Machinist Help, Advice?

The internal threads on this Ohlins top cap that thread into the damping rod have pulled out (the threads that go into the fork tube are fine). After exhaustive research and phone tag with Ohlins USA, this is an obsolete, sold out part, NLA. Have tried to go the used route to no avail. Still searching.

I am wondering if the threads in the cap can be repaired. I thought about a helicoil or something, but it is a very goofy size, M12 with a thread pitch of what I believe is .80 or .75 (hard to tell using my metric thread file as a gauge). Is it possible to weld and re-tap the aluminum center portion? (should have take a pic of the underside, will do that when I get home).

Looking for any ideas to repair at this point. Thanks.

 

Are some of threads just burred or what? Easy fix would be to turn it down, press a sleeve on and rethread, but that would probably be a bad move for that application without an engineer sorting out the numbers. Know anyone who can tig?? Weld that sucker up and start cutting!

 

Never heard of a .75 pitch but Ohlins does crazy sh*t just to keep their stuff "special". Good luck, welding and tapping on this precise scale is way over my skill level.

 

My first thought would to weld in an oversize alum plug after opening the hole up all the way through(so it can be welded in top and bottom), then redrill and tap to proper size.

 

I'd be surprised if that tap exists! Interesting project, for sure!

 

This appears correct: https://www.amazon.com/Metric-Right-Thread-0-75mm-Pitch/dp/B008574PXS

 

Learn something every day!

 

Lots of odd pitch taps availible at MSC.

 

Unfortunately the part I am referring to is a stand alone "pillar" on the underside of the cap (I know a picture is worth a thousand words!). Cannot be drilled all the way through because it will compromise the adjuster unit. My untrained eye tells me there is enough meat for a helicoil or insert, but they don't make them that size as far as my research reveals. Again, being a complete layman, the only possible solution I see is filling the "pillar" with weld and re-tapping, but don't know if that is feasible.

 

Wow.... Never thought of using amazon for work

Thanks... great stuff!!!

 

Very common thread pitch in the medical field of parts we machine.

 

Fine thread with soft material is never good. I don't like coils for repairs. I make my inserts solid by threading ID and OD out of solid stock. If you fill and redrill and thread, welding will shrink all other dimensions. So, if there are other bores near the weld, they will shrink or distort.

 

If you timesert/helicoil the pillar could you rethread the damping rod? Or connect the two via another means?

Why did the rod strip out of the top cap anyway?

 

How strong does the repair have to be? Could you line the old threads with QuikSteel and retap? QuikSteel has a shear strength of 740 pounds per square inch and a tensile strength of 6,200 pounds per square inch.

 

I don't know if it would be possible to re-thread the damping rod, as it is already cut for the fine thread?

The threads were slightly questionable when I first disassembled the forks to rebuild. I think what happened is the cap stopped screwing down on a bad thread about 1/8" of the way in, then the damping rod spun as I thought the cap was tightening. The cap and rod spun until they came into contact with the lock nut. So everything seemed in order when I re-assembled, not knowing there were very few threads actually holding the rod in place.

This is on a hare scrambles bike that also sees occasional duty on motocross tracks. I was in the middle of a race last weekend and found the forks topping out over ever bump and I knew what had happened.

 

I thought about this as well. There are actually at least 1/2" of good threads towards the top of the cap, I might try tapping and some Loctite.

 

Make a complete new part

 

Get a thread pitch gage and check the threads on the rod (external) to make sure the pitch is correct. If you're sure it's m12X0.75, you're limited in your potential fixes, because there aren't any threaded inserts available that size (I checked 5 different manufacturers).
If it's a m12x0.75 thread, I'd start with buying that size tap (you'll need it anyway) and clean up the threads, see if it holds the torque. If that doesn't work, clean it real good, fill with JB weld or Quicksteel and re-tap. If that doesn't work, you're going to either have a threaded insert made custom or remake the piece. Can that part be removed from the cap?

 

Unfortunately it is a single aluminum piece.

 

i'd go ahead and machine the threaded boss , make it a sleeve, and then create a press fit over the old boss which has been machined down for that press fit. fix it wit a few tacks of tig.