knobs

[clayton991]

My A model has a classic piano key panel in great condition except for one damaged mixture control knob. All knobs match, sort of a pink-tan color....any ideas where I could find a replacement?

[Mike Smith]

Wouldn't you want the mixture to be slightly different? Something like a red knob might be the ticket. ...just a thought, good luck in your search.

[GAHorn]

Clayton, you might be in luck. Let me look in the hangar when I get home on Wed. (Remind me if I forget. Convention Host dementia.)

[doug8082a]

The knobs are fused to the cables so you can't replace just the knob. If you want everything to match original, then you'll have to scrounge up an original control (cable w/knob) either through folks in the Association, on the forum, or the salvage yards.

If you are willing to go with a non-original replacement, then there are a couple options:
1. You can get a replacement from the Cessna supply houses (Hill Aircraft, Yingling, etc) that is nearly identical. The only difference being a knurled edge around the circumference of the knob. You would have to paint this to match the rest of the knobs if that is what you are after.

2. Switch to a vernier control

[doug8082a]

Looks like George is already on the case...

[GAHorn]

My own airplane was "matched up" by the owner having all the knobs coated at the same time to the desired color. (Plain old ordinary Dupont Imron paint does a great job and, because it's a urethane, is durable because it bonds to the plastic knobs.)
Another way to achieve the desired result is to obtain a used cable, pull out the knob (and all it's solid-wire core) and insert it into an existing cable-housing (if the replacement knob/wire is not too short.)
Don't assume I've got his problem solved. Anybody else who can help out....speak up!

[clayton991]

Thanks to all--
Good suggestions....acft salvage yards are some of my favorite places and paint matching is do-able, although it is an odd color.

[doug8082a]

Make sure you measure your current control length and verify the length of the one you are buying. Just because you get "an original" mixture cable from a salvage yard does NOT mean it's the proper length... been there, done that. Just because it fit the donor 170 doesn't mean it'll fit yours... even though you'd think it would. It's not unusual to wind up short by some annoying little amount... like 1/2 inch.

[GAHorn]

Very original looking control cables are available new from both Univair and Spruce. I recommend the ACS A-730 (glide/smooth) and A-740 (ratchet) types. The ratchet types are especially good for mixture, cabin air and heat. The knobs can be urethane-painted any color if you don't like the red or black they come in. (Mine are painted light grey as they were originally from Cessna. They have not chipped or worn-thru in 10 years.) <edit:20 yrs now and the exact color is dupont “gull” grey>
http://aircraftspruce.com/menus/ap/throttle.html

[clayton991]

Alright, George, thanks. That ACS-730 looks good. Thanks for the link. Doug, your advice is well taken....I restored a Cessna 140 a few years back and ran into every 'been there, done that' imaginable.
Thanks,
Rich

[gwillford]

I also had this problem. I had two bad cables that I wanted to replace but I wanted my knobs to match. I came up with a solution that I thought I would share. I came across a cable print. I noticed a note that states "heat knob and press on". I had an old cable with a good knob and a new one with a non matching knob. I tried an experiment. I removed the inner wire of the old cable and placed it in a vice. I wrapped tape around the knob as well as a pair of pliers so I would not scratch the painted knob. I heated up the wire with a propane torch. Heat a few inches away from the knob! Bring the heat up slowly and begin pulling on the knob. When soft, the knob will come off. Mount the new wire in the vice and then heat it up. Press the knob against the wire. When soft enough, the knob will slip on. Hold in place until cool. Final cool under cold water. Print says pull test to 100 lbs. I gave it a 163 lb pull test (big lunch). If you are patient and careful, you won't even have to repaint the knob! Mine turned out great. Good luck!

George

[clayton991]

Excellent. Thank you very much.

[n2582d]

Reviving an old thread here.

On the pre-53 C-170’s Cessna designers made an concerted effort to have the panel as symmetrical as possible. Cut an imaginary vertical line through the centerline of the panel, splitting the throttle in two. Now, fold one half over the other and the instrument holes, piano key switches and the push-pull knobs would perfectly align with their opposite half. To maintain this symmetry, Cessna went so far as to add a fake piano key in the second from the right position and a “optional control provision” knob next to the mixture control knob. These knobs match the symmetrically placed primer and cigar lighter knobs as well as the smaller but similarly-styled wing root vent knobs. It’s a shame to see this design element compromised with a mish-mash of random knobs replacing the original ones. I’m sure you too have seen on a lot of C-170 panel photos a “variety pack” of knob shapes and colors. This mix of knobs is unfortunate but understandable as the original bowden cables / knobs are not available new. Here’s my solution.

Groov-pin generously sent me several samples of their 10-32 Barb-sert threaded inserts, p/n B19032-30. A quick Google search shows that these type of inserts are readily available from various companies.

Clamping the old inner wire in a vice, I heated the metal rod which the knob is attached to as George Willford describes above. The infrared thermometer indicated it was around 225°F before the knob softened enough to pull off.

Next I drilled an 1 1/16” hole about 1/4” deep in a scrap piece of plywood which was clamped to the drill press table. Without moving the plywood jig, a 1/4” drill was used to slowly drill the knob, taking care not to drill any deeper than the length of the insert. The head of a 10-32 bolt was cut off to make a mandrel for the Barb-sert to be screwed onto. Finally, with the mandrel chucked in the drill press, I heated the mandrel/insert to around 225°F and quickly pressed it into the plastic.

Click to Enlarge
Barb-sert on 10-32 mandrel

Click to Enlarge
Pressing heated Barb-sert into knob

McFarlane sells two versions of bowden cable that have 10-32 male threads for the knobs. MCU224-048 is a creep resistant control cable with a high temperature Teflon lining and MCU124-48, their regular (less friction) one. I found my original color knobs attached to worn out cables at Faeth Aircraft Salvage.