Cowl Trimming Question

[N223JH]

Following the advice of forum posters, I am deferring cowl installation until the engine/prop are in place. I would, however, like to trim out the cowl beforehand as much as possible. Aside from the obvious (oil door, etc-) how much can be done? Are the critical final trim clearances at the spinner bulkhead a function of cowl/firewall hinge or the upper/lower cowl?

Just want to get an idea how much fibreglas dust I can get rid of early on...

TIA Jim AKA120264

 

[Geico266]

The scribed trim lines are amazingly accurate. IMHO, you could trim up to the lines and be fine, and final positioning of the cowel for spinner positioning and clearance after the engine is hung.

 

[jte65]

agree on accuracy of scribe lines

I agree with Larry on the accuracy of the scribe lines on the cowling. I just completed my cowling installation. Like several other builders have suggested, I waited until my engine and spinner were installed to do the fitting and final trimming. This worked beautifully- I was able to get a nice, consistent gap between the cowling and the spinner backplate without any need to trim the backplate as some others who installed the cowling first have had to do. I really encountered no problems whatsoever with doing the cowling install after the engine was hung. I did install the spinner without propeller blades in order to minimize interference and decrease the risk of damaging the blades. You also need to be careful that all ports on the engine are covered, as drilling for the hinges, etc. generates a lot of dust and shavings that could find their way into a place you don't want them to go!

Jeff

 

[BigJohn]

Jeff,

Thanks for the encouraging words and advice. I will be at that stage in a week or two. I have roughed out the cowl pieces and trimmed to the lines.

BTW, did you do any pin hole filling yet on the cowl halves? I'm about to use the Epoxy skimming method to try and fill the pin holes beofore I mount the cowl.

John

 

[N223JH]

The scribed trim lines are amazingly accurate. IMHO, you could trim up to the lines and be fine, and final positioning of the cowel for spinner positioning and clearance after the engine is hung.

OK, so the final trim to set spinner clearance will be inside the scribe lines?

 

[Geico266]

OK, so the final trim to set spinner clearance will be inside the scribe lines?

It should be right on the scribe line. I was one of the 1st builders and followed the instructions. My spinner gap looks pretty good even though the cowel was done first. You could leave 1/32" and use that amount to adjust the cowel. I think some builders got into clearance issues when the "man handled" the cowel in to position. The plans say to avoid that, but it is easy to do when there is nothing to "aim" for.

All I'm saying is the scribe lines are really accurate, use them as your guild to get close.

 

[jte65]

Filling pin holes

BTW, did you do any pin hole filling yet on the cowl halves? I'm about to use the Epoxy skimming method to try and fill the pin holes beofore I mount the cowl.

John

Thanks John. You and I are at very similar points in the build process and have been for quite some time now!

No- I have not attempted any filling yet. Firstly, this should probably wait until the cooling shroud is installed given that you have to drill and fill holes in the fiberglass (I am just starting the cooling system installation). In addition, the instructions suggest no sanding/painting or other work until this work is done, as you need to be able to see through the fiberglass for a few steps and don't want to obscure the transparency.

I have not yet decided whether I will do any fill work before my first flight. I plan to have my -12 professionally painted after initial flight tests are done and may just leave the fiberglass finishing work to the painters.

Jeff

 

[jte65]

spinner clearance

OK, so the final trim to set spinner clearance will be inside the scribe lines?

In my case, I had to sand a bit beyond the scribe line on the aft portion of the upper cowling in order to get consistent clearance around the spinner backplate and to get the spinner lined up nicely with the contour of the cowling. This will probably vary a bit from plane to plane. However, as Larry states, the scribe line seems to be a good starting point (I started about 1/32 inch outside the scribe line and then repeatedly fitted and sanded from there). I had mine on and off (particularly the top) many times and used a lot of duct tape before I finally got to the point where I was happy enough with the fit and clearance to finally drill for the hinges. Fortunately, it is pretty easy to sand off small amounts. Just be careful- those cut fiberglass edges are pretty sharp until they have been sanded (ask me how I know)!!

Jeff