Belly Screws - '67 F

[gwav8or]

I was spending some quality time with my F this week, cleaning the belly.  I noticed I have several screws missing from the belly panels.  It's a stock Mooney, no speed mods.

Anybody know what screws they are and where I can get them?

Pic attached but kinda blurry.

Edited July 9, 2023 by gwav8or

[EricJ]

Take out one of the adjacent screws and you'll see what's in there.   Some are machine thread, depending on where they are and what nut plate is behind it.

You might try taking one of the adjacent screws off and make sure it grabs in all of the empty holes, just to make sure that there's a functional nut plate behind each one.   Sometimes they move or get broken or whatever, but you may need more than just the screws.

Something I did early on in my Mooney ownership that has helped a lot was to buy a stainless screw kit from AS.   It pretty much has stainless replacement screws for everything on the airplane.   There is a stainless and non-stainless kit available for the F.

https://www.aircraftspruce.com/catalog/pnpages/04-02850.php

[N201MKTurbo]

Those screws may be PK (Sheetmetal) screws. They probably fell out because the tinnerman nuts are messed up. Usually because someone put in the wrong screw or over torqued the screws. I would take the whole panel off and and check the tinnermans one at a time with the panel off and replace as necessary. Check the PIC to get the correct numbers for the screws and tinnermans.

More quality time with your belly.

[McMooney]

6 hours ago, EricJ said:

Something I did early on in my Mooney ownership that has helped a lot was to buy a stainless screw kit from AS.   It pretty much has stainless replacement screws for everything on the airplane.   There is a stainless and non-stainless kit available for the F.

https://www.aircraftspruce.com/catalog/pnpages/04-02850.php

I asked an AP about swapping all the screws for stainless 2 days ago, he said he'd recommend against it due to galvanic action.

any truth to this ?

[MooneyMitch]

1 hour ago, McMooney said:

I asked an AP about swapping all the screws for stainless 2 days ago, he said he'd recommend against it due to galvanic action.

any truth to this ?

Maybe consider swapping out the AP person!

[EricJ]

1 hour ago, McMooney said:

I asked an AP about swapping all the screws for stainless 2 days ago, he said he'd recommend against it due to galvanic action.

any truth to this ?

Stainless and aluminum are dissimilar, so they can corrode galvanically in the presence of moisture.   Since the nuts and nutplates aren't aluminum, it's not a huge deal for the actual grip surface, but if there's bare aluminum in contact with the screw in a humid environment for a long period of time, there is a possibility that it might corrode at that site.    I live in the southwest, so it's not an issue here and I've never had a problem with it.

 

[gwav8or]

2 hours ago, M20Doc said:

Without knowing exactly which panel, it’s hard to say.  Mooney used a wide variety of screws in the belly.  AN526-1032 truss head machine screws, AN507-632 countersunk machine screws, #4 and #8 and #10 self tapping sheet metal screws.  Then add fifty years of maintenance and who knows what you’ve got.  A picture further back would be helpful 

Yeah, I didn't realize how terrible the picture was until I got home.  I'll try to get a better pic(s) later this week and post back here.

[MooneyMitch]

My experience with SS screws with various Mooney models over a 35 year period of owner assisted annuals.

I prefer the looks of the SS screw as opposed to the cad plated screw.

After some initial experiences, I never tightened a SS screw so that the cross hatch [Phillips]head was destroyed or difficult to remove.  I don't recall ever losing a SS screw unless the spring nut [Tinnerman] broke or became unable to hold the screw in place.

I never left a screw in place long enough for it to corrode.  Possibly keeping my airplanes hangared was another plus for no corrosion to occur.

 

[MooneyMitch]

7 hours ago, bluehighwayflyer said:

and that this is where the myth comes from that Mooney belly panels are hard to remove.  Once you get it all restored to original they really aren’t that bad.

Yes, always one my favorite tasks was removing  the 10,000 plus belly panel screws!! 
 

The only reason I purchased the Ovation was because the belly panels are held in place by a minimal amount of camlocks ….. another 

[Ragsf15e]

With how cheap they are, I would order enough of the machine screws, and the sheet metal screws to do all the belly panels. Also, order the Timmermans. All new, all the right size from the manual. Do it once, they will last for a long time. There’s a good chance, The ones in there right now are all different sizes and the Timmermans are worn out. I did this once, and it’s real nice.

Edited July 10, 2023 by Ragsf15e

[A64Pilot]

Galvanic corrosion is real, but honestly it’s not that bad, then add in the belly is usually well oiled and that certainly helps prevent corrosion.

I’m only talking SS fasteners and 2024 aluminum

Maule intentionally build their wings and flaps, ailerons etc with el-cheapo hardware store SS pop rivets, once an aircraft that’s kept in Florida and other higher corrosion areas get about 20 years old it’s very common to see a ring of paint missing from the aluminum around those rivets, small ring you have to look for it, but it’s from galvanic corrosion from those SS rivets.

‘One theory is Maule did it so that in the Bush the airplane could be easily repaired with readily available materials. I think being easy, cheap and fast as well as requiring almost no training had a lot to do with it.

But it takes a couple of decades to show up and even then it’s not an airworthiness issue and I think in dry areas of the world it doesn’t occur at all on a Maule.

On edit, should you decide on Cad plated steel screws, and honestly they are better, no or less corrosion, stronger etc. Stick a bunch of them in a piece of cardboard, then spray paint them the same color as the aircraft before you install them.

The proper screw for a Tinnerman nut plate is a Type B screw, not the sharp pointed Type A screw you so often see used.

Edited July 10, 2023 by A64Pilot

[PT20J]

Galvanic corrosion occurs when an less noble metal (anode) is in contact with a more noble metal (cathode) in the presence of an electrolyte (e.g., water with salt or other impurities). The corrosion removes material from the anode. So, in the case of stainless steel in contact with aluminum, the aluminum is the anode and would corrode. So, why is it that you can get away with stainless screws on an aluminum airplane without damage? There are two reasons:

1) If the airplane is mostly hangared and not subject to warm, moist, salty air then there is no electrolyte which is a necessary ingredient for corrosion. 

2) There is an area rule: If the area of the anode (aluminum skin) is much greater than the area of the cathode (screw heads) then the corrosion proceeds at such a slow rate as to be essentially unmeasurable.

Skip

[EricJ]

11 minutes ago, PT20J said:

Galvanic corrosion occurs when an less noble metal (anode) is in contact with a more noble metal (cathode) in the presence of an electrolyte (e.g., water with salt or other impurities). The corrosion removes material from the anode. So, in the case of stainless steel in contact with aluminum, the aluminum is the anode and would corrode. So, why is it that you can get away with stainless screws on an aluminum airplane without damage? There are two reasons:

1) If the airplane is mostly hangared and not subject to warm, moist, salty air then there is no electrolyte which is a necessary ingredient for corrosion. 

2) There is an area rule: If the area of the anode (aluminum skin) is much greater than the area of the cathode (screw heads) then the corrosion proceeds at such a slow rate as to be essentially unmeasurable.

Skip

Since most of the contact between screw head and the aircraft is on paint, the main risk is a potential thin unpainted edge that may contact the edge of a thread on one side of the screw.   It seems to be a small risk, but not zero.

[A64Pilot]

If you want to see galvanic corrosion put Carbon Fiber against aluminum.

[MB65E]

On 7/10/2023 at 3:36 PM, A64Pilot said:

Galvanic corrosion is real, but honestly it’s not that bad, then add in the belly is usually well oiled and that certainly helps prevent corrosion.

I’m only talking SS fasteners and 2024 aluminum

Maule intentionally build their wings and flaps, ailerons etc with el-cheapo hardware store SS pop rivets, once an aircraft that’s kept in Florida and other higher corrosion areas get about 20 years old it’s very common to see a ring of paint missing from the aluminum around those rivets, small ring you have to look for it, but it’s from galvanic corrosion from those SS rivets.

‘One theory is Maule did it so that in the Bush the airplane could be easily repaired with readily available materials. I think being easy, cheap and fast as well as requiring almost no training had a lot to do with it.

But it takes a couple of decades to show up and even then it’s not an airworthiness issue and I think in dry areas of the world it doesn’t occur at all on a Maule.

On edit, should you decide on Cad plated steel screws, and honestly they are better, no or less corrosion, stronger etc. Stick a bunch of them in a piece of cardboard, then spray paint them the same color as the aircraft before you install them.

The proper screw for a Tinnerman nut plate is a Type B screw, not the sharp pointed Type A screw you so often see used.

Have you seen the sheet metal crack along the ribs on your Maule? I been maintaining and flying a ‘76 M5-235. Awesome airplane. But about 6-9 spots on each wing I’ve had to do a little sheet metal dollar patches. Some have even cracked rivet to rivet. I think the skin is so thin, a little stress from the old Hartzell causes the issue. 
 

-Matt 

[A64Pilot]

Are these cracks right back near the trailing edge?

If so they are from using too much flap at too high an airspeed. I have seen those many times.

[MB65E]

47 minutes ago, A64Pilot said:

Are these cracks right back near the trailing edge?

If so they are from using too much flap at too high an airspeed. I have seen those many times.

No all over, random spots mostly along ribs and external around the tanks. Oil canning areas mostly. Most of the wings are pretty thin skinned. Just curious as you have mention Maule several times. Really a neat airplane. Much better utility than anything out of Yakama WA. ;-) 
-Matt

[A64Pilot]

No, if it’s all over I don’t know.

Maybe the washout on the wings is excessive? Just guessing. Of course that’s adjustable by the length of the aft strut.

There shouldn’t be much oil canning, some yes as you say the skins are thin.

Very first Maule had fabric covered wings and I think later they just skinned it.

David Wright in Moultrie has an unusual set of M-5 Wings. Old man Maule built a set with full span flaps and used spoilers for roll control, story is he flew it once  

Edited July 18, 2023 by A64Pilot

[MB65E]

Washout was good and I just put new struts on it. Who knows. But neat airplane! 
I’ve enjoyed your posts and thanks for your service!

-Matt

[A64Pilot]

Maule is a cheap airplane that’s hand built.

That's why I like them, headliner is just fabric with bows like a 67 VW, anyone can replace it. The interior is literally sheet aluminum with vinyl glued to it and a few screws attach it, it’s not even aircraft aluminum it’s roofing flashing available at any home improvement store, it doesn’t break like plastic. When you replace the interior it’s not worth removing the vinyl, just cut more aluminum and glue on new vinyl, or leather I guess if you want to be fancy.

The floor is Luan plywood, also available at the home improvement store with carpet glued to it. It’s in three pieces I think, laid on the fuselage tubing with studs welded to the tubing that stick out between the pieces, fender washer and a nut holds the floor down.

No special molded plastic pieces, it’s all easily hand fabricated and actually looks pretty good  and anyone can do a complete new interior in a day for $100 I guess?

Wife misses the Maule, keeps hinting that she wants it back. But it burnt a LOT more fuel than the Mooney, it’s MPG was identical to the C-210 we had and it was 20 kts slower.

But there are times that I miss it too, flying around with the windows open or the doors off and landing in 300’ in cow pastures has its attraction.

Little boy in the picture is approaching 30 now so this was a few years ago at Sun-N-Fun, the C-140 right beside of us I still have

 

Edited July 18, 2023 by A64Pilot

[MB65E]

How did you get ribbed tundra tires? We just put ABI 26’ on my buddies. 
Sure beats a 400k super cub! 
-Matt

[cliffy]

Thinking of screws and Tinnermans-

Any screw going in that fastens with a Tinnerman nut needs "Type B" screws and NOT standard sheet metal (PK) screws. 

The thread pitch is different on Tinnerman nuts and Typs B screws match that thread pitch'

PKs will go in but they somewhat ruin the Tinnerman nuts. 

Type B screws are blunt on the end where PKs are sharp pointed for self tapping into sheet metal. 

[Andy95W]

This must be some sort of record for thread drift.  From missing belly screws on a vintage Mooney to Maule tundra tires in less than 1 page.

[A64Pilot]

8 hours ago, Andy95W said:

This must be some sort of record for thread drift.  From missing belly screws on a vintage Mooney to Maule tundra tires in less than 1 page.

I’m bad about thread drift, but honestly if one doesn’t drift, they usually die, and so long as the original question was answered I think it’s fine and if it wasn’t then it will come back.

The tires are Thrush crop duster tires, Thrush tires are 10 ply, these 4 Ply, Maule doesn’t need 10 ply it would just be excess weight. The wheels are Alaskan Bushwheels 10” wheel. They are STC’d of course. Tire is 29” tall, 11” wide. Normally the tires have the tread buffed off for weight, I didn’t as I was based off of pavement. I believe full tread 29” tires would last until they dry rotted. They did knock off 10 kts on cruise and killed climb rate too. But the wheels are beautiful quality and the entire brake fits inside of the wheel so no rock can damage the brakes.

https://www.airframesalaska.com/ABI-10-Inch-Wheel-p/abi-1010.htm

This is the Thrush tire, but if you call they have a 4 ply too and I’m sure it cost less too.

https://shop.desser.com/29x11.00-10-10-ply-specialty-tires-of-america,-air-hawk-tire/ecomm-product-detail/323315/

The type B screws body is thicker and the thread pitch different, supposedly a Tinnerman nut plate is self locking with a type B screw because the ears of the nut plate are pulled against the body of the screw, type A not so much.

Edited July 18, 2023 by A64Pilot

[cliffy]

Nice way to bring back the thread drift :-)