Fasteners question: head sizes and drive options, is it important?

[musiclover]

I had some problem with my derailleur hanger: for some reason the bolt that held it attached to the frame destroyed the thread on the hanger. This also damaged the thread on the bolt so I discarded it.
Then, I have been using a standard phlips round head bolt (non countersunk) in the place of the old countersunk bolt with a nut on the other side to hold it in place.
I thought it was incorrect as there was a limited amount of contact of the bolt head on the frame with this set up (there was a countersunk hole in the frame for the bolt head).

I decided to replace a derailleur hanger so that I get one with new threads and am able to use a countersunk bolt again.

Now, to the question:
The hole in carbon frame for the bolt head is quite a bit bigger than the bolt head, so the bolt head does not sit flush to the frame but goes in. The original bolt head for hex drive is a little bigger (probably around 0.1-0.2 mm) the countersunk philips head bolt that I found comes in a smaller size.

Does the size of the bolt head affect the stress on the countersunk hole walls? Can a smaller head damage the epoxy? There is definitly some marks from the fromer round head bolt. It would be nice to find a bolt with a bigger countersunk head with otherwise the same size to make it seat flush against the frame...

Do the countersunk heads come in different sizes?

PS All countersunk bolts do not sit flush in the frame. It seems that the countersunk hole that they made in the frame is a bit bigger (deeper) than it should be for a standard head countersunk bolt.

[mikemelbrooks]

A smaller head will concentrate pressure over a smaller area. It depends on how tight the bolt is wether it damages the frame, too loose is not good either as movement can wear away the epoxy. Most frames the derailleur hanger is secured by the skewer or thruaxle with the countersunk screws there just to align it.

[musiclover]

A smaller head will concentrate pressure over a smaller area. It depends on how tight the bolt is wether it damages the frame, too loose is not good either as movement can wear away the epoxy. Most frames the derailleur hanger is secured by the skewer or thruaxle with the countersunk screws there just to align it.

This is the way it should be, but my Boardman frame has a defective design in that sense.

[musiclover]

Anyway, looks like the head size is pretty standard: https://www.boltandnut.com.au/socket-al ... key-screws

Was not able to find anything with a bigger head.

[musiclover]

Maybe, a nylon washer?..

[spartacus]

https://www.mcmaster.com/

[Cleaner]

Make sure that you match the angle of the screw head to the countersink it will mate with. There is more than one standard for countersink holes and if the angle is not matched it cause high stress in the screw and bolted parts.

If you have not already, I would confirm with the manufacturer the specification for the screw and threads.

[Attermann]

if it's a metric thread, with it's probably is, then it uses the iso standard of 90.

[musiclover]

if it's a metric thread, with it's probably is, then it uses the iso standard of 90.

Correct. And then the size of the head is just dependant on the drive type. But the difference in sizes is very minor.
The only possible way to improve the fit a bit is probably a washer. But then again - most washers are just flat. There is no 90 degree washers to match with countersunk head.