Looking for opinions on how to handle the freeplay that results from using the .090 hinge pin in the 1/8" hinge for the top cowling attachment hinge. I am preparing the drill the top cowling to that hinge, but the hinge can move fore/aft by about 30 thousandths. Should you push in all the slack, pull it all out, or accept the natural/random lie of the hinge? The cowl has been trimmed for 3/16" spinner gap on the front end and a nice butt joint to the forward top skin.
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Drilling the top cowl firewall hinge
- Thread starter Girraf
- Start date
PaulvS
Well Known Member
I think you should push it in since the butt joins need a gap of 1/32" to 1/16" to allow for paint, otherwise the paint will chip. Once the side hinges are fitted between the top and the bottom half of the cowls there is not any noticeable movement.
The ideal way to deal with the remaining minor movement issue is to use 3/32" hinge and not the 1/8" hinge along the straight section at the top of the cowl. The curved part of the cowl uses the 1/8" hinge. This will prevent the straight section of hinge from moving and smoking due to vibration.
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HFS
Well Known Member
I'm sure someone else will chime in, but ...
Use Drill Rod instead (McMaster-Carr among other suppliers). To the uninitiated, the name makes you think of high speed twist drills who are as hard as the devil and just as brittle. But not so grasshopper - the name comes from the diameter sizing of the round wire, using AWG (American Wire Gauge) specs. To put that in perspective, if you have, or are familiar with a 1 - 60 drill index, you have access to all the pertinent sizes we can use to tighten up or free up hinges on our cowling (as well as flaps - if need be).
For the standard (.090) hinge pins, there are three sizes that may be of help - #46 - .081", #45 - .082" & #46 - .086". For the .125" hinges that go on the upper cowling, where typically the pin that comes with the hinge just won't go around the corner, here are the sizes that can help (reduce the freeplay of a .090" pin), #36 - .106", #35 - 110", #34 - .111", #33 - .113", #32 - .116" & #31 - .120".
Because the sizes are so close together, you can also size "up", as your hinges wear in over time, and recover some of the "looseness" that just comes with time in service.
For the "truth in advertising" part - drill rod is not as "stiff" as the .090" pin material. So, it can be bent if you are not careful with it - but I have used it for more than 30 years in my -4, and Rocket, and have never had to replace one for a bending failure. Replaced them as conditions allowed for "sizing up" - but o/w, no failures.
So there is a path out there for tightening up those loose ones if you choose.
HFS
As an addendum - In my experience I have seen a lot of "sag" over time in cowlings mounted with the undersized pin; and, when building you would (should) have already allowed for a paint "gap" for the future ...
Use Drill Rod instead (McMaster-Carr among other suppliers). To the uninitiated, the name makes you think of high speed twist drills who are as hard as the devil and just as brittle. But not so grasshopper - the name comes from the diameter sizing of the round wire, using AWG (American Wire Gauge) specs. To put that in perspective, if you have, or are familiar with a 1 - 60 drill index, you have access to all the pertinent sizes we can use to tighten up or free up hinges on our cowling (as well as flaps - if need be).
For the standard (.090) hinge pins, there are three sizes that may be of help - #46 - .081", #45 - .082" & #46 - .086". For the .125" hinges that go on the upper cowling, where typically the pin that comes with the hinge just won't go around the corner, here are the sizes that can help (reduce the freeplay of a .090" pin), #36 - .106", #35 - 110", #34 - .111", #33 - .113", #32 - .116" & #31 - .120".
Because the sizes are so close together, you can also size "up", as your hinges wear in over time, and recover some of the "looseness" that just comes with time in service.
For the "truth in advertising" part - drill rod is not as "stiff" as the .090" pin material. So, it can be bent if you are not careful with it - but I have used it for more than 30 years in my -4, and Rocket, and have never had to replace one for a bending failure. Replaced them as conditions allowed for "sizing up" - but o/w, no failures.
So there is a path out there for tightening up those loose ones if you choose.
HFS
As an addendum - In my experience I have seen a lot of "sag" over time in cowlings mounted with the undersized pin; and, when building you would (should) have already allowed for a paint "gap" for the future ...
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Further research last night suggested putting the 1/8" pin in the hinge for drilling. The instructions said this was impossible but I've proven that untrue. Its probably not possible to do it through the oil door as it would be done in the field, but for the purposes of marking and drilling during construction, it works. I'll drill the top cowl to the hinge this way and swap out the pins once the hinge has been riveted.
PaulvS
Well Known Member
FWIW, and this may be consistent with what you are going to do, I drilled the cowl hinge with some high tensile farm fencing wire as the hinge pin. The diameter was slightly larger than the 3/32 hinge pin that came in the kit, but smaller than 1/8. I was unable to get a 1/8 hinge pin around the bends. I used the fence wire as an interim mainly because I did not want to cut the real hinge pin too early. It seems to have turned out well enough and there is no apparent movement in the cowl.
One other idea that I still intend to try out is to use the 3/32 hinge pin, but with some glue lined heatshrink over the section of pin that passes through the straight part of the 1/8 hinge. So the hinge pin will in effect be tapered.
