Restoration Questions

[Ryan Smith]

Hi all,

I’ve decided to double down on my insanity with my annual-turned refurbishment/restoration. While replacing my gear box extrusions, my IA pulled my front floorboards and it afforded me a little better glimpse up front with the rudder pedals and associated bits out of the way.

I planned to do a vacuum system delete and replace the boot cowl since the venturis on my airplane were installed on the wrong side, and while I’m doing that I have ruminated on my displeasure with the state of my firewall and lofted the idea of replacement. While I’ve got everything exposed, I decided to go for it.

Never one to not go for broke, I decided to upgrade to the later model heater manifold, largely driven by the fact that Preferred Airparts had the later model firewall in stock.

I bought a salvage firewall manifold and measured it, drew it up in CAD minus the defrost duct outlet and had it cut and will take it to my mechanics to bend and then paint and install.

My issues are these:

1. How have folks addressed the difference in the mixing valve ducting with the older plenum-style baffling? There isn’t a lot of room beside the blast tube to add the fresh air intake with the lower profile of the plenum and I don’t want to mutilate my brand new powder coated AirForms baffles which have increased in cost 44% since I bought mine less than a year ago. Could I relocate the blast tube mount? Is the later style blast tube different from the early style? I assume both blast tubes are directed at the oil temp probe? The only person I recall seeing do this is Richard Pulley and he had a scab plate made and put the mixing valve over his old one on the left side of the fuselage.
2. Are there drawings or dimensions or does anyone have any factory original upholstery backing panels that they wouldn’t mind loaning me for non-destructive duplication for the heater ducts for the later model heater system?

Unrelated to the heater system, I’ve got two instrument panel questions.

At the next annual I plan on an avionics update. I was lucky to get a fixed piano key style panel that’s in excellent shape to use at the basis for my upgrade. I will be using Aerospace Logic gauges in place of the 1 7/8” gauges and want to replace the keyway for the clock. I’ll obviously have to remove some of the radius to allow the slightly larger gauges to fit, but I want them to be round holes. I’m hoping someone has a damaged panel or one they cut out for a center stack upgrade that they wouldn’t mind selling one of the lower instrument holes so my metal guy can use that to weld the repair rather than him having to form it.

Lastly, I’m looking for a piano key bracket 0412475 to borrow to draw up some circuit breaker mounts. Alternatively, if someone could give me the diameter of the switch holes and the fuse holes, as well as the distance between them and an approximate bottom corner radius, I’d be forever grateful.

Kindest regards,

Ryan

[brianm]

Hi Ryan,

Regarding the fuse holder, I've attached a tracing of mine. It was scanned at 600x600 dpi and I included a machinists rule for reference. You should be able to import and trace it in your CAD package. It was made while everything was installed in the airplane, before I decided to take it all out to clean it up and replace the grommets and switches, so it doesn't include the switch holes.

My measurements have the fuse holder holes at .630" diameter and 1.15" on center. The distance on center between the innermost holes is 4.7".

I drew up an adapter that holds 10 P&B fuses and had an online service cut and bend me two of them out of .040" aluminum. Total cost was $31 shipped to my door (and they threw in a third copy for free). Can you see my mistake? The breakers (or 3d printed substitutes for now) sneak into the slots that fit around the bends on the fuse holder panel. My actual fuse holder is in pieces in the hangar until it warms up a little around here so I haven't been able to see if it fits or if I'll need to redo it.

[n2582d]

I drew up an adapter that holds 10 P&B fuses and had an online service cut and bend me two of them out of .040" aluminum. Total cost was $31 shipped to my door (and they threw in a third copy for free).

Considering the time it would take to fab that up, $31 seems quite reasonable. Could you share the “online service” you used?

[brianm]

Ryan, got to the hangar today to take some measurements. The switch holes are 15/32, same as the boss on an AN switch. The spacing is the same as the fuse holder holes.

Gary, the service I used is SendCutSend.

And it fit:

Just need to paint it and drill a couple of holes some screws to help keep it located.

[Ryan Smith]

Brian, can you measure the distance from the center of the switch to the center of the circuit breaker?

Part looks great! I used Send Cut Send to cut my heater manifold and they did a very nice job. I’ve used Xometry for making a beacon mount for my Whelen 7108051 as well and was very pleased.

[Ryan Smith]

For anyone remotely curious…

I have fretted and analyzed and believe I have a solution with the baffling situation. After speaking with my IA and a DAR, they agree it’s a non-issue with respect to the legality.

I’m going to delete the blast tube on the RH side and install a union from the later model baffling, and then upsize the LH blast tube to 1.25” ID from .090” ID to approximate the same volume of air. After speaking with a buddy that’s a talented aerodynamicist he felt that was sufficient despite the loss of velocity with the increased diameter of the tube.

This is the most streamlined solution of the ideas I had and yielded the least modification and retained the most number of stock parts.

[n2582d]

Ryan,
Thanks for online metal fabricating leads. Now what do you all do for a CAD program? I'm not paying $2030/year for an AutoCAD subscription.

This really is a baffling question. Here are a couple of random thoughts:
-Don't know if you're going with a Tempest oil filter or not. Cessna Accessory Kit AK172-68A provides a new right side blast tube, p/n 0550224-4, with this oil filter kit. The new tube is aimed at the adapter. I think I bought one of the last NOS ones in existence.

-You've probably already examined this but it appears there are three spots on the rear baffle that would fit a 3” duct. Mounting it inboard of the right blast tube would require relocating the oil breather line. Mounting it on the left side of the baffling would involve getting the ducting around (or over?) the accessory case to connect to the valve on the right side of the firewall.

-Cessna used an oval duct adapter, p/n 1253031-3, on the C-172 heat duct manifold. This might be an alternative to a 3" round duct adapter. Would there be enough room to mount this above the rear baffling right in the center, attached to 90º adapter which would be riveted to the top center plate of the plenum?

[Ryan Smith]

I use AutoCAD. I use it enough to justify the yearly premium, mostly because the things I draw up are unobtanium. My next drawing projects are a mirrored 0523106 landing light bracket assembly to install a RH landing light in my airplane and inter-cylinder baffles for my Bonanza.

I’m happy to draw up anything you need, Gary. It’s the least I could do for the amount of info you’ve given me over the years.

I bought a 1253031-3 off of eBay and will see about using that and keeping all the blast tubes intact, and putting it to the right of the RH blast tube, but I’d be happy with the single larger blast tube. My baffles are powder coated and I don’t want to make a mess out of them with relocating and adding a bunch of holes. I initially thought I’d mirror what Richard Pulley did, but I don’t want a bunch of SCAT hoses running everywhere. I also don’t want to install an oil cooler for weight and complexity reasons. My airplane has a screen only; no filter adapter. I’m a bit of a simpleton and would like to keep it that way.

That said, I have a filter adapter on the E-185 on my Bonanza, but I will keep the 170 stock in that regard.

[brianm]

Gary,

For 3d work I use Fusion 360. They have a free hobbyist tier. For 2d I use QCad. There is a free version and the pro version is something like $50. It is more than adequate for my projects.

Ryan,

I measured 1.28 - 1.29 edge-to-edge between the switch hole and fuse hole. By my math that comes out to about 1.84 center-to-center. It varies a little and my calipers aren't great.

[n2582d]

Ryan,
That's very generous of you to offer your drafting services. Thanks! I'm sure you and Brian are not the only ones in the Association that have these skills that the rest of us envy. What I'd love to see is a section in the Mx Library with a C-170 part number listing of CAD drawings that you all have drawn up. Members could then email or PM the author of the drawing to buy a copy of the file which they could then send to SendCutSend, Xometry, etc. If one looks at the simplest of parts like the various landing gear shims to more complex parts like the fuel valve block and what Cessna charges for them, a service like this would be invaluable for members. In my book -- if the appropriate boxes are checked -- these parts would qualify as owner-produced parts rather than SUP (suspected unapproved parts).

I'm doing the same conversion/upgrade as you on the heating/ventilation system. Here's some further thoughts I've had regarding this while waiting at the dermatologist to give my ear the Vincent Van Gogh look:
-If the oval duct is 1.5" wide with the same circumference as a 3" circle, the area will be 25% less than a 3" circle.
-At the Continental factory training course, to emphasize the importance of maintaining tight baffling, the instructor said that "a finger-size hole in the baffling results in a 20º F rise in CHT." So, should we be concerned about cutting a 3" diameter hole in the rear baffle for cabin air? In this conversation Richard seems to show that Continental's rule-of-thumb may be suspect.
-It sounds like you are abandoning the venturis. If so, you might consider adding the fresh air valve on the right boot cowl and using the single heat duct on the firewall. Approval for this alteration might be more challenging than just updating to the late C-170B style heat/vent system. Over the years Cessna used several variations of this firewall valve. Most fiberglass ones I've seen online look like crap - the resin has been cooked. The best I've seen is p/n 0453016-1, a cast aluminum version that is used on later 172's.

p/n 0453016-1

p/n 071315-2