut inside the crank pulley up front) is not a little bit wrong. My thinking is that this takes up slack in the engine, in a manner that's not the case when the rotating works is being driven by the pistons. So an engine turned manually will be "off" by whatever stack-up occurs in and between the rotating parts. This might be particularly the case when trying to find TDC for the distributor, being that it's at the far end of whatever "stack-up" occurs.

But what might a professional say about stack-up in this situation?
 
Ever try to get the engine to stop on tdc by itself? It ain't going to happen, and if bumping the starter that's the same as turning with the jesus nut.
 
Well that is really the minutia.
Remember only one piston at a time is loading the crank, everybody else is along for the ride. Loaded vs. unloaded.
I do use the big nut on the front to turn the motor.
I am currently setting my M30 3.5 up with a degree wheel and dial indicator on the cam and an adjustable cam gear. See photo.
This will be as close as I could get the timing set perfectly. I'm actually setting it to what the cam card says.
Paul Burke N31 cam and 10.5:1 pistons with triple Weber 45's and a 123 ignition.
You can take a regular cam gear and have a machinist elongate the holes so you can move the cam around relative to the crank.
I can actually hold the engine with the big nut and with another socket move the came forward and backward to change the timing. Manual Vanos.
On my S38 M5 engine with dual overhead cams I can move either cam to tweak it.
I really don't know how they do a modern engines except they get a base and then a computer can vanos the cams for best power and best mileage.
 

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