Thorp Air Command - T18.net

I want to move my battery (Odyssey PC-680) to the firewall. I am not willing to drill through and mount the battery directly to the firewall unless I remove the fuel tank and install some re-enforcement. I would like to avoid that option if possible. I have what appears to be an unused engine mounting point left over from the V-6 that I could attach the outboard end of a battery box to. The bracket next to the engine mount and the existing unused nutplates have no support behind them. What is the general opinion for using the “A” frame to support the inboard end of the battery?

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Hello Jeff, what I ended up doing is drilling into my firewall and used 1/4-20 stainless steel nutserts, I understand not wanting to drill into your firewall but removing the fuel tank was not a option for me so far the battery box is still solid just as the day I installed it....Ira

Hi Jeff,
I'd use nutserts like Ira did also, as necessary, but the firewall can be pretty flexible in areas. In my opinion, using the landing gear A- frame with Adel clamps is a fine idea and about as solid a mounting point that there is in that area.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

My firewall is really soft. The drill kind of mushed through it when I made the hole for the new fuel line last winter. I haven't stripped any of it to identify the metal but it isn't stainless and it isn't magnetic.

I don't have a problem drilling into my firewall. I DO resist any desire to install nutserts. When I bought my airplane, the lower cowl could not be removed because of several bad nutserts. Some were replaced with like fasteners and others were replaced with any alternate I could make work, to include nutplates, hard to reach lock nuts and tinnermans. As long as they hold, nutserts work well but, when they fail, they are hell. Sometimes there isn't a choice but I would rather pull the fuel tank than install one in my firewall... just the opinion of a career aircraft mechanic.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

You're right, Jeff. In my experience it's often hard to drill a nice, clean, close tolerance hole in thin stainless, especially without some backup on the opposite side, and you need a good hole, exactly sized for material thickness, for a reliable nutsert installation. There are also differences in quality with different brands of nutserts. "Tric Nut" was one of the originals, and they are much better than some of the others on the market now, some of which are not even serrated at the base where they contact and expand against the hole. That is a mandatory feature in my opinion. I like to use some cylindrical retaining compound during installation in some instances also. Similar situation with Rivnuts, where some knock-offs of the original Goodyear brand don't even have the "key" provision, requiring a notched hole. If installed properly they can be very reliable. Probably like you, I've seen some poorly installed examples of both types of blind nuts... sometimes makes you wonder, doesn't it?

Your firewall should be either galvanized steel or S.S., as per the prints. It could be annealed, soft stainless - easier to form the flange. Unlikely that it's titanium! If it's aluminum, someone did a very stupid thing, IMHO.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

That firewall better be stainless! I hate drilling stainless. Lots of pressure and very low rpm. Be advised, the gear does move especially during landing.
Cubes

It could be annealed stainless. I don't have any prior experience with stainless that soft. Other firewalls I have cut into were harder and easier to make a clean hole. So far, as near as I can tell, the builder stayed pretty true to the drawings.

I know there are rubber spacers at the bottom of the firewall but isn't the "A" hard bolted at the top? It looks like it is but I don't have the drawings handy to confirm it.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Yes it is. But it all moves a bit. That's why you see crack discussions pop up. I'm not saying not to do it. Just be advised. And don't put anything on the gear or engine mount unless it has Adel clamps.
Cubes

Thanks for the input guys.

John, thanks for the additional info on the rivnuts and nutserts. I have always made sure I used serrated or keyed but I didn't know anything about the brands. I probably wouldn't be so sour about them if I didn't work on Cessna's. Cessna likes to use them next to tinnermans on some airframes and it seems like people are always forcing tinnerman screws where they don't belong.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I removed the 27 pound battery from behind the baggage compartment and removed several pounds of battery cable, box, and auxiliary power connector.
I fabricated the battery box in the picture, and found enough structure behind the firewall to support it. The box is riveted together and adds strength when riveted to the firewall so it's not flexible. It's not complete, I still have to fabricate the battery hold-down.
The box fits a PC680 as well as this EarthX LiFePO4 so I have battery options later.

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Scott Emery
EAA Chapter 326
T-18 N18TE

I dropped my tank and used reinforcement on the inside of the firewall for mine. It was a pain to do but I didn't want to risk riveting into the firewall with no additional support.

Put a doubler plate on the inside of the firewall and used domed , self sealing , adjusting nut plates . Bolted the PC-680 battery box to the firewall . Muey solid !

RB

I installed an remote oil filter on the firewall and put a doubler on the outside using the two 3'4 angles that are on the inside of the firewall located one on the side and the other in the middle. Drilled out the rivets ,used cherry max. This is for guys that have ham hocks for hands and not little dainty ones like Rich that can get behind the gas tank. I bet you did this before the tank install?

This would be my preferred method if I remove the tank. If I have help when doing the work I would opt to save a few bucks and use nuts or AD rivets instead of nutplates to hold the doubler. The box would get nutplates in case it ever needed to be removed.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Hmmmm, just tried to pull some info from a drawing and the search lead me to one I don't have.... again. Is there any information on drawing 580 that might be of use to me for moving the battery? Structure spacing on the firewall is what I was trying to find without going to the hangar and opening up the airplane to pull a couple of measurements.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson