First post-new question: cork in bolt holes

[ggodshall]

hello, I'm curious as to why the bolt holes around the hub are filled with cork. It's a Curtiss model MF flying boat. I've done significant research and the piece has been in my family for almost 100 years now. original condition, not restored and i don't think it's ever been installed on a motor. I got all the markings and know it's a USN prop. it's the cork that I'm baffled by. Any ideas?

Thanks!

[Dbahnson]

I certainly don't know the answer, but my guess is that it's to reduce ventilation within the hub. The central bore is painted, which resists moisture transfer, but the bolt holes are too small to paint (and paint could reduce their diameter and make bolt passage more difficult) so they are bare wood, which allows moisture in and out much more readily.

Like any wood, the hub will contract and expand perpendicular to the grain and remain very stable parallel to the grain. If the bolt holes allow easy airflow the hub is more likely to expand and/or contract from its original size, so lining up the holes with the hub could be more difficult if it has spent much time in storage, as many of these props did.

Just out of curiosity, you might try to very accurately measure the distance between two holes that are parallel to the grain and compare that to two that are across the grain and see how much variation there is.

[ggodshall]

Thoughts

I was thinking something installed at the factory to protect the threads. I'm sure this is an example of a prop never used. just my educated opinion is that the cork was normally plucked out before installation. Like a fine wine.

[Dbahnson]

I was thinking something installed at the factory to protect the threads. I'm sure this is an example of a prop never used. just my educated opinion is that the cork was normally plucked out before installation. Like a fine wine.

Yes. By necessity the cork had to be removed, either by "plucking" as you say (e.g. even with a corkscrew) or simply by running a drill bit through the original bolt hole. I suppose you could even just hammer it out with the bolt itself or a drift of some kind, but no matter what the cork did not have a permanent purpose.

[Bob Gardner]

Two thoughts.

Firstly I was told by someone knowledgeable that a cork sheath reduced the ovality in a worn bore hole caused by the thrust plane and vibration.

Secondly, cork plugs in bolt holes prevented the egress of insectidae to lay eggs when props were in store.

With kind regards,

Bob