Flexplate bolt thread conundrum

[Einbbx]

You may have read my story..

Automatic Z24 ST manufactured date 6/84, Flexplate failed had to replace, during removal flexplate bolt snapped.

 

Now I am trying to re-thread the holes on crank, but have found issue.

 

I have 3 different flex bolt sets

Each have different signs on head

*Original bolts= upside-down T & 12

*Removed from a manual= MT

*Removed from D21?= upside-down T & 13

T12 shortest (tapered end)

MT slightly longer (no taper)

T13 longer still (tapered)

5 of 6 T12 bolts are UNUSABLE, trying to find suitable substitute.

 

ISSUE

Bolt is either

12mm with 1.25 thread or

1/2 with 20 thread.

Foreign... should be metric...

Used a thread checker and thread looks interchangeable

1/2-20 tap does not fit bolt hole, logic says it's 12mm tap.

 

PROBLEM

Bolt seems to fit 12mm-1.25 die but will NOT thread

Bolt fits 1/2-20 die loosely but threads nicely.

 

MY CONCLUSIÓN

After trying several dies and reading up on my measurements

BOLT:

Appears to be 12mm, 12 slightly smaller than 1/2 explains loose fit in 1/2 die and tap not fitting in crank.

THREAD:

Appears to have a standard 20 thread, if it can fit the 12mm die but wont thread with 1.25.. then it must be so?

.. is this possible? Is this a real thing?.. or am I crazy!?

 

I need to re-thread but don't want to damage crankshaft, ANY advice is welcomed wholeheartedly

Please and thank you all

-E.C.V.

[Draker]

It's metric, for sure. I don't know the correct pitch.. but its metric.

[wayno]

You should be able to spin the proper tap into one of the other good crank holes with your fingers.

Metric for sure, Draker has that covered.

[Einbbx]

Thanks for the speedy reply

I agree logic dictates it.. but the closest and ONLY die that should work is 12m 1.25 thread. I measured 1.5 and 1.75 thread and it's not even close. But it won't fit the 12mm die with correct thread. Bolt threads are beautiful so it makes absolutely no sense

[wayno]

Take one of the good bolts to the hardware store tomorrow and find another one that is exactly the same and you will know what it is.

[Lockleaf]

Crank bolts are super fine thread. Very good possibility it's an m12x1.0.

[Charlie69]

Take a good bolt to a local Ace or True Value hardware store or any place that has metric bolts and taps.  Use the thread finding tool to determine the thread and the get your self 2 taps in that size. ! a tapered tap and 1 a botoming tap. 

 

Run the tapered tap first with lots of thread cutting oil.  if you have never tapped a hole then search it out on the web.  You don not run a tap straight through you turn it in a little the back it out, then turn it in a litle past where you stop the last time, then back it out, add more cutting oil as you go.  Keep repeating this process until your tap comes out the backside.  Becarefull not to try to thread to much at a time or run the tap into the back of the engine block.  It is easy to break off a a tap but difficult to remove the tap.  after you finish with the tapered tap (when the tapered tap runs the entire depth off the threads then switch to the bottoming tap and repeat the process.

[Charlie69]

I will see if my tap and die set has the correct size for the bolts.  I f it does I will post thread size here to morrow evening.

[Einbbx]

I'll try the tap 12 1.25 tap and see how it goes

[wayno]

I'll try the tap 12 1.25 tap and see how it goes

No, take a good bolt to the hardware store and find out what you have, do not guess as you will not be able to fix it if your wrong, then you WILL need a new crank.

[Charlie69]

I'll try the tap 12 1.25 tap and see how it goes

No it is a fine thread and it may or may not be 1.25 pitch.

[datzenmike]

Go to an over size bolt in metric or standard, it doesn't matter. If you tap the same metric size there is a chance that you will slice the original threads off the crank. Go to a larger hole so your tap cuts new metal threads. Get the corresponding drill and tap for the bolt you pick.

[Draker]

My guess is M12x1 also.

[Einbbx]

TAP CONFIRMED it's 12m 1.25.

Only two holes need thread, the broken bolt one and another has first half of thread flattened as it dethreaded half the bolt..

[Einbbx]

I assume broken bolt hole will need new thread. What grade is this bolt? Maybe 8?

[G-Duax]

Metric bolt strength doesn't go by the same numbering system as SAE bolts.

Where as an SAE grade '8'  is about the same strength as a metric grade 12.9.

[Charlie69]

The bolt thread is 12 x 1.25.  These flex plate bolts are shorter than the flywheel bolts.

 

I have torn down over the past 25 plus years 6 automatic cars and trucks.  This is how I know that all use the same hardware even the 3 and 4 speed auto matics.  The flex plates are different between the 3 and 4 pspeed automatics but they also use the same bolts for the torque converter.  The 2 speed automatics behind the torbo engines have the same hardware also.  When I say this I am talking only about the L engines 4 & 6 cylinder 6 hole cranks, the 4 cylinder Z20 Z22 Z24 engines and the 4 cylinder KA engins all take the sme spacers, flex plate to crank bolts, and flex plate to torque converter bolts. 

 

 

 

You will have these soon.

 

If you have any questions on how the spacers go on or any thing please ask.

 

Einbbx I noticed in the picture above that there were 2 differen link shoulders.  So I unboxed and made sure all the T12 bolts had the shorter shoulder so you have a matched set of 6.

[thisismatt]

Holy crap, just count the threads across a metric ruler.  You don't have to drive to a hardware store to figure out the thread pitch

[Einbbx]

No I had the measurements covered, have a little nifty tool that has all metric and standard thread sizes to test bolts. Only issue was my m12 1.25 die had damaged thread and made me think the threading was different.

 

I'm really thankful @Charlie69 the local LKQ yards always have 720s & d21s, but recently they've all been manual.

[Charlie69]

I just remembered why I have 6 bolts with longer threads.  These came out of a 97 D21 Hardbody tha only uses the outer spacer plate and actually fit one of the 4 speed automatic Transmissions I have for a project.

 

 

I sorry about the confusion but I will send you the shorter threaded bolts that fit the dual spacer setup.  All these bolts have T12 on the tops of them.

[Einbbx]

D21s have T12 bolts? Strange.

[Charlie69]

1986 to 1989 D21s ran the Z24I engines and the KAs used the similar 4 speed automatic but these automatics had more electrical in them than the ealier L motor 4 speed automatics.

[Charlie69]

Parts shipped.