Which way to install motor mount bolts.

[Greg Bockelman]

Generally accepted practices for installing bolts is to follow the “IDA” philosophy. In, Down, or Aft. On my motor mount, the bolts through the firewall were installed from the inside, opposite of that. I kind of like that idea because they are easily looked at during preflights.

The manuals have no guidance on this. What say you? Nuts on the cabin side or engine side?

 

[Gary Ward]

Do you have any pictures before it was taken apart?
My nuts are facing forward on my mount. It never has been apart I can say with confidence.

 

[Bell206]

Nuts on the cabin side or engine side?

I usually follow the direction shown in the OEM MM or even in the IPC if MM not shown, first. And while standard practice is bolt head up or forward in direction of flight or rotation as you mentioned, bolt clearance, access, and application of proper torque are another considerations in most cases.
For example, Cessna shows engine mount bolts installed in opposite directions between the 172 and 182. I don't remember which one had bolt heads forward, but I would follow what the MM showed regardless the standard practice of bolt head forward.
What aircraft?

 

[BillTIZ]

Looking at the PA-25, from memory, the bolt heads were on the cabin side of the firewall and nuts forward. No space to put a torque wrench to the bolts inside the fuselage and manipulate it. Looking at the PA-25 Service Manual, it does not specify other than bolt size about fastening the engine mount to the firewall. It does specify the orientation of the bolts and lord mounts attaching the engine to the engine mount. The bolt heads against the lord mount and the nuts against the engine mount. No spinning nut while tightening against the rubber lord mounts. Top down, bottom up.

 

[Dan Thomas]

I usually follow the direction shown in the OEM MM or even in the IPC if MM not shown, first. And while standard practice is bolt head up or forward in direction of flight or rotation as you mentioned, bolt clearance, access, and application of proper torque are another considerations in most cases.
For example, Cessna shows engine mount bolts installed in opposite directions between the 172 and 182. I don't remember which one had bolt heads forward, but I would follow what the MM showed regardless the standard practice of bolt head forward.
What aircraft?

That right there. The information is in both the maintenance and parts manuals, and many of those are available online, free.

There's a story about some B-17's (I think it was) that were crashing during WW2. It was finally determined that a factory worker was using the in-down-aft philosophy on a certain bolt instead of the orientation shown in the drawings and instructed to him during his training. His obsession with IDA killed a bunch of people. The bolt end was striking something in certain scenarios, limiting control travel of something critical.

 

[Clip4]

Generally accepted practices for installing bolts is to follow the “IDA” philosophy. In, Down, or Aft. On my motor mount, the bolts through the firewall were installed from the inside, opposite of that. I kind of like that idea because they are easily looked at during preflights.

The manuals have no guidance on this. What say you? Nuts on the cabin side or engine side?

Unless you have guidance the bolts are in the wrong direction now, I would not change it.

 

[Let'sgoflying!]

 

[Greg Bockelman]

OEM MM is a joke, for the most part. 1947 Manual not helpful. But what WAS helpful was Bill’s comment about getting the wrench on the nut. That is not possible if the nut was on the cabin side of the firewall. So, in my case the nut has to be on the engine side.

Thanks all for the input.

 

[Dana]

Down or Aft makes sense (In less so) in cases where the bolt is loaded in shear; if the nut is lost or missing gravity or airflow will tend to keep it in place so it can still carry loads. On a motor mount where the bolt sees mainly tension loads, it doesn't matter; if the nut falls off the mount is gonna come loose either way, so it makes sense to have the nut where you can inspect it (and get a wrench on it as others have pointed out).

 

[Checkout_my_Six]

Most cases the nut is always on the trailing edge of the free stream flow......cept engine mounts where access is important.

 

[Brad W]

OEM MM is a joke, for the most part. 1947 Manual not helpful. But what WAS helpful was Bill’s comment about getting the wrench on the nut. That is not possible if the nut was on the cabin side of the firewall. So, in my case the nut has to be on the engine side.

Thanks all for the input.

But could it be that you just don't have whatever specialty wrench was designed for that application?

Maybe it's a low profile wrench...a stubby wrench.... a right-angled wrench...etc....​

​

I'm not at all suggesting that this is the case here....just saying that I'm not going to be always convinced that just because I don't have a wrench to fit a nut one way, that doesn't necessarily mean it's the wrong direction.

 

[Silvaire]

Well, since the engine is tugging on the airframe having the bolts face aft wouldn't do much for you anyway. At least this way if a nut comes off you have a chance of seeing it when checking the oil. When is the last time you had your head crammed up under the panel looking at the backside of the firewall?

 

[Dan Thomas]

When is the last time you had your head crammed up under the panel looking at the backside of the firewall?

Hah. I have found awful stuff in there, proving that many mechanics never crawl under the panel during an annual.

The bolts in a Cessna are inside a hat-section tunnel, nearly impossible to see through the small access hole. One needs a socket on a short extension on a U-joint on a ratchet to hold it while someone torques the nut. Have to take the glove box out to get at the top right bolt, and usually both rudder bar covers come off to get at the lower bolts.

 

[Greg Bockelman]

But could it be that you just don't have whatever specialty wrench was designed for that application?

Maybe it's a low profile wrench...a stubby wrench.... a right-angled wrench...etc....​

​

I'm not at all suggesting that this is the case here....just saying that I'm not going to be always convinced that just because I don't have a wrench to fit a nut one way, that doesn't necessarily mean it's the wrong direction.

We’re talking torque wrenches. I’ve never seen a torque wrench that fits those descriptions.

 

[Greg Bockelman]

Well, since the engine is tugging on the airframe having the bolts face aft wouldn't do much for you anyway. At least this way if a nut comes off you have a chance of seeing it when checking the oil. When is the last time you had your head crammed up under the panel looking at the backside of the firewall?

You would be surprised. But never during a preflight.