Sealing bolt holes on a wood prop

I recently bored the bolt holes on my old spare Great American prop so I could use my 7/16” extension (the prop was previously 3/8”). What should I use to seal the inside of the bolt holes? I’ve read about spar varnish and sanding sealer, although I’ve seen arguments for and against each. Is there a definitive product for an unpainted surface like this?

sealer

the standard used to be a silver material that did not seem to stick to the bolts. I think some aluminum paste mixed with epoxy varnish would be good. The silver paste is used in the dope used for UV protection on fabric airplanes.
The poly fiber epoxy varnish without the silver is probably good also.
when I varnish the holes in wood structures I thin the first coat so it soaks into the wood. Then after the second coat if necessary run a reamer thru the hole turning the reamer backward by hand.

My opinion

Nothing….you want a tight fit that will not wear away. I recently removed a Warnke wood prop after 28 years in continuous use, nothing in the bores. Wood swells and contracts as seasons change, there should be no movement (tight fit). I see no value in sealing the bores but possibly making prop bolts difficult to remove.

Potential source of expertise

There was a website / forum for wooden propellers. Maybe your Cub, Luscombe, Champ etc buddies can be helpful. The Air Power Museum also knows a lot of smart people, call Brent.

Prop

Wood props depend on the crankshaft bushings and clamping force on the flange. They DO NOT depend on shear loads on the bolts. Spool type prop extension works the same way.
The bolt holes should be protected similar to the rest of the prop. On one of my props the bolt holes were not drilled accurately. I reamed the bolt holes .010 oversized. If the bolts bind in the drilled holes you will not achieve the proper torque value.

Unless

In my case, I was torquing nuts, not bolts in a threaded flange where a tight hole would have any influence/impact. Most RV use a spacer, not attached to crank directly.

Not sure if this will be helpful but...

Clark Lydick of Performance Propellers once told me to use nail polish to seal a small scar (caused by poor spinner clearance) in the wood of one of his propellers. A clever and simple "factory approved" cure. YMMV

jrs14855 said:

Wood props depend on the crankshaft bushings and clamping force on the flange. They DO NOT depend on shear loads on the bolts.

Click to expand...

Jim is correct as usual. I think the actual torque transfer numbers would surprise most readers. Henry Rose, then chief engineer at Sensenich, published the basic calcs in Sport Aviation way back in 1975. I've attached tables from the article. Compare the "Q" torque value given for our familiar SAE2 hub, flange friction vs bushings. The bushings are a backup, and only come into play if the primary torque transfer mechanism (static friction between the prop and the flange) should fail. FWIW, the calcs currently used at Sensenich are a bit more conservative.

Carved some props here and sealed the bolt holes with System Three Clearcoat, a very low viscosity, no solvent epoxy. Soaks in nicely. The holes can be reamed after cure.

The torque capacity can be increased by coating the rear face of the prop with a urethane rather than a traditional varnish, raising the coefficient of static friction.

rmarshall234 said:

Not sure if this will be helpful but...

Clark Lydick of Performance Propellers once told me to use nail polish to seal a small scar (caused by poor spinner clearance) in the wood of one of his propellers. A clever and simple "factory approved" cure. YMMV

Click to expand...

Nail polish would certainly be the easiest solution. The majority of the time this prop will be a wall hanger but it is my spare and of course, if on the airplane the bolt holes will not be directly exposed to the elements, seeing as they would have a bolt shank inside and a bulkhead on both faces.