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Looking for bolts

Ed_Wischmeyer

Well Known Member
So here's the deal...

Time came to overhaul one of the Matco brake master cylinders, and it turns out that they will overhaul all four for cheap -- $125, including shipping them back to me, versus $52 for four overhaul kits.

They told me that the conventional way of mounting the master cylinders to the rudder pedals -- one long bolt with a washer stackup(s) -- can put side loads on the master cylinder and lead to wear. What they recommend is a bolt with threads all the way to the head so a nut can be threaded on all the way to rigidly mount the bolt to the rudder pedal. This eliminates standard AN bolts in the lengths needed. The rest of the washer stackup would be the same.

They recommend a Grade 8 steel bolt, and to be compatible with the AN hardware, that would be 10-32 diameter and thread. Plans call for 7/8" and 1 1/8" length, four each (dual brakes).

Can't find em!! Closest I could come was stainless, and my IA said no. I did find some 10-24 (IIRC) hex heads at Lowe's, but not enough of them and no matching nuts. They were out... (I did find a lot of empty plastic envelopes in the drawers, indicating significant pilferage...)

Anybody got any *specific* leads that they know work?
 
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McMaster has some slightly longer ones (1" and 1 1/4). Out of curiosity what's wrong with stainless?
 
So here's the current "solution"...

Normal stackup is bolt head, rudder pedal, washers, master cylinder, castellated nut.

Matco recommendation is bolt head, rudder pedal, nut, washers, master cylinder, nut.

Suppose we turned the bolt around and used two nuts to hold the bolt firmly against the rudder pedal? Then I'd get the bolt firmly mounted to the rudder pedal and be able to use AN hardware.

McMaster *almost* has the parts I need, but for the bolts longer than 1", the threads don't go close enough to the head.
 
What kind of forces are there on the pedals? 10-32 stainless bolt will support well over 1000 pounds in shear I think.

Also, this looks like a fully threaded 1 1/4" grade 8 10-32 bolt: https://www.mcmaster.com/92620A203/. Is it not what you're looking for?
 
You might consider...

You can look at Bumax stainless or A286 stainless. They are nearly twice as strong as plain stainless...
 
What kind of forces are there on the pedals? 10-32 stainless bolt will support well over 1000 pounds in shear I think.....

It looks like the bolt is loaded in bending for this application. That means that the bolt has to carry shear, bending and tension from the preload, all at the same time.

Dave
 
What kind of forces are there on the pedals? 10-32 stainless bolt will support well over 1000 pounds in shear I think.

Also, this looks like a fully threaded 1 1/4" grade 8 10-32 bolt: https://www.mcmaster.com/92620A203/. Is it not what you're looking for?

most of the fleet uses AN bolts for this application and they have a tensile of 125K and shear of 75K.

There are numerous SS bolts the meet or exceed this strength, though some are less. Mcmaster should list the strengths for each bolt type. I would not put a grade 8 bolt in that application. Their strength also comes with a significant increase in brittleness over a grade 5 or AN bolt. I would rather my brake attach bolt bent than snapped in two if excessive force was applied. Grade 8 bolts don't bend at their yield like Grade 5, instead they snap in two.

Just like the addage that bigger is not always better, stronger is not universally better. There is a reason that AN bolts are not Grade 8. The industry would rather upsize the bolt spec'ed to support a load than specify a bolt that could snap instead of bend at the yield.

Larry
 
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:eek:
It looks like the bolt is loaded in bending for this application. That means that the bolt has to carry shear, bending and tension from the preload, all at the same time.

Dave

Big Thumbs UP

Which is exactly why there is a moment generated at on the master cylinder housing. A spherical joint would work, as the steel tab is going to deflect too.
 
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Hmmm...

Attached pic.

Maybe replace the 4 AN960 washers with a single 1/4" (.275") spacer that has a nice "cup" machined into the outside face...

What am I missing here?
 

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Attached pic.

Maybe replace the 4 AN960 washers with a single 1/4" spacer...

What am I missing here?

Timely post! I’m currently working on brake pedals and master cylinders, and also considering using a spacer here instead of a stack of washers.

Also, to mount each brake pedal to its respective rudder pedal I used a long 3/16” 4130 steel rod secured at each end with a washer and cotter pin. That method has been in use on my 170 for nearly 3400 hours over the last 65+ years, so I figured it would be good on my RV-7. At least it should help alleviate the binding common in that area when short bolts are used on each side.
 

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Tensile?

There is really no tensile load in this application because the last nut can’t be torqued, otherwise the pedals will bind. The problem with using a fully threaded fastener is that the threaded shank is subject to bending loads which can cause failure. Applying the recommendation of using a jam nut to secure the bolt to the pedal does reduce the bending moment quite a bit and puts the bolt mostly in single shear, but there will still be some bending load beyond the nut.

Skylor
 
So I got bolts from McMaster and cut them to 7/8 and 1 1/8, just like the plans call for. Oops! Looks like 1” and 1 3/8” are better lengths. Expensive mistake, but it’s so much easier to cut the bolts at home, very difficult or worse in the hangar.
 
My cost on overhauling Matco master cylinders is less than $0.50 in o-rings.
Just did a set of four leaking master cylinders for a customer of mine a few weeks ago.

Side loading scratches the shafts which causes the o-rings to leak. I typically chuck those up in my lathe and polish the scratches out with 600 grit emery cloth.

Van's brake attachment methods could definitely be improved upon using heim joints and a fully threaded screws at the bottom so the bottom "pivot" can made more rigid. There are AN screws that can be used for this purpose. I used both of these parts on my F1's brake pedals.
 
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NAS Bolt/Screw

I was working on this exact issue. Pulled rudder pedals, overhauled brake cyls and parking brake, installed the upgrades on the rudder pedal torque tube, rebuilding pax rudder pedals, powdercoating.

This is a test fit up of an NAS bolt (fully threaded) with a thin jamnut and the Pegasus saddle washers. I was contemplating a thin nylock for the jamnut on final assembly. Once tightened, the NAS bolt acts like a solid pin so I'm thinking the brake cylinder will not be sideloaded as much.
 

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The standard installation has the brake cylinder eye bearing on the smooth shank of the bolt, whereas some of the modifications discussed in this post have the eye bearing (and pivoting a little) on the threads. Isn't that likely to be an issue for wear?
 
Yes, it's a compromise. Don't later RVs have the full moment arm bracket on both sides so you can run a bolt/shaft full width across the rudder pedal?

The standard installation has the brake cylinder eye bearing on the smooth shank of the bolt, whereas some of the modifications discussed in this post have the eye bearing (and pivoting a little) on the threads. Isn't that likely to be an issue for wear?
 
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