Upper cowling blisters to accomadate the spark plugs...

[JohnSoar]

Folks-How many cowling blisters are required? Four? Two? I have some aluminum cowling bumps but they are not long enough to easily cover both spark plugs...
Thoughts? There must have been different suppliers of these parts over the decades?
Thank you!
John B
N95605

[Hank Jarrett]

ALL kinds of companies made blisters, but I think most were never intended for airplanes. The plastic ones are mostly junk and the heat causes them to split and fall apart. There are long ones that cover front and real plug holes and shorter ones where you end up with four. The other ones I have seen are formed directly into the top cowl sheet metal by metal bumping the cowl into a tear drop cut out in a form.

Hank