Cowl to firewall hinge attachment using full diameter pins

[Stevea]

Starting a new thread so as not to totally hijack this one:
https://vansairforce.net/threads/alternatives-to-089-hinge-pin.241031/

The above discussion is about trying to fit a 0.089" diameter pin in the top cowl to firewall hinge attachment on a -12. Maybe someone has been able to help Bob solve his problem with that. In that thread I mentioned I had been able to install a full sized (1/8" nominal) pin across the top of a -6 cowl. Here's how I managed to pull it off.

The original -6 instructions for the cowl to firewall hinge attachment say to use a 0.089" diameter pin. In other words, do not use a full sized (1/8" nominal) pin, but change to an undersized approximately 3/32" diameter pin. Have to admit, my first thought was "No....I'm going to see if I can make a full sized pin work, before I give up on that idea." So, after a fair amount of trial and error fit up, it worked. Here are some tips on making this work, along with some photos.

1. Bottom cowl: side and bottom hinge pins have fully rounded over (bull nosed) ends.

2. Top cowl: hinge pins are tapered (mostly on the top side) over the last 2 -3 inches from the tip.

3. Top cowl, pilot side hinge pin: pre-bent to follow the hinge bend (sort of.....see photo below)

4. Top cowl: hinge pins do not wrap completely around the cowl curve to the end of the hinge. Pins are 2 - 3 inches shorter than the hinge.

5. Pin size: 0.117" diameter stainless steel.

 

[JonJay]

You can use a full length full size pin if you chuck the rod up in a drill and using a disk sander taper the last few inches.
I wouldnt want to leave the pin short as that assumes the designers don’t factor that in. There is quite a bit of pressure inside the cowling. Might get away with it I guess.

 

[rv6n6r]

You can use a full length full size pin if you chuck the rod up in a drill and using a disk sander taper the last few inches.
I wouldnt want to leave the pin short as that assumes the designers don’t factor that in. There is quite a bit of pressure inside the cowling. Might get away with it I guess.

? My recollection is that that's what the plans said, but maybe I'm just remembering what someone figured out and shared with the rest of us. Anyhow that's what I did (and everyone I know).

EDIT: I pulled out my plans and I stand corrected, Van's recommendation as of that writing was for the undersized pin. That said I would advocate the taper method instead. There's a lot of vibration to wear out those eyes, and even with the full size pins I get a some gray weeping back from there.

 

[PhatRV]

You can use a full length full size pin if you chuck the rod up in a drill and using a disk sander taper the last few inches.
I wouldnt want to leave the pin short as that assumes the designers don’t factor that in. There is quite a bit of pressure inside the cowling. Might get away with it I guess.

This is what I did to narrow the tip of the cowl pins in my RV8

 

[Stevea]

I wouldnt want to leave the pin short as that assumes the designers don’t factor that in. There is quite a bit of pressure inside the cowling. Might get away with it I guess.

Well it has held up for 1400+ hours. The back edge of the cowl is very stiff in that area. Plus, the top is attached to the bottom cowl at the top to bottom seam. Then the bottom is attached to the firewall. So… it doesn’t move in that area. Also, the designer specified an under sized pin as an acceptable choice. That under sized pin isn’t providing much, if any, support out there either.

The pressure difference across the cowl (from inside to outside) is about the same as from top to bottom of the engine, a few (2 - 3) inches of water column. Not worried about delta pressure blowing out the back corner of the top cowl.