I recently bought a Mazdatrix underdrive pulley.

Here my problem:
I checked the TDC ignition mark, which should be the second from left (-10, 0, 10, 20), against some 12A pulleys lying around here.

I realized that the TDC is not matching the TDC of the original 12A pulleys.

To be sure I checked it also on a spare engine.

Here what I did:
I set the original pulley to the TDC mark.
Locked the engine in this position and removed the original pulley.
Then put the mazdatrix pulley on the engine.
There is a difference of 3-4 degrees.

Which time marks are correct?

 

Could someone confirm please, that the second time mark from left on the original 12A pulley is at TDC?

 

https://www.rx7club.com/general-rotary-tech-support-11/finding-tdc-414191/
In this thread they are talking about 5 degrees difference between a 12A pulley and a 13B pulley put on a 12A engine, if I understood well.

Is it possible, that the Mazdatrix pulley is designed for 13B engines?

I do not understand why there should be a different TDC on 13B engines than on 12A engines.

Or is there a difference on the pulley itself (bolt holes) and not on the whole setup that is fixed onto the eccentric shaft?

 

hey tom i asked mazdatrix and they couldnt help me or under stand the problem they said it should be the same as the stock pulley but i said it wasnt. They keep telling me maybe the keyis not at the right location.... idk wierd.

 

It looks about right to me. The factory mark should be at -5, and based upon where the old pulley was lining up and where new pulley is lining up, it looks to be about -5, but if you want to be entirely exact, than it looks more like -4. It looks fine to because the marks according to the picture are -10, 0, +10, and +20 from left to right. In other words, you're ok. I used a small chisel and made a mark where -5 is on my pulley.

 

If I understand well, the factoty mark is not TDC but -5 (ATDC) for 12A pulleys?

 

DTC? Do you mean BTDC (before top dead center)? All 12A's and 13B's from 79 - 92 have the mark at -5 ATDC

 

Uups, I meant TDC (= 0 degrees)

So, I was wrong with all my previous configurations.

I thought that mark is TDC.
I always used this mark as 0 degrees to add additional marks on my pulleys.

When I set my locked ignition to 24 BTDC it was 19 BTDC in reality.

 

In another thread I found this:
(Confusing)

If I find the time today, I'll measure the distance in degrees between the 2 timing marks.

 

Wacky (A nice guy, who sent me a lot of material to Luxembourg) explains that TDC is exactly on the yellow mark.
Is he wrong or just inexact?
(Once again, confusing)

 

I measured 3 pulleys out of my unnecessary big stock of original 12A pulleys.

Here is the result:
All pulleys are the same.
The angel between the first and the second mark is 15 degrees.

If the first mark (left, red) is at 20* ATDC (I think I read that somewhere, again …) then the second mark (right, yellow) is at 5* ATDC.

So I was wrong all the years, assuming that the yellow mark is TDC

Conclusion: The marks on the Mazdatrix pulley are OK. (-10, 0, 10, 20)

If someone has another theory or wants to correct me, please let me know.
A yes you are right, to assure me, is also welcome

 

yes

 

That's what I wanted to know.

Thanks.