dweenk 

I picked up a Paramount PDG Series R-50 for cheap. I would like to replace the Bone Deth replacement fork with a rigid fork. I have a threaded fork that will work and looks remarkably like the original in the catalog - but it is threaded. The headset is in great shape, so am I risking steerer tube failure by clamping a stem to it?

Ronno6

I would not recommend using a threadless headset if the top race contacts the steerer tube in the threaded portion.
The tube may be weaker in the threaded area, creating a stress point where steerer exits the tapered collar.
The threaded portion of the steerer is typically reinforced by the threads of the threaded top race
on the outside and the quill stem on the inside. That moves the stress point inside the steerer below the top portion of the headset (inside the head tube of the frame.
What type of stem arrangement wold you use?

Possibly the more learned minds will have other opinions ??

dweenk 

The top bearing race is on a part of the steerer tube that is not threaded. The stem will be clamped to the threaded portion.

Ronno6

I do not believe that that is recommended.
The threaded portion will see the stress point at the bottom of the stem clamp.
I have inquired in the past about using a threaded fork using a quill/threadless adapter and stem to
load the threadless headset instead of the star nut, and was informed that it was a bad idea....
I believe that the quill stem or adapter reinforces the steerer by moving the stress point to the bottom of the quill portion inside the head tube (between the headset bearings.
Plus, I don't believe you would get suitable clamping force on the peaks of the thread pitches with a clamp stem.

I could be incorrect..............

dweenk 

I will not be using a quill stem. I intend to use the threadless clamp-on stem that came with the bike. The ONLY difference is that the steerer tube has threads at the top. I repeat - "The ONLY difference is that the steerer tube has threads at the top". There will be a star nut inside the steerer to hold things together.

Ronno6

I have to believe that clamping to the outermost peaks of the threads will have a difficult time grabbing with
adequate force to keep the stem in position. There is probably only 20% of the clamping area as compared
to an unthreaded tube. (in addition to the stress point/possible fracture scenario.)

I will yield to responses from those who are more knowledgeable than I...........

dweenk 

I am thinking that some people have tried this and have either had catastrophic failures, or not. I have a greater understanding of my own mortality now, so I am in no hurry to meet my maker. At the same time, I have always been the type to push things and try different ways of accomplishing things. That's why I am asking.

HillRider

The only safe way to use a threaded fork in a threadless arrangement is if the steerer is so long the threadless stem clamps below the threaded section, which can be cut off. Otherwise the threads seriously weaken the steerer and clamping a stem over them is not a good idea.

dweenk 

Thanks. When I put it together, I wondered about the reduction of material where the stem would clamp. If I were the only one to riding it, I might dare - but if I forgot about it and passed it on later - I would want to make sure it was sound.

rmfnla

It will not be sound.

It is not a matter of reduction of material; any cut into a surface creates a stress riser which is where a failure will begin.

Just don't do it...

dweenk 

Thanks. I will either put the Bone Deth fork back on the bike, or buy a threaded headset.

HillRider

Or, buy a true threadless fork.

FastJake

There's one possible solution to this problem that is solid. But it involves using bits of a threaded and threadless headset and a quill stem. Basically, if your threaded fork is a bit too long you can use the top pieces of a threadless headset, add whatever spacers you need above, and then use the upper bearing race and locknut from a threaded headset to keep the bearing adjustment. The stick a quill stem in the fork.

I've got a fixed gear setup this way that I've put thousands of miles on without issue. I can take a picture if desired.

FBinNY 

Yes. and possibly worse.

Threaded steerers have the wall thickness reduced by about by the threads. The design 100% depends on the support of a quill stem inside to buttress the cut tube, and carry stress below the lowest thread.

The possibly worse comes into play if the steerer snaps, while you're riding leaving the bar in your hands, but no longer attached to the fork.

fietsbob

threads remove material , that is backed up by the quill of the Stem.

An option: BBB BHP 20 BHP-20 - Headset parts - BBB **

Is a quill + spacers and the bolt that pulls up the wedge, is itself threaded thru the hex, in the bolthead.

and with that cap bolt the thread-less headset, is functioning normally,


** not sold in US, currently , but ordering overseas is common on this forum.

dweenk 

OK - this is the post to end the post. I am not going to do it. I went down to my LBS to pick up a part I needed - the new mechanic was Russian (Jackie had given the regular guys a day off). His English is a little better than my Russian, so I am pushing the solution into next week.

I can't blame him. He was assembling bikes and hoping no one would ask him questions that he didn't understand.

I am going either full threadless, or threaded. Parts availability will determine the path.