Lock-o-seal or stat-o-seal for brake cylinder?

[Rubycoowner]

All,

Rebuilding an original brake Master Cylinder on our 170B. The rebuild kit appears to include both a two-part lock-o-seal (a brass washer and a small o-ring that fits inside the washer) and a single piece stat-o-seal (aluminum, with the o-ring built into the washer). I assume that these are interchangeable? If so, which should I use? If you all recommend the lock o seal, do I insert the o-ring into the washer? Or rest the washer on top of the o-ring?

Thanks!
Alex

[n2582d]

Alex,
Is this the rebuild kit you bought? As you can see the kit is applicable to 15 dash numbers of base p/n 0541138. The Stat-O-Seal (p/n 600-015-6 superseded to p/n 600-0115-6) is used on the master cylinder for the Cessna T303, (p/n 0541138-24). My guess is that it would work as a replacement for the Lock-O-Seal but technically the correct part for the C-170B is the Lock-O-Seal. I'm a bit puzzled by your question regarding the position of the Lock-O-Seal seal to the washer as you correctly describe it as "a small o-ring that fits inside the washer". The piston to Lock-O-Seal clearance is illustrated in S.N.L. 6-5-52.

[Rubycoowner]

Gary,

First: yes, that’s the kit I bought. The lock o seal is in the center of the arrangement (in two pieces). The stat o seal is at the 10 O’clock position. Aluminum, with embedded seal. It appears to be an alternative to the lock o seal, as I don’t see any other home for it

The reason for my question about whether the o ring goes inside the washer (in the lock o seal) vs resting beneath it is that when I took the piston apart, the washer was “on top” of the o-ring. And, in fact, not large enough for the o-ring to fit inside of. I thought for a moment that this was the design, ie when there is slack in the piston, the washer separates from the o-ring, allowing fluid to bypass. When compressed, the washer pushes against the o-ring, creating a seal. But your puzzlement suggests that this is not how it works, and that my brake cylinder may have had a jury-rigged improper lock o seal inside. Which might explain my issues.

[n2582d]

Alex,
Okay, that makes sense — the previous rebuild of your master cylinder was faulty. The master cylinder is a very simple mechanism, but — as you have seen — won’t work if not repaired correctly. I think it would be well worth the expense of having an A&P looking over your shoulder as you rebuild this considering the potential damage that could be caused by a brake failure. The aphorism “penny wise, pound foolish” comes to mind. For example, in addition to getting the .040” gap correct, it would be easy to overlook the need for a vented fill plug.

Bruce has a lot of good information here. There are a few corrections that need to be made though. He wrote,

Lock-O-Seals have other names like Stat-O-Seal. There is also a MS and NAS equivalent.
When searching for Lock-o-Seal 200AC-6, 800-001-6 comes up most often.
Most of the 800-001-6 seals I found where in the $6.50 range.
If you search Aircraft Spruce for 800-001-6 or Lock-o-seal 200AC-6 you get STAT-O-SEAL NAS1523-8B for $2.45

As this Parker catalog shows, there is a difference between the Lock-O-Seal and the Stat-O-Seal, the primary one being that the Lock-O-Seal has two separate parts, an o-ring and a washer, while the Stat-O-Seal incorporates those two parts in one unit. That catalog shows that Parker’s Lock-O-Seal 800-001-6 is now 800-0001-6. Further, that p/n is not the same as NAS1523-8B. The Parker catalog shows that the matching NAS p/n is NAS1523AA06B. Whether one is looking at the Lock-O-Seal 800-0001-6, Stat-O-Seal 600-0115-6, or NAS1523AA06B, the OD of the aluminum washer part and the ID of the seal/oring is the same, or nearly so, on all of them. The Cesssna T303 master cylinder (which uses the Stat-O-Seal) has the same part numbers as the C-170 B master cylinder (which uses the Lock-O-Seal) for the piston rod, piston rod sleeve, and piston. In other words, I believe that, while the Lock-O-Seal is the correct part, any of those three part numbers would work.