Fastener Orientation

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pfarber

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The FAA textbooks are no longer published as AC's. However, in FAA-G-8083-30A, AMT General Handbook, it's on page 7-49:
"Whenever possible, place the bolt with the head on top or in the forward position. This positioning tends to prevent the bolt from slipping out if the nut is accidentally lost."
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Dan Thomas

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I am putting the engine mounting bolts with the nuts facing forwards. They are easier to install that way and also far easier to inspect. If I had them with the nuts to the rear, they will be a pain to even see, let alone install them on the bolts.
That is typical on the Cessnas and many others. The bolts are inside hat-section tunnels that transfer stresses to the skin, and just getting the bolts into those holes is fun.

While you're at it, make sure there's no paint on the engine mount bolt tubes. You want the best contact possible between that mount and the firewall surface and nut/washer contacts. Poor engine grounding leads to poor starter cranking, alternator noise, and electrical engine instrument over-reading.
 

pfarber

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While you're at it, make sure there's no paint on the engine mount bolt tubes. You want the best contact possible between that mount and the firewall surface and nut/washer contacts. Poor engine grounding leads to poor starter cranking, alternator noise, and electrical engine instrument over-reading.
Shouldn't the engine have its own ground strap? Or are the vibration mounts electrically conductive??? Every certified plane and car I've ever seen has always had a direct ground.

Ideally you want to ground to a spot close to the battery's ground on the firewall, but that may lead to an overly thick/heavy strap.

Engine mounts are reamed to size, so you don't want paint in there to cause a poor fit.
 
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Dan Thomas

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Shouldn't the engine have its own ground strap? Or are the vibration mounts electrically conductive??? Every certified plane and car I've ever seen has always had a direct ground.

Ideally you want to ground to a spot close to the battery's ground on the firewall, but that may lead to an overly thick/heavy strap.

Engine mounts are reamed to size, so you don't want paint in there to cause a poor fit.
The engine's ground strap typically clamps to the engine mount, and it's at the firewall/mount interface that trouble arises. In fact I've found very poor clamping of that strap on the engine, too. Paint, grease and oil under it, clamp stretched so it's not tight anymore, paint on the engine under the strap at that end.

The alternator often has its own ground cable from the alternator case to the firewall, and when the engine's grounding is bad that cable ends up carrying starter current. It's usually a smaller than the alternator output, too, so it can get warm during cranking and really slow the starting. If that ground cable is poorly bonded, the engine's ground strap takes the alternator ground current, and the engine instruments end up going wonky if that grounding is even the tiniest bit resistive.

Worst-case would be the engine control cables carrying those hundreds of amps of starter current to the instrument panel. I've heard stories of those cables getting cooked.
 

TFF

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On my Triumph Spitfire, the clutch hydraulic line ended up as the block ground. I burned my hand on it one day. Why is that hot? Exhaust is on the other side of the compartment. Nice big ground strap on the block to chassis not doing a thing. Ran a different one.
 

Dan Thomas

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I have personally arrived right after an engine start was attempted on a homebuild. All the wires were in place apart from a ground strap from the firewall to the engine. There was apparently quite lot of sparks and smoke from the throttle cable housing when it had to carry all the starter current!
And that throttle control would be trashed. All the hardness and temper in the wires that make up the housing and push-pull wire would be eliminated, and even if the thing still worked it would fail soon enough.
 

Pops

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On my Triumph Spitfire, the clutch hydraulic line ended up as the block ground. I burned my hand on it one day. Why is that hot? Exhaust is on the other side of the compartment. Nice big ground strap on the block to chassis not doing a thing. Ran a different one.
Is the Spitfire's hydraulic clutch built like the TR-3 ? I used to help my neighbor when I lived in Pittsburgh, Pa with his 3. Unbolted the floor hump from the inside to get to it. He had a lot of trouble with the hyd clutch. I loved driving it.
 

MadRocketScientist

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And that throttle control would be trashed. All the hardness and temper in the wires that make up the housing and push-pull wire would be eliminated, and even if the thing still worked it would fail soon enough.
I think they didn't try cranking it for very long at all and fortunately quit before any damage was done! Everyone involved did look a bit sheepish about the incident.
 
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