Thread pitch questions

[Anonymous]

fine threads used where vibration may be a concern, as fine threaded fasteners are less likely to loosen off from vibration because of the difference in geometry between fine & coarse thread pitches…

or the fastener has to thread into a thin wall.

In this case – perhaps a combination of both.

And heed Frank’s advice on the repair. If you can’t do the Heli-Coil, consider brazing to fill in the holes, then re-drill and tap to original 1/4-28 NF. Threads will be every bit as strong as the original part. I did that with a 1955 Elto 22.5 HP, worked just fine.

Hope this helps.

Best,
T2

[frankr]

Tell ya why we see stripped ones. It has nothing to do with the thread pitch. It happens because people don’t have the notch in the shift rod perfectly aligned with the screw hole before inserting the screw. If not aligned, the screw drags on the edge of the notch, which wipes the threads off the screw. Then attempting to tighten the damaged screw wipes the threads out of the connector piece.

Moral: Use a flashlight to peer in there and make sure the notch is perfectly aligned before inserting the screw. Do this and you won’t be stripping them.

[frankr]

Yet another option to salvage stripped connectors. Solder a brass nut on the backside of the connector. Then you don’t have to hold it while tightening the screw and it won’t fall off when you remove the screw.

[opposedtwin]

Thanks everybody for your input! Lots to learn and know! Many good ideas here!

[PugetSoundBoater]

Dosen’t a fine pitched thread hold tighter and not loosen as easily as a courser thread? Along with new star washers, maybe those engineers had a good reason for using that setup ,even though we are running into problems with the connector that has been reused many times. Brass threads just wear, and we tighten them up really well for the umteenth time.

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[Anonymous]

One could always read up on some of the characteristics of NC and NF threads

http://www.katonet.com/article/coarsevsfine.html

https://www.google.ca/search?q=coarse+versus+fine+thread+usage&ie=utf-8&oe=utf-8&gws_rd=cr&ei=pW3lVqWZBuuRjwSk15vwAw

Hope it helps.

Best,
T2

[Mumbles]

To help get the shift rod installed correctly inside the connector, try painting the groove in the rod with some really bright nail polish! Hot Pink seems to work best for my tired eyes. To help prevent destroying the threads, make sure the internal star lock washer is on and just snug the bolt down firmly. It doesn’t have to be horsed down tight!

While you’re buying the Hot Pink polish, also pick up a jar of Day Glo Green. They will help to identify opposed twin port and starboard side connecting rods/pistons on assembly day, especially if they have been sitting under the bench for a year or two and the dog ate the notes you had taken.

[johnny-infl]

Having spent a good many years in the aviation community, I found that
most, if not all, of the threaded fasteners are fine thread.
This is to lessen the chance of vibration loosening the fastener.
A coarse thread fastener coming loose on a vibrating aircraft at 20,000 feet is
not a good thing.
Neither is a shift rod linkage that fails miles from the ramp.
Looking closely at the brass clamp, I see that the bolt actually fits
in the groove of the shift rods and that holds them together.
When properly torqued down, that was OMCs design to make a secure fitting.
Personally, I have never seen one fail (if installed correctly) maybe one of you mechanics have seen it ??

Edit: LOL – Mumbles beat me to it – so we are on the same page.

AWESOME photos Mumbles !! great presentation.

hmmmmmm now where can I hide neon pink and neon green nail polish in my tool box ?? lol

.

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