Engine Mounts

[Doug G.]

Anyone know how the rubber isolators are torqued or otherwise tightened? I can't seem to find it in any of the Rotax or FD manuals.

[Tom Baker]

Without a torque spec specific to the engine mount I would tighten based on the size of the bolt. That is what I have done in the past. When you are tightening you are not pulling against the rubber. You are tightening against a steel bushing inside the rubber pieces.

[Doug G.]

That would have been my go-to option, but wondered if I was missing something. Thanks.

[Roger Lee]

The 4 engine mounting bolts that go through the rubber isolators are torqued to 200 in/lbs. The 6 firewall bolts are the same.

[Doug G.]

Thanks Roger.

[Tom Baker]

The 4 engine mounting bolts that go through the rubber isolators are torqued to 200 in/lbs. The 6 firewall bolts are the same.

 

Roger, where does the torque for the 4 engine bolts come from? IIRC the bolts are different sizes, and should be different torques based on the size without having a specific torque value listed.

The torque for the six bolts sounds right based on memory, but I always check the value in the book before torqueing anything.

[Roger Lee]

Hi Tom,

 

The 4 bolts are the same size in diameter. Same torque. The aluminum spacer in between a couple are a different lengths to give the engine the correct angle. I was given this torque by FD back in 2007 and you won't find it in the book unless something changed.

[Doug G.]

The manuals give a 35 nm standard for M10 bolts which equates to 300 in. lbs. Is there some instruction to the contrary?

[Roger Lee]

None of the engine mounting bolts gets 300 in/lbs. This may deform the rubber mounts and the aluminum spacers.

 

If you torque any of the engine mounting bolts to that torque you may damage something. 

[Doug G.]

OK. 200 It is. I am amazed that nothing gives the torque. I realize Rotax deals with different mounts, but you would think FD would fill in the blank. Officially Rotax says 35nm "if not specified otherwise." (35 nm is shown as standard torque in the heavy maintenance manual for M10 bolts section 10.2 and the bolts that attach the mounting frame to the engine are 40 nm according to the parts manual)

[Roger Lee]

Found it in the Rotax Installation manual and in the FD Maint. manual.

[Tom Baker]

Hi Tom,

 

The 4 bolts are the same size in diameter. Same torque. The aluminum spacer in between a couple are a different lengths to give the engine the correct angle. I was given this torque by FD back in 2007 and you won't find it in the book unless something changed.

Maybe I didn't make myself clear. The 4 engine mount bolts are a different size compred to the 6 that mount the frame to the firewall.

[Anticept]

They are different sizes at Tom said. The bolts attaching the engine MOUNT to the engine FRAME are M10s, the bolts attaching the engine mount to the firewall are M8s.

 

For clarification of terminology:

 

The engine FRAME is the circularish piece of tube steel that bolts directly to the engine.

 

The engine mount is the T with the nose gear attached.

 

Unfortunately, flight design doesn't want engines to be dismounted or mounted except at FD Service Stations per section 5.2 of the CTLS MM, hence the difficulty in finding the torque. Call them.

[Doug G.]

Roger, page numbers?

[Roger Lee]

Hi Doug,

 

I have to retract the comment on the Rotax installation manual. I was looking at an older manual and I was tired after a long day of riding and failed to read a little better.   The new manual has no such value for those 4 engine ring mount M10 bolts. I called Rotax this morning and talked about those. The reason Rotax doesn't have a torque spec is because they don't supply the bolt or spacer. The aircraft Mfg's and owners/builders use different bolts and different nuts and the spacer inside might  steel, but FD used aluminum. So Rotax's stance is since there are so many variations they don't publish a spec. I ask about torque values in the 16-20 ft/lb range and they said that is in the normal range depending on the hardware. If you used too much torque on an aluminum spacer it could be deformed and depending on what bolt and nut it could get over stretched. This would then change the angle of the engine on its axis. Using Nyloc nuts will help with torque reduction over some standard nuts. Using grade 8 bolts over AN hardware will change the value too.

 

So for those 4 M10 bolts as they are in the CT I couldn't find a published torque and have just what I was told years ago by FD.

 

As far as the 6 through the firewall M8 bolts those are in the FD manual. Depending on which manual you have (SW or LS) those are in section 1.12 and 1,13. and  I had confirmed these years ago with FD during the first firewall installations. 

[Doug G.]

Thanks again Roger.

(I hadn't looked for the firewall bolts yet.)

[Tom Baker]

For the design of the rubber engine shock mounts and the aluminum spacer when the bolt is tight there won't be any more compression. Tightening to 300 in/lb compared to 200 in/lb likely won't make a bit of difference. I really don't think the aluminum spacers will collapse or give any at 300 in/lb. BTW the rubber will not deform due to over torqueing, unless the aluminum spacer deforms.

[Roger Lee]

You're right Tom. The rubber won't suffer unless the aluminum spacer starts to deform.
200 in/lbs (16.5 ft/lbs) verses 300 in/lbs (25 ft/lbs) most likely not, but FD may have had their reasons. With the Nyloc nut it won't move so higher torque is probably a moot point.

[Doug G.]

mute point / moot point ????

[Roger Lee]

Hi Doug,

 

Mute,  I guess it means I wasn't going to talk about it any more. 

[Anticept]

As an additional point: Many nylock nuts cannot take as much torque as regular hex nuts. In many generic torque spec tables, they specifically have their own entries which are considerably lower than standard shear and tension bolts/nuts.

[Doug G.]

Called FD. I was told to use standard torque, but they were OK with 200 or 300 in. lbs. I used 200.

[Roger Lee]

Good choice. 

[sword_guy]

The torque is not listed in the maintenance manual, but it is 35nm per an e-mail from Flight Design.  Roger, please feel free to spread the word.

[Roger Lee]

35nm is almost 310 in/lbs. Way too high. I got my info directly from FD years ago during the first round of hose change. At 310 in/lbs you might damage the aluminum spacer and overly squash the mounts to be poorly functional. I wonder if they are thinking of the 4 bolt engine ring mount  with the bolts on the side?