Engine Mount Alignment Ajustment

[Baloo]

We are in the midst of swapping the io-360 for the io-540 in the 4 place. The mounts for both engines were ordered from Avipro Bearhawk. The 360 mount fit perfectly and aligned with all 5 firewall holes. The 540 mount on the other hand aligns perfectly with the 4 corners but the top centre mounting hole on the mount sits 1" higher than the hole in the firewall.

Ive tried 2 separate Avipro 540 mounts and had the same issue on both. I've talked to 2 people one said to cold bend the top portion of the mount in place and the other said to heat it up and bend it to fit stating that the top mounting hole is for added security and not critical.

Im looking for some more input on this because we are stalled on this project until we figure it out. What would you do?

engine mounts on Cessna 172s, Piper Navajos even the venerable radial on the DHC2 Beaver all attach in 4 places.

Input appreciated.

[jaredyates]

I recall guidance about mounts often needing to be sprung a little into place, though that was easier on the longer 360 mount. Are you able to bend it into alignment by hand, enough to get a bolt through?

[S Lathrop]

There are 2 sides of the engine mount issue. I expect that Bob chose to add the fifth mount because the fuselage may be more the structural issue than the mounts them selves.

I don't know what your fabrication skills are but I would modify the engine mount so that all the bolts fit easily into place. If there is 1" difference and the top mount is high, I would cut the boss out of the mounts and bend the tubes and cope them to fit the new location. I would use a heat to do the bending.

I built the mounts for my IO540 from scratch. I took the time to make aluminum inserts to replace the Lord engine mounts for the construction process. I worked from drawings for the mounts to make the inserts. You should not have to do that because you won't be welding close to the rubber mounts.

[Baloo]

Jared, it is out a little too far for a little hand bending. Being a shorter mount it is stiff as hell. How about welding a small extension under the current hole to drop it to the level of the hole in the firewall. Thanks for the suggestions please keep them coming.

[Battson]

Oh wow - that is a mismatch! Bugger.... sorry to see that it wasn't a bolt-on part, that is always a painful moment...

I guess we should clarify, you own a scratch-built fuselage which isn't matching the kitset mount?

My kitset mount did need springing to match up with the kitset fuselage. It wasn't anything like the problem you are facing.

It sounds like you have considered just using four bolts. I think you should install all five the way Bob designed it, unless you do a redesign. Using four bolts would increase the flexibility (strain) on the engine mount, increase stress in the welds, and increase vibration compared to a five bolt situation. The engine departing the plane in-flight is probably the second worst consequential failure, after a wing falling off.... Just don't go there!

I would not cold-bend the mount unless you plan to stress-relieve whatever part bends first, that is quite a bend. I would not want to spring it 1" under any circumstances, it just adds to the total stress at the welds and increases the chances of cracking the mount in future. The chances of cracking may slim, but higher stress in the fuselage / engine mount will increase the chances quite a bit.

Hopefully you haven't reamed the bolt hole yet. The mount should have been supplied undersize from Bearhawk Aircraft to address this exact problem (at least they used to be).

I suggest you try the following:
1. Carefully measure the misalignment.
2. Remove the engine mount from the plane, secure it to something solid.
3. Set up a marker which indicates how far the bolt hole needs to move so it lines up perfectly.
4. Find a buddy with a decent gas torch or borrow an Oxy welder, and get his help too - you want two heat sources and two people
5. Heat the welds for the top part of the mount to a nice cherry red colour.
6. Using a long bar, bend the top mount hole into the correct location, watch for spring-back.
7. Keep the welds hot for another 2 minutes, gradually reduce the heat (don't shock-cool the welds).
8. Test fit

An aside for kitset builders reading this:
I found the order of installing the mounting bolts was important. It was hard to do the 4 corners and leave the top mount until last, it was very hard to spring.
I did the top one first, and then sprung the rest into place. That was much easier.

[whee]

I thought the o360 mount only used the four corners? The O470 mount only uses the four corners but I would certainly make your mount fit as Bob designed.

First thing I would do is verify the thrust line is correct with the mount bolted up with the 4 corners. Then I'd follow Battson's recommendation using two O/A welders to heat the 5th bolt hole tubes at the engine ring/5th bolt hole tube welds. You want to bend the 5th bolt tubes not the engine mount ring. Also, I'd remove the engine from the mount and use the fuselage as the jig. Once the tubes are heated they will bend easily and you can just hand bend them so the bushing aligns with the fuselage bushing. Slip a bolt in to hold it in place and cool it as slowly as possible.

[Zzz]

My 540 mount fit up to my quickbuild fuselage perfectly with minimal pressure to slide the bolts into place. I suspect your issue is from the fuselage being a scratch build? I personally like having the 5th attach point. That's a lot of weight hung off the front and that upper/center support could prevent flexing as you bounce along a rough strip.

If it was me, I would go for the serious solution: Cut and reweld the upper/center arms at the end. 1" is a long way to bend without distortion. Any bending you do, hot or cold, is going to wack the alignment of the bushing on the mount. You may end up having to bore an oversized tube for the new bushing to cover the gap. Battson's idea is worth a try too.

If that sounds scary, find an experienced fabricator/welder. That stuff is child's play for a lot of guys who do it daily.

[JimParker256]

I thought the o360 mount only used the four corners?

Correct, but this is for the IO-540 he's installing to replace the O-360 that's coming out...

[tbaylx]

Why not cut out the bars attached to the top mount, move the mount boss to the correct location and fabricate and weld in new bars to attach the one boss in the correct spot? A bit of welding and fabricating a couple of bars but should be far easier than building a mount from scratch

[Bdflies]

I agree completely. It's about an afternoons work.

Bill

[bearhawk2015]

The easiest and I don't see any reason not to is to just weld on the extension below it like you mentioned. I also tried to use a factory mount on a scratch build fuselage, every hole was just close but not close enough so I ended up building my own from scratch and it wasn't that big of a deal.

[alaskabearhawk]

Lots of good suggestions, but Whee hit it on the head. You need to verify the thrust line is correct. It will cause problems if it isn't. Doing extensions, bending, etc doesn't do any good either because of the load path(s) not being straight. There is so much going on up there. A 400+ pound engine with a heavy prop spinning like a gyroscope in front of that inducing all sorts of torque and stress on those five points, add some moderate turbulence...well...I would want everything to be just right. It may be the thrust line isn't compatible and you need to fabricate a mount from scratch. It's not difficult, but so necessary to get it right. That offset thing will do absolutely no good.

[Battson]

It sounds like the fuselage boss for the top bolt is probably welded in a slightly lower location than the factory kit. That seems like the most probable cause of the bad fit. I think it unlikely the thrust line will be seriously wrong because of the engine mount alone, particularly because the lengths and angles are all still correct, also the other 4 bolts all match closely. If there is some error, it will almost certainly be minor, and will probably be in part due to the scratchbuilt fuselage (which is a hard thing to adjust).

It would be nice to check the thrust line nonetheless, but his finished fuselage and finished engine will make accurately checking the thrust line a very difficult task. It will be hard to know if there really is a minor error, or if it's just a measurement error. If anyone has some ideas on how to measure this without removing the crankshaft and firewall, it might be worth posting them?

I guess that having a correct thrust line is mostly about efficiency of the airframe, relative to the main aerodynamic surfaces. I suppose that very minor imperfections in engine mounting angle wouldn't materially change the stresses in the structural parts - the factors of safety in the design wouldn't be degraded dramatically, I guess. I bet most scratchbuilt models don't get the dimensions perfect for that matter. Someone with more knowledge might like to correct me on those last couple of statements, as I say I am just going by gut feel.

[JimParker256]

I think I would re-check the measurements of the engine mount points on the fuselage against the plans, and if they are off, contact Bob for guidance.

I agree with Battson that the most likely scenario is that the upper mount point boss on the scratch-built fuselage was "off" target, given that the other four points mate up properly with the factory-jigged engine mount...

I don't know whether Bob will recommend changing the length and angle of the upper engine mount tubing, or changing the upper boss on the fuselage. But my engineer brain just cannot "align" with "bending" those upper tubes of the engine mount and ever getting them to properly align with the boss as it is now. It would require some kind of an "S-bend" which seems like would weaken the rigidity of the mount... (Not to mention being incredibly difficult to make such a bend.)

[DRLPatrol]

How can the fuselage be wrong if both mounts came from Avipro and one fits and the other does not?

Something get bent in shipping?

Doug

[Baloo]

Thanks for all the great answers. Just to clarify, the mount that did fit was the original Avipro 360 mount but the 540 mounts don't.

I had an entertaining conversation with Bob yesterday. I told him that my Avipro 360 mount had 5firewall mounting points and he refused to believe it. He even convinced me I was wrong until I went to the Bearhawk site to verify with the picture in the BH store that I wasn't crazy. I called Bob back and told him that the Factory 360 mount has 5 holes and that I wasn't crazy Bob said "I have no idea why they would do that, my drawings only call for 4 holes and 5 for the 540."

It looks like the solution that we are going to go with is to cut off the top tubes and re weld them at an angle that works using the firewall as a jig.

Thanks again for all all the tips I'll let you know how it goes. The next question is where to relocate the oil cooler to and what to do about the fact that the oil filter sits so close to the firewall that it would be impossible to remove. Looking at options at the moment for that one.

[James]

Yeah, mine has 5 bolt holes as well. All the holes in the mount are 3/8" diameter, but the top centre cluster on the airframe is only 5/16".
I'm a long way from worrying about the engine, so I just put that in the "things to work out later" pile.
There's probably an obvious reason.

James