how critical is TDC
- Thread starter 4tford
- Start date
Bored&Stroked
Well-known member
Everybody has pretty much flogged this horse (for an 8BA) to death now haven't they! 
On 32 to 48 engines, I'm not aware of there ever being a timing mark (at least that I've seen). So, I usually create one during the build process - using a degree wheel as well as a dial-indicator or dead stop before the heads go on.
Here is my latest creation for a 42 Merc engine that is going into my 32 Cab (being put in today . . . after 2 years of work . . . yippee!).
I modified an original 32 motor mount (fabricating some steel and a pointer to make it look like it came that way). Then I marked the crank pulley with TDC as well as 10,20,30 degrees.
On 32 to 48 engines, I'm not aware of there ever being a timing mark (at least that I've seen). So, I usually create one during the build process - using a degree wheel as well as a dial-indicator or dead stop before the heads go on.
Here is my latest creation for a 42 Merc engine that is going into my 32 Cab (being put in today . . . after 2 years of work . . . yippee!).
I modified an original 32 motor mount (fabricating some steel and a pointer to make it look like it came that way). Then I marked the crank pulley with TDC as well as 10,20,30 degrees.
flatjack9
Well-known member
It doesn't really matter how you install the distributor, but once it is in, #1 has to be the one the rotor is pointing at with #1 piston at TDC firing.
Mike51Merc
Well-known member
To answer the original question as simply as possible, there are 360 degrees in a circle, and there are eight cylinders that need firing so each cylinder gets 45 degrees of arc to play with. Since the cam turns at half the speed of the crank, this means you have about 90 degrees of crank turn that belongs to each cylinder.
So, as far as TDC and initial ignition timing (for the purpose of distributor installation), TDC is critical only to the extent of the 45 degree "range" of the #1 cylinder, otherwise you'll be in the range of a different cylinder. The fine tuning is going to occur in that 45 degree range that "belongs" to that particular cylinder.
So the bottom line is that if your #1 cylinder is somewhat close to the top (which is close enough to the crank timing mark) and you stab your distributor in so that the rotor is pointing to the 6 o'clock position (Assuming your cap is wired with #1 at 6 o'clock), you stand a good chance at having the engine start (unless you're 180 degrees off which is a whole other thread to talk about). Once it starts, then you do final tuning.
So, as far as TDC and initial ignition timing (for the purpose of distributor installation), TDC is critical only to the extent of the 45 degree "range" of the #1 cylinder, otherwise you'll be in the range of a different cylinder. The fine tuning is going to occur in that 45 degree range that "belongs" to that particular cylinder.
So the bottom line is that if your #1 cylinder is somewhat close to the top (which is close enough to the crank timing mark) and you stab your distributor in so that the rotor is pointing to the 6 o'clock position (Assuming your cap is wired with #1 at 6 o'clock), you stand a good chance at having the engine start (unless you're 180 degrees off which is a whole other thread to talk about). Once it starts, then you do final tuning.
Last edited: