SonexBuilders.net

My plans called for an AN4 bolt through the engine mount and leg. I had clearance issues with the bolt and the firewall. Extra washers gave me clearance, but then I had threads in the hole. I wanted the grip length greater than the hole depth because the shear action would be between the titanium rod and the steel tubing. The mount hole depth was 2.615 inches; two standard washers brings the maximum grip length to 2.739. The AN bolt options were:

AN4 bolt grip lengths
An4-30 2.5625
AN4-31 2.6875
AN4-32 2.8125

The AN4-31 has the correct grip length. The overall length includes .500 thread length and is 2.6875 + .500 = 3.1875 inches. This creates interference with my firewall. I researched options and selected a NAS shear bolt. The NAS6204-42 specs are:

Grip Length: 2.625
Overall Length: 2.950
Thread length: 0.375

I used an AN363-428 metal locking nut and two AN960-416L washers. This left me with two exposed threads.

My research found that the NAS6204 has 160,000 Dsi tensile/ 95,000 psi shear vs AN at 125,000 Dsi tensile / 75,000 Dsi shear. I'm considering replacing the AN bolt at the gear also. Additionally, I used the standard torque value for an AN4 bolt with a shear nut, measured with a torque wrench. The plans called for tightening, then backing off. Thoughts on the substitution and torque?

Zack

I drilled holes in the firewall so I could pass the bolts through from inside. That's the only way I could see to eliminate threads in the gear leg area and nut-to-firewall interference.

Did I read that someone had replaced these bolts with taper pins? Is that a good idea or a bad idea?

Gordon

Just my opinion, but a taper pin is not appropriate for this location.

Since Sonex instructions are not to fully tighten these bolts, shear bolts should be a viable alternative. However, i also drill access hoes in firewall and installed small cover plates over each. And used 5/16" bolts at both top and bottom of the legs. The tubing that surrounds the upper bolts is thin, so need to be very careful drilling that for 5/16".

David A.

Update:

The original service bulletin from Sonex was published because we in this country had reported 4 separate bolts break on different aircraft in rapid succession. The original SB calls for AN-4 bolts, but if you operate on rough grass fields, these could break as well after a while. Having the aircraft standing on its nose on the runway at regular intervals while ATC is watching from the tower quickly looses all novelty, so since around 2010 we ream out the holes to ø5/16’’- 8,0mm and secure the legs with 120 grade capscrews. Torque is set to bolt limit- that is a lot of torque.

Some naysayers told us that we would now instead shear off the engine mount, but so far no issues with this mod and almost 600 hrs on one of the planes operating on a tough grass field.

However- it should be noted that a broken bolt in itself does not neccessarily mean the leg rotates in its engine mount fixture. This only happens if one bolt half falls out of its hole. If the lower bolt half (the upper half is obstructed by the firewall) is denied the possibility to fall out by for instance securing it with safety wire, the immediate problem of nose diving on the runway could be avoided.

Thanks
Kai

When all was said and done with mine, there was no way to fit a nut between the firewall and engine mount. There was ,however, just enough space to fit two fender washers between the mount and firewall to fill that void and then drilling a 1/4" hole in the firewall, I put a washer and castle nut on the inside of the cockpit. Obviously, the bolts were installed from the engine side. I did change out the SS bolts for new AN ones as per the SB about 100 hrs ago. I put a drop of "torque Seal" on the bolt heads and check them at every oil change, 275 hrs total time without issue. I have operated off of improved fields only. Good Luck ! George.