Please explain timing marks to me - M189 engine

[scoot]

I get really confused with timing marks and I know this is an easy question (which I can't say for the MB cars that have two rows of timing numbers stacked on top of each other).   I'm trying to set the timing for the 1965 300SE that hasn't run in probably 10 to 15 years or maybe longer.  The BBB says that the "Basic" setting (which is what you use when you install a distributor) is 4 degrees BTDC (which I assume to mean before top dead center).  Then it appears that at 800 RPM the timing should be 8 - 15 degrees.  It doesn't say BTDC so I'll assume it is not "before" TDC.  Then it says at 4500 RPM it should be 26 degrees w/o vacuum advance.  That's all well and good. 

Here's a picture of the harmonic balancer with the timing marks.   It reads 20 - 10 - OT - 10 - 20 - 30 - 40 - 50.   Is the 20 - 10 - OT range the "BTDC" range?

thanks
Scott

[Benz Dr.]

20 and 10 is ATDC, OT is TDC, and all the numbers right of OT are BTDC. BTW, the injection pump mounts at 60 degrees ATDC on number one piston - don't mount it at 20 degrees ATDC like a 113 engine or it won't run properly. There's a 60 mark on the pulley for this.

[scoot]

20 and 10 is ATDC, OT is TDC, and all the numbers right of OT are BTDC. BTW, the injection pump mounts at 60 degrees ATDC on number one piston - don't mount it at 20 degrees ATDC like a 113 engine or it won't run properly. There's a 60 mark on the pulley for this.

Thanks Dan.  What I don't understand is why the BBB says that initial timing at starting RPM (not idle, just starter) is 4 BTC and running at 4500 rpm is a much higher number than the ATDC shows on the scale...

Scott

[Cees Klumper]

That's because (I think) it is indicating 26 degrees at 4,500 RPM  BTDC, not ATDC?

[scoot]

That's because (I think) it is indicating 26 degrees at 4,500 RPM  BTDC, not ATDC?

The feedback I'm getting is that ALL of the timing specs are BTDC and apply to the 0 - 50 scale.  Does that sound correct?  Is the 20 - 0 scale just there to confuse people? 
thanks
Scott

[Raymond]

The 20 to 0 ATDC range is there so you can see how far off you are.  If you got to 20 it wouldn't run at all so you wouldn't need any other marks. 

[scoot]

The 20 to 0 ATDC range is there so you can see how far off you are.  If you got to 20 it wouldn't run at all so you wouldn't need any other marks. 

OK, that's good.  So in a perfect world I can ignore the 20 - 0 range entirely and focus on the 0 to 50 scale...   thanks so much!
Scott