Thread type in block

[jhowes]

A few days ano it was posted that the thread type in the Model A block was not standard but a type 4. If you chase it you can harm them and reduce them to a type 1. So how do you clean out old threads in the block?

[Tom Wesenberg]

I cut or grind 4 notches into an old bolt or stud using my Dremel with a thin cutoff wheel. Run it in and out and the notches will pick up the junk. Then spin a small bore brush in and out and use compreesed air to blow out the junk.

[CarlG]

The next question is: Once they have been chased with a standard tap, what now?

[Kevin in NJ]

It should be noted that MANY threads on the A are the tighter tolerance type.

Brake rods are a good example. They are nice and snug if you just clean the threads nice. If you chase the threads they get loose and sloppy.

Even the firewall to radiator rods are tighter tolerance threads. I made the mistake of running a die on one pair of rods and remember noticing a small amount of metal coming off. The nuts seemed to have a normal sloppy fit, but this was before I learned of the thread types. More recently I clean up the set of rods I am going to use. The nuts fit on nice an snug.

Why worry?

Well tighter threads means less need for lock washers and higher strength. If you have a car that is constantly running down bumpy roads you will want everything to just hold together.

One other thing to keep in mind. Some nuts are VERY hard. Run a file across some of the original nuts. Nuts used to hold important things like the springs are very very hard and they use the tighter tolerance threads.

I did what Tom suggested for my rear axle banjo threads. I took a bolt I knew was too loose and put a notch on it to clean the threads. Once I had all the bolts and the holes clean, I tested the bolts in one of the clean holes. Some bolts would go all the way down by finger. They were put in the do not use pile. The bolts that only went 2/3 or 3/4 the way down by finger were considered good. As I recall, a NOS bolt in a NOS banjo only goes about 1/2 way down by finger.

Those who just wholesale put all new nuts and bolts on their cars. Please send me the old original nuts and bolts. I have no problems using a slightly rusted nut or bolt over modern hardware. I like my car to stay together.

[Kevin in NJ]

Originally Posted by CarlG

The next question is: Once they have been chased with a standard tap, what now?

Hope that they stay in when you tighten.

There are a lot of holes tapped out to the looser standard.
Just a fact of life.
Just means the hole can not support as much tension as there is less thread to thread contact surface. If you are doing a high compression head then I would guess it would nice to find a block with tight threads.

[Bruce Lancaster]

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Anyone have any history on the standards?? I believe threads in top of block and in banjo are extra tight; in modern terms I believe class three fit, but was that in use long ago? Regular hardware and any tap that is easy to find are class two, easy fit with just a trace of slop like a regular nut and bolt.
Someone here must have an archival drawing of top of block threading...
With original fit you get not only strength from greater thread engagement but usually good protection agains leaks and a properly aligned stud...all of those holes were tapped simultaneously in a huge machine that hel all its spindles right.
I think about the only reasonable field-expedient cure for #2 tapped head stud holes is your choice of sealing goo. Like some other people on here I am old and primitive and use Permatex stickum...when threads are set, just firmly bottomed, not torqued, temporarily drop on head and gasket and tighten head nuts enough to firmly hold them vertical and let the goo dry. If you skip this, the tapped-out holes will leave them mostly at slight angles and make head installation way too much fun.
A nice cleaning tool is a gun cleaning kit, with brushes in both brass and bristle for a wide range of sizes.

[d.j. moordigian]

This chart is nailed to the wall at work, I'll use 7/16" as an example.

7/16-14, 75%, U drill
7/16-14, 70%, 3/8 drill
7/16-14, 60%, .382 ( what-ever-the-hell-that-is ), NOT on any charts here.

Remember this, 2 cut threads will leak, 1 cut an 1 rolled has a better chance
of sealing, do to thread form.

Dudley

[just plain bill]

The interchangeability problem was found during WW1 but threading size standards evolved slowly between the wars.
The current unified thread series was developed during WW2 because of lack of interchangeability between the allies. They were made official in 1949. There are currently 3 classes but some companies have had their own in house standards. There was a common use of "Interference fit" threads. Those fits were by necessity held to tighter tolerances in most cases by the use of what are called "off lead" taps ground for a certain fit. We used an off lead tap to machine solid lifters for Chrysler engines as there was not enough room for a lock nut. Of course we can also go into "J" and "M" threads. I have made extended screws/bolts for the single locker tappets which use a different locking system than the offlead tap.

[Bruce Lancaster]

OK...have some early thread specs in here, "Mechanic's Vest Pocket Reference Book", one of the editors is John H. Wolfe, Sc.D., director of the Ford Apprentice School. This is a '45 edition, but I think actual compilation was a bit older so considerably pre-unified thread. Next, look at all those itty-bitty numbers 'til my head hurts!

[d.j. moordigian]

Vince,
Interesting, the print calls out for a " V "@.376-.380", and a "u" @.368". Ford put
more clearance than my Morse Cutting Tools chart, post #7. Need to check a
NOS stud with a thread mick, studs could have a larger diameter.

[Bob C]

What does the 5 after the 14 in Vince's drawing mean, is that the fit?

Bob

[Marco Tahtaras]

Originally Posted by fordgarage

Interesting! My drawing shows the same spec but different description.

[Bruce Lancaster]

And NC 4 versus NC 5 taps...trying to get that explainrd via the little Ford reference book...

[d.j. moordigian]

From the Machinery's Handbook, 16th edition, 1962.

Classes " A "....external.

Classes " B "....internal.

It lists classes 1,2,3.

Class 5 is classified as a "Interference-Fit", WITH 5 sub classes.

I didn't find a Class 4.

[Bruce Lancaster]

Those may be too new. My modern source is Carroll Smith's great book on bolts and nuts for racers...I think he says 2 is normal, 3 extra tight, other classes rarely used in automotive stuff.
Ford school book lists classes 1-4, which may or may not be the same as post unification. 1 loose fit, 2 free fit, 3 medium, 4 close fit...no 5 apparent.

[Marco Tahtaras]

Originally Posted by fordgarage

Yours appears to be an earlier drawing, before the oblong water hole was added in the top center of the block. Mine is October 1, 1929.

Mine is 3/28/29

[d.j. moordigian]

To make sense of the hole size/thread class size, you need a print of the stud.
Bruce, do you agree, looking at half the knowledge.

[just plain bill]

Some more stuff to ponder about Bruce!
Also H designation mark on tap means the higher the number the larger the thread it will cut. H 3 is standard H 5 is normally called a plating tap as it cuts oversize to allow for plating buildup. Prior to WW2 we had Coarse and Fine designations. If I remember correctly the fine series was called SAE by some.

When I made some longer bolts for the single locker tappets I found the threads on the bolt/screw were cut/rolled at the top limit for a class 3 pitch diameter and after slotting and heat treating they worked or tightened the same as the original bolt/screws. But a variation of .001 .002 minus wouldn't give the same grip. This was my experience.

[Bruce Lancaster]

Yes...the studs are special with bottom formed to fit the recess shape. Thread specs for that would certainly be nice to go with this. Looking at dimensional info on modern thread classes might well cause my head to explode, and I don't even know if the classes are the same...
Anyhow, it is pretty clear that '28-48 Ford studs (same threads all the way through) go all wonky when tapped with modern class 2 tap and also that few repro studs actually seat in the recess.

[Bruce Lancaster]

OK...weeks of research...finally went to what should have been the obvious first stop, before the Ford apprentice School reference book, before the Ford engineering book..."Shop Theory". I believe this book was first put together about 1934, and it is very easy to find because huge numbers were reprinted during WWII and used nationwide to train new people flooding into war production.
So...most modern threads are classes 2 and 3 of the current system, class two being most of the hardware on your car and nearly everything else around us, class three being tight fit.
Tha 1930's standard used by Ford and described in the "Shop Theory" book has FIVE classes. Scroll up and look at the Tap descriptions in the drawings...Marco's early 1929 drawing shows class 4 tap, Vince's slightly later one shows class 5 tap. I am assuming the number represents class of fit designation as I do not know what else it could be!
Ford cited the National Screw Thread Commision (Stand up and salute!) as the source for its classes, and as I said I believe this list was probably first put into this Ford book in 1934. My copy is from 1942 printing, still before unified thread and SAE classes and such:
Class one: Loose fit
Class two: Free fit
Class three: Medium fit
Class four: Close fit
Class five: wrench fit

This means, as experience shows, that the 5 used on most A's and probably up to 1948 (same everything on studs) cannot be cranked in by hand on good threads with proper rolled original studs.
Probably the majority of A-B and V8 block threads have been wallowed out by now by well meaning machinests wielding modern class 2 taps, hence leaks, slanted studs, need for Loctite and Permatex, etc. I've killed a few in my past...

Of course this all goes with rolled thread, carefully made studs formed to seat into the surface formed by that second shallow drilling noted in the drawings.

Question for those with lots of archival drawings...which class is the "normal" one for the 1930's, used for less critical clamping with common bolts??