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Engine mount bolts are really, really tight!

ArVeeNiner

ArVeeNiner

Well Known Member
I have a conical mount and I remember back when I hung the engine that the AN8 bolts were VERY VERY tight in the engine mount. So tight that I believe I had to drive them in with my rivet gun.

Well fast forward to today and I'm going to reshim the engine because it didn't droop at all like I was expecting. I'm back at these VERY tight bolts. My hangar mate suggested that I ream the holes in the mount a bit so that they can slide in and out easier. The ones in his biplane slide in and out of the mount quite easily.

Has anybody else ever run into this? Has anybody else reamed their mounts?
 
the interference fit, why they are hard to get in, work hardens the hole giving it better crack resistance and greater strength, if you do ream the holes, you will then require oversize bolts to achieve the same work hardening of the original bolt.
 
I believe they make "U" shaped washers to be used as shims here so that the bolts don't need to be fully removed. Maybe look into that and see if it helps.
 
Unless the print calls for an interference fit/close tolerence bolt (which I don't believe it does) than it would be acceptable to ream the hole to the correct size.
 
bret said:
Can you shim the mount on the fire wall?
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I thought about that and I have washers and longer bolts in case I need to shim here. My thought is that shimming here would preload the mount since there isn't any rubber to take up any misalignment in this area and that's not a good thing.
 
assuming also you are taking the weight off the bolts by supporting the engine weight with a hoist and finding that sweet spot where the wheels aren't off the ground but the front end is light
 
Walt said:
Unless the print calls for an interference fit/close tolerence bolt (which I don't believe it does) than it would be acceptable to ream the hole to the correct size.
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Adding to Walts comment, I think the OP said mounts, is that right? Were they new or used? Could they have been over torqued and the sleeve compressed? My new Lord mounts and the engine case (new) allowed the new bolts to slide easy, but not loose.
 
BillL said:
Adding to Walts comment, I think the OP said mounts, is that right? Were they new or used? Could they have been over torqued and the sleeve compressed? My new Lord mounts and the engine case (new) allowed the new bolts to slide easy, but not loose.
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Just to clarify, the bolts are tight in the weldment near the rubber mounts. They are not tight in the rubber mounts themselves. Each bolt goes through a small tubular piece, much like a sleeve. They are tight at the metal to bolt interface.

I could drive the existing bolts out but I don't know how I'll get them back in if I don't ream. I do have replacement bolts since I'm sure I'll mash up the olds ones. I don't see a way to get a rivet gun behind the new bolts in order to drive them back in. I have a lot more things back there now (mags, hoses, wires, etc) than I had when I initially put them in.

My gut (and Walt) tells me that they shouldn't be so tight. I bet it has something to do with the welding process.
 
ArVeeNiner said:
Just to clarify, the bolts are tight in the weldment near the rubber mounts. They are not tight in the rubber mounts themselves. Each bolt goes through a small tubular piece, much like a sleeve. They are tight at the metal to bolt interface.

I could drive the existing bolts out but I don't know how I'll get them back in if I don't ream. I do have replacement bolts since I'm sure I'll mash up the olds ones. I don't see a way to get a rivet gun behind the new bolts in order to drive them back in. I have a lot more things back there now (mags, hoses, wires, etc) than I had when I initially put them in.

My gut (and Walt) tells me that they shouldn't be so tight. I bet it has something to do with the welding process.
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Walt is right and it is very common to have to ream these after welding, remove as little material as possible, I think it's a 1/2" hole so start with a .500" ream, if that does not do it go up a couple k.
 
+1 for Walt's suggestion to ream to the proper size. There's no reason that you should need, or want, to drive these bolts in.
 
If you choose to use a reamer, I would make an aluminum block about 1"thick with a hole reamed straight through it to use as a guide so the reamer starts straight. Line up the block with the hole using a pin that fits both the guide block and the mount then clamp it to the mount. A 1/2" reamer needs to be turned very slowly or it's cooked on the first hole.
You might get lucky running the reamer in the hole with out a guide but I wouldn't want to ruin my mount trying it. I would try to get some measurements with precision gage pins (machine shop measuring tool). There might just be a tight spot in the holes from welding that you could file with a round file or chainsaw sharpening file.
 
aerhed said:
Or... Just use an adjustable reamer and wa
K it up. Molehill.
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adjustable reamer,,,, Cool, I checked it out, learn something new on hear all the time!

As far as guide blocks etc go, you don't need anything like that, just the ream and a 1/2" drill, there very easy and the ream follows the hole just fine.
 
Excess powder coating

I had to ream the motor mount on the inside of bolt tubes, be cautious.
 
Thanks for all the advice. I shall ream. Ken at Van's concurs but he had never heard of a problem like this before. Weird.
 
Before you start doing anything, how do you know the engine is or isn't sagging? Is the cowl done? I've only ever seen engine mounts shimmed after a few years when the old rubber mounts start to sag. If the cowl isn't done, you build the cowl off the engine, not the other way.

We're talking the engine mount bolts and not the firewall right?

Disregard, I see you're through phase one. Washers behind the mount are an easy way to adjust motor position.
 
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Sig600 said:
Before you start doing anything, how do you know the engine is or isn't sagging? Is the cowl done? I've only ever seen engine mounts shimmed after a few years when the old rubber mounts start to sag. If the cowl isn't done, you build the cowl off the engine, not the other way.

We're talking the engine mount bolts and not the firewall right?

Disregard, I see you're through phase one. Washers behind the mount are an easy way to adjust motor position.
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Yea, I totally expected the engine to sag A LOT and I allowed for that. After 2 years, it looks like it's hardly sagged at all. It makes for a very ugly nose with the spinner above the Sam James cowl.

It looks like a couple of washers will get it down to where it needs to be and make for a much sexier nose.

I have a reamer on order but I will try to use a wooden dowel wrapped in sandpaper first.
 
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I don't think you will see much "sag" with conical mounts. Most "sag" I've seen is with dynafocal mounts.
 
Bevan said:
These worked great for me...

http://www.averytools.com/prodinfo.asp?number=282

Whack them through with the bolt you're trying to insert.

Bevan
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Just thinking out loud here....

Those alignment pins are 7/16" not 1/2".

What I am getting at is that these pins correspond with AN7 bolts not AN8 bolts.

My engine is mounted using AN7 bolts (Dynafocal, IO360C1C). And those AN7s were a perfect fit.

Perhaps the OP might want to double-check the requirement for AN8 bolts for his set up?

Again, just thinking out loud here...

http://exp-aircraft.com/library/alexande/hardfig2.html
 
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