Found this great site.

Figured I would share as this subject has been covered multible times.

I just think this site sums it all up.



http://members.optusnet.com.au/~onec...-%20part%20one

I've seen this before and it is not accurate. If you mark your front pully like this your timing will be several degrees retarded.

Yes it is not a completely accurate way to find TDC. Especially considering Mazda saw fit to move the placement of the spark plugs among different years. I have however used this method on occasion to get the engine close to TDC. Recently I did a Haltech install on a 1st gen. Zeroed the timing and it wouldn't run right. Obvious that the timing was way off. Found TDC in this manner only to find out the pulley was 90* off.

TDC is when the pulley keyway is at 9 o´clock right.



Why is marking the pulley like that an inaccurate way to do it?

I just dont get it, you have a 360degree pulley that you mark at a certain point, lets say 20degrees.

Why is that inaccurate? Do you need to subtract or add the degrees between the TDC (center of the rotorhousing apex) and the leading sparkplug.?

Yes the keyway is at 9 o'clock but at true TDC the 2 pulley bolts mentioned do not line up with the pointer, they are off to the side a little. From memory the difference is 5 deg or so. I made a master pully using a partly assembled engine, this is the most accurate way I can think of.

The location of the spark plugs has no affect on the true location of TDC.





Lynn E. Hanover

I always use the factory timing marks to GUIDE me, in marking new timing marks on a factory pulley. factory timing marks are -5 for leading spark plugs and -20 for the trailing spark plugs.. u can't argue against the mazda factory when they marked the pulley. -5 and -20 marks are marked to perfection as far as before and after top dead center on a rotor. start there using -5. then, find top dead center, that would be 0 then +5 +10 +15 +20



-5 means atdc after top dead center

+5 and anything with a + means BTDC before top dead center..

I mark my pullies no more then 30 degrees BTDC that would be +30. I run my naturally aspirated engines at +30

It was the S4 plugs that were moved, but because it didn't really improve anything Mazda went back to the original position. So the plug positions are the same on engines before '86 and after '88.? I'm pretty sure(please correct me if I'm wrong) that the leading plugs were retarded(cant remember by how much though).



I've seen another do-it-yourself timing article where the rear housing plugs are removed, and then the engine hand-rotated until an apex seal can be seen in the trailing hole. After making a mark on the pully in line with the timing post the engine is rotated again untill the same apex seal is seen in the leading plug hole. Another mark is made on the pully, and suposedly the distance half way between the marks made is TDC for the front rotor.



https://www.nopistons.com/forums/pub...1047683785.gif

Yes I realise that. TDC is when the combustion area is at its minimum.

are this timing marks for both 12a and 13b engines alike