I'm upgrading a car with AB valves to ABD. The new valves didn't come with studs and I know I have to change at least some of the studs. I can measure the clamped length and come up with an approximate stud length, but I'm wondering if there is a specification floating around with the "standard" lengths.

Is there any "rule" against using all-thread?

Greg

Don’t take this to the bank, but I think AB to ABD is a direct swap and you only need to change the top stud to a shorter one on the service portion. You can order studs and nuts from your brake shop - they’ll know what size. I haven’t run across any data specifically on the studs online.

Curious question: Why change it? AB is still pretty reliable.

WABCo went to great lengths to ensure that the apparatus they made couldn't be assembled incorrectly. So if you are changing to ABD, you also have to change the pipe bracket because the two models of valves use slightly different bolt patterns.

All thread is junk. Besides, with studs the threads on the end which goes into the casting are a tighter class of fit than the ones for the nut. This helps ensure that when you remove the nut, the entire stud doesn't come loose. It's also why there are no threads between each end: slightly different thread geometries.

As jayrod said, why bother to change?

Bit of an over simplification there I'd say.

Yes the "all thread" rod that someone in a hurry might "grab" at a "big box" store might be "junk" when applied to a precision application. But there are many "grades" of "all thread" on the market;

https://www.mcmaster.com/all-thread-rods/

Yes, replacing specifically designed studs in a precision mechanism with generic "All Thread" is probably asking for trouble long term. But in a museum operation it could be appropriate for a temporary solution.

Perhaps the OEM of the brake parts specified different thread classes, the stud was probably specified for a "tight" fit on one end and a "loose" fit on the other end. Design intent was to drive the "tight" fit into a casting with a lot of torque and then use a nut on the other end of the stud with a "loose" fit so the nut could be removed without disturbing the studs fit into the casting,

This was referred to as a "Tap Stud";

https://www.mcmaster.com/tap-end-studs/

In internal combustion engine practice the end of a cylinder head stud that went into the cylinder block was often a coarse thread (requires more torque to seat/unseat) and the portion of the stud above the cylinder head that received a nut was a fine thread which is more vibration resistant.

From McMaster: "Inch studs have a Class 5 thread fit on the friction-fit end and Class 2A thread fit on the machine-threaded end." "Inch" referring to non-metric ones. Thanks for the link, it saved me from having to thumb trough my Machinery's handbook.

WBCo's use of these types of threads predates internal combustion engines by at least 10 years.

I thought I read somewhere, like maybe an older WABCO publication or possibly in something like TM-55(?) that the ABD was meant to go on the same bracket to make upgrades easier. Could very easily be wrong, though. It might have been the pipe bracket maybe? I haven’t seen any diagrams of the bolt patterns. My hands-on experience has been with D22 and recently ABD.

To expand my comment on the AB - unless you’re running heavy tonnage in 100+ car trains, AB is just fine. Unless parts and cores for refurb are getting rare.

I took a look in the part catalogues. The EQ portions of both valves use the same pipe bracket gasket. However, the ABD EQ portion functions quite differently than the AB one both in releases after service applications and in emergency applications and releases. The SVC portion gaskets are two different part numbers, which means these portions have differing porting and stud arrangements.

A further note on studs:

> Perhaps the OEM of the brake parts specified different thread classes,

This could be, but WABCo was rather proprietary about its engineering standards. All the part catalogues show are item piece numbers.

> the stud was probably specified for a "tight" fit on one end and a "loose" fit on the other end.

There are two ways to install or remove a stud: jam two nuts together on the nut thread end and use a wrench on the outer nut or search for a stud installation tool.

If you are going to do that, please don't scrap the old AB valve. You might not have any use for it (and neither do I, for that matter) but it is useful to others in the preservation community as a "core"

You could sell it or trade it back to Multiservice.

ABD is not my thing (there should be plenty of people here to help you with the bolt pattern questions, maybe they just haven't read your post yet) but regarding studs vs threaded rods, don't ever use the latter for air brake component mounting! You'll regret it when you are trying to get that heavy portion onto the studs and the bores of the mounting holes keep getting snagged on the threads. Or when you unscrew the nuts and the "studs" come out of the bracket. Or when the the soft threaded rods get bent and tweaked.

As others have said, proper studs with a tight fit on the one end are readily and cheaply available from places like McMaster. I've changed out many pipe bracket studs which have gotten damaged and I've never had a problem finding the right one. Sometimes the WABCO length was oddball like something and 7/16 of an inch, but usually the next standard size up works just fine. If it causes a problem, it is pretty easy to trim it down on a lathe, restore the rounded or chamfered end, and clean up the thread with a die.

If you use the vise-grip on the unthreaded part method to install the new stud, clean up the mess with a file

As to why I'm considering changing from AB, basically because the UP is suggesting I have to. That's not the final word, and I'm not changing out until I have to, but I want all my stuff ready to go.

I wasn't really considering using all-thread, more curiosity if it was allowed. I see the studs at McMaster, and will go that route once I confirm the length.

No, not getting rid of the AB valves. I actually have other uses for them too.

Greg

Greg,

We've had the same stud length issue here when swapping AB for ABD, and this was simply solved by using a pipe nipple or cut off length of pipe as a spacer to make up the difference.

I think the notion that there is a necessity to change the pipe bracket is incorrect. It is true Wabco often changed stud patterns from one size and type of valve to another to prevent incorrect application, but as far as I'm aware all valves ABD+ were meant to be a direct swap with the good old 1933 vintage AB, and therefore all mount on the same pipe bracket - with the caveat of the stud length. Either that or the pipe bracket gods have smiled on us all these years...

-Erich Armpriester
Strasburg Rail Road Mechanical Services

Greg - I can understand UP wanting at least ABD if they’re moving your cars in freight. I would also guess that UP car inspectors would frown on a pipe spacer in interchange, though probably fine in captive service. You can get the correct studs from Multi-Service Supply and likely other brake shops. I don’t recall the exact size or part number but I can maybe dig it up from a past quote. They’ll know which stud you’ll need, anyway.

Erich - thanks for validating my sometimes not-so-wonderful memory. My biological memory storage device is full and sometimes files get deleted to make room for new.

The stud that is needed to convert the AB pipe bracket to use with the ABD valve is WABCO Pc. 568998. This is a 5/8-11 X 3” mill stud.

Figured I’d share this here.

Ah, yes. AAR Field Manual, Rule 4.

Thank you.