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Firewall bushing measurements, + or - tolerance?

Marty57

PATRON
Nipomo, Ca
I am getting ready to weld in the four 1/2 x .058 bushings at the four corners of the tubes that will make up the firewall. The 2+2 plans call for the on center measurements of these 1/2" tubes to be 20 1/2" at the top . How much tolerance is there in the measurements? My distance from the left and right vertical tube is off, about 1/16-1/8". Looks like the on center of the two top bushings will be between 20 3/8" and 20 7/16". Is this a major problem? How much tolerance is there when mounting the engine mount later? This is uncharted waters for me and I do not want to re do the tubes if not needed but the engines mount needs to fit later. This is my first project so I don't know what is "good enough". The plans do not give a + or - tolerance regarding the location of the bushings. Any thoughts?

Thanks,
Marty
 
I'm at the same stage on my 12. If you have the engine mount you would fit the bushing locations to that, I would think, as close as possible to the drawing dimension. If you're going to build a mount you could make it to fit, however if the mount were ever to be replaced, then a factory built one might not fit?

I have the Piper drawing for PA12, and can check for tolerance, but it's not with me this weekend. I'll check Monday if the tolerance hasn't appeared here before then.

Here's a question for you - - what about the folded gussets at each corner, that the bushings weld to. Did you make them or were they part of a kit? I'm wondering if I can purchase them or if I have to whittle them out of sheet - - -
 
Gordon,
I have to make the corner gussets. The kit came with most of the parts needing bending, but flat parts they supplied just the sheet steel. Time to cut some steel. Let me know what you find regarding the tolerance.

Thanks,
Marty
 
Tolerance is usually noted on dwgs. It applies everywhere unless specifically noted otherwise. I think you could live with 1/16", but 1/8" might put too much stress on the rubber bushings (The engine mount will warp). Just my 2c.
 
I would recommend taking a round file and rounding out the 1/2" tubes to get your centers as the drawing calls out. If you dont get the spools in accurate dont count on any Piper mount fitting without some trouble with alignment. You dont want wallowed out holes to fit an engine mount. Since this area is boxed on the 2+2 with .050 it will not be a problem, just maintain your centers across the centerline of the fuselage. The PA12/14 uses "U" shaped pieces with engine bushings instead of flat plates and the short diagonel tube that the 2+2 uses.
 
Cub junkie,
If I understand you correctly, the .050 you are refereing to is the corner gussets, right? Am I using the round file to enlarge the hole in the bushing after welding in place to get the correct distance from the center of one bushing to the center of the other bushing and then welding in the gussets? Or, am I using the file to take down one interior side of each bushing to get the proper distance? If the second is correct, how much can I file the .058 side wall before it is too thin? The plans call for the measurements to be taken from the center of each bushing to bushing. As I type this, it seens to me that the strength of the mounting is not the 1/2" x .058 tube, but the gussets and the short tube welded corner to corner against the bushing. I feel real dense in the head right now trying to wrap my brain around this. Thanks for helping.

Marty
 
Marty, What I mean is file or grind the airframe tubes on the firewall sta. to allow you to make centers. I wouldnt take any from the .058 eng. mount bushing. And yes the .050 I was referring to is the corner gusset on the 2+2. As I said you want to be as accurate on the eng. mount bushings as possible,then you have the option of using a PA12 engine mount or the store bought Wag Aero. Even if you are an 1/8" off that means you can take .062 or so from each side to get your dimension called out in the plans. I use drilled angle to hold my eng. mount bushings, that keeps them perpendicular to the firewall face and they stay put when welding. If I can help more PM me.
 
12 Geezer & Marty

The PA-12 engine mount bushings with the folded gusset are available at Univair. The numbers are 20291-00 for the uppers at 103.90 each and 20301 -00 for the lowers at $96.30 each.

On an original 12, the weld is not visible on the outside of the U shaped gusset and I can see the outline of the bushing, so it must be welded inside of the folded gusset.

Do all 4 bolts have to fit snuggly inside the bushings or is a bit of "slop" okay? I have one (lower left) that the bolt fits with a loose tolerance, unlike the other three with a tight tolerance, easy to insert but no wiggle room.

Wayne
 
Thank you for the Univair part #'s. I'm going to try to salvage the original ones, but - - -.

I'd think a few thousanths clearance would be alright, but not more than .010? But I'm not an A&P so take that with several grains of salt.
 
I resolved the problem regarding location of my engine mounting bushings, I replaced one of the firewall tubes, welded in a new one with a much better fit. Before I weld in the bushings, I want to locate the engine mounting brackets. I bought a set that is used with the Dynafocal mount shown in the plans. I guess I am a bit lost here because the 2+2 plans do not seem to show how the brackets are mounted. Simple question: what type of bolt/nut/washer combo holds the bracket to the bushings? I assume the bolt goes into the bracket and then through the bushing with the nut on the inside of the fuselage, right? Is there a washer between the nut and the gusset? There isn't a lot of room for a washer with the way the lower tubes are welded in place. Is the bolt a special engine mounting bolt or a regular AN bolt? I am trying to understand this before I weld in the bushings. Needless to say, I am a bit frustrated because the plans don't show anything here. This is so unclear to me, am I missing something? Where does a builder go to get a clear picture of this? Any help here would be greatly appreciated.

Thanks,
Marty
 
You might try looking at the setup on a similar plane. On my certificated 12, the bolt (AN, drilled for castle nut) installs from inside the fuselage, and there are small tabs welded to the aft side of the bulkhead to fit against a flat on each bolt-head, so that the bolt doesn't turn while installing the nut. On mine, there is a washer between the nut and the engine mount, though with a washer-face nut it doesn't seem that would be necessary?
 
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