Contact between air duct and cylinder head #2

During my annual I found that the lower cowl had a black spot on the air duct/engine side due to abrasion of the fiber glass. Checking the engine I found that the cylinder head #2 had a corresponding spot at the bottom aft corner where the green paint had been stripped. Interestingly I checked another RV-12 that was in the shop of my local AN&P: it had exactly the same problem! Checking the air duct I found that at the point of contact there is a small bulge in the fiberglass that does not make sense to me. I cut a 2 inches diameter hole around this bulge and laid a new fiber patch removing the bulge. After installing the lower cowl I have now 1/4 inch clearance and by the way I felt that the installation of the lower cowl was easier too. I am convinced that most RV-12 have this problem unless there was a change of design in the air duct (My serial # is 395).

You are not alone

I did the same thing. It's a "feature".

Yep been there done that to. How is the clearance on the front left header?

Made same patch/repair at 25 hour engine check

I looked at several RV-12s at Oshkosh that had top cowl's removed and some clearly had a little more clearance between value covers for Cyl 2/4 & the air duct than others.

When I originally installed the air duct there was a little flexibility on where exactly it fit to the lower cowl. Add to that some variablity on fitting the cowl itself to the firewall and that might explain why some of us have seen this and others haven't.

Jetguy said:

Yep been there done that to. How is the clearance on the front left header?

Click to expand...

Clearance on front is the same as aft now, about 1/4 inch.

As this issue seems common as I suspected, VANs ought to consider removing this small bulge on their fiber glass duct channel mold.

Thank you all for posting as I am just about to head into this part of the build.

Yeah, the rule of "thumb" is that you should be able to get a finger between the cylinder head and the air duct at that point. If you can't or it feels a bit tight, go ahead and do the fix while you are still building. It's easier that way than on a finished plane and the idea is that you are doing a better layup as you might have just finished your canopy at that point .
On mine I could barely squeeze the tip of my index finger in there and I decided to take a chance with it. I opened up the area and did the layup on my first oil change as I had clear marks on the duct .... Should have done it during the build...