I have a somewhat pricey bicycle part that requires repair because it is no longer made. It is an aluminum crank arm in which the threaded hole for the pedal has been cross threaded and the thread destroyed. From my research, it seems likely that the part was cold-forged, but I don't know what aluminum alloy was used. It was made by Sugino for Ritchey, if that helps anyone who has expertise in bike parts with a more educated guess on alloy. I had to ream out the remainder of the damaged threads to get to clean metal, using a carbide burr. I'm now to shiny metal, and now I need to fill the hole to original size in order to restore the pedal thread. Hole is now just over 5/8". Crank arm thickness is 1/2 inch or so... Restored thread size is 9-16/20. Cannot use a threaded insert because hole is oversize...
Two questions: (1) what TIG alloy rod would you recommend? 4043? 5356? something else? (2) would you fill in the hole completely, or try to just fill the circumference? I'm thinking filling in the hole completely would allow a better likelihood of centering the replacement hole in the crank arm, but would also make the surrounding metal much hotter, and maybe affect the alloy in some negative way...
Any suggestions/experience on best method of filling a hole this size in an aluminum part would be useful.
Also, recommendations to avoid overheating the crank arm while doing so, if that is likely to change the characteristics of the part.
I have a Miller 350DX available for the job.
All replies appreciated!
This might sound a bit complicated, but it is how I have done similar repairs in alloys with unknown specs.
Take a solid piece of 6061 and thread the outside of it. Drill your damaged hole a size larger than now, then thread it internally. Use LocTite, and thread the solid bar into the old hole.
Then drill and tap the new “slug” to the correct thread size and pitch.
You can buy a thread die and tap for this one time use without breaking the bank. And you’ll save yourself some headaches if the filler you use doesn’t quite standup to the stresses of the crank against the threads because porosity will be a huge concern with older cast alloys.
Take a solid piece of 6061 and thread the outside of it. Drill your damaged hole a size larger than now, then thread it internally. Use LocTite, and thread the solid bar into the old hole.
Then drill and tap the new “slug” to the correct thread size and pitch.
You can buy a thread die and tap for this one time use without breaking the bank. And you’ll save yourself some headaches if the filler you use doesn’t quite standup to the stresses of the crank against the threads because porosity will be a huge concern with older cast alloys.
Thanks for your reply and suggestion.
I considered that option, but since the hole is not quite centered due to the existing thread damage and reaming, I am concerned that if i get the hole much larger it will be too close to the outside edges of the part for the external threads to hold. I'll give that some thought and see if I can figure out how to hold the part so as to enlarge it so that it is precisely centered side-to-side and the same distance from the end radius of the part, and have dimensions that would allow a standard size thread to be tapped. I'll also measure to see if there is enough room to drill and tap the necessary internal threads leaving enough to keep it from failing.
I'd like to be able to TIG fill it completely, mill the outside surfaces flat, and be able to find center side-to-side and the same from the end radius of the part, assuming that the metallurgy would permit. If you were making an educated guess, which filler would you use knowing that it would have to withstand tapping at 9/16-20?
Thanks for your suggestions and recommendations.
I considered that option, but since the hole is not quite centered due to the existing thread damage and reaming, I am concerned that if i get the hole much larger it will be too close to the outside edges of the part for the external threads to hold. I'll give that some thought and see if I can figure out how to hold the part so as to enlarge it so that it is precisely centered side-to-side and the same distance from the end radius of the part, and have dimensions that would allow a standard size thread to be tapped. I'll also measure to see if there is enough room to drill and tap the necessary internal threads leaving enough to keep it from failing.
I'd like to be able to TIG fill it completely, mill the outside surfaces flat, and be able to find center side-to-side and the same from the end radius of the part, assuming that the metallurgy would permit. If you were making an educated guess, which filler would you use knowing that it would have to withstand tapping at 9/16-20?
Thanks for your suggestions and recommendations.
1. By slugging the hole you can locate the new hole anywhere within it to relocate the correct center. I get it that you’re concerned about the edge thickness, but that will also be a concern with welding it up then retapping it.
2. Before I venture a guess on filler, the 9/16 tapped hole threads to what? Steel? Alloy? I prefer 4047 for cast, but that REALLY depends on the application. I’d probably go with 4043/4943 if it was a case of “best guess”. Make sure you peen the weld during build-up.
You can post a picture by using the attachments button at the bottom. Might help to see the problem.
2. Before I venture a guess on filler, the 9/16 tapped hole threads to what? Steel? Alloy? I prefer 4047 for cast, but that REALLY depends on the application. I’d probably go with 4043/4943 if it was a case of “best guess”. Make sure you peen the weld during build-up.
You can post a picture by using the attachments button at the bottom. Might help to see the problem.
Thanks again for your comments.
The end product 9/16 tapped hole's internal threads attach the bicycle pedal to the crank arm. Most quality pedals have a steel axle; most of this type are threaded 9/16-20.
I've attached pictures of the crank arm and close ups of the portion with the smoothed hole. The user did not appreciate that the pedal was loosening until it nearly fell out, thus the damage was to the threads, and the shape of the through hole was affected. One common method of repair is to ream out the hole to 5/8-24 and using a steel thread sert of that outer size to mate with the crank arm. The inside of the thread sert is tapped the proper 9/16-20 size. The problem is that I cannot use that size threadsert because the existing hole had to be reamed larger than the proper exterior dimensions for a 5/8-24 tap. The steel threadsert is a good engineering fix, but I'd still have to fill the hole exterior diameter and redial to proper size for the 5/8-24 tap to use the threadsert. Holding the crankarm in a vise to get a perfect position on redialing the hole is complicated because the exterior surface of the crank arm is tapered with the "backside" about .100 wider than the "frontside"... Nice original design, but poses some challenges to repair.
Now that you can see the workpiece, would you still try the slug method? I do have 1" 6061 aluminum solid bar with a 1/2" center hole that I could machine the slug from...
Again, thanks for your thoughts on best alternatives.
The end product 9/16 tapped hole's internal threads attach the bicycle pedal to the crank arm. Most quality pedals have a steel axle; most of this type are threaded 9/16-20.
I've attached pictures of the crank arm and close ups of the portion with the smoothed hole. The user did not appreciate that the pedal was loosening until it nearly fell out, thus the damage was to the threads, and the shape of the through hole was affected. One common method of repair is to ream out the hole to 5/8-24 and using a steel thread sert of that outer size to mate with the crank arm. The inside of the thread sert is tapped the proper 9/16-20 size. The problem is that I cannot use that size threadsert because the existing hole had to be reamed larger than the proper exterior dimensions for a 5/8-24 tap. The steel threadsert is a good engineering fix, but I'd still have to fill the hole exterior diameter and redial to proper size for the 5/8-24 tap to use the threadsert. Holding the crankarm in a vise to get a perfect position on redialing the hole is complicated because the exterior surface of the crank arm is tapered with the "backside" about .100 wider than the "frontside"... Nice original design, but poses some challenges to repair.
Now that you can see the workpiece, would you still try the slug method? I do have 1" 6061 aluminum solid bar with a 1/2" center hole that I could machine the slug from...
Again, thanks for your thoughts on best alternatives.
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I don't believe that part is cast. It's forged or other manufacturing process, likely from a higher grade aluminium bar stock like 6000 series. For welding 6000 series, 5356 is indicated or 4943 would be good too. I wouldn't use 4043 for anything, certainly not for this repair. 6000 series also regains alot of it's strength over time, so if you go ahead with the repair, let it sit for a couple of weeks after completion before putting it back into service.
Flat out like a lizard drinkin'
Thanks, cj737 and Coldman, for the quick replies.
Yes, Ritchey's specification page said the crank was cold-forged, but didn't specify the alloy. I've asked Ritchey to check their files, but no word back yet.
The crank arm is about 1.25" across, so I'm thinking that I could use a 6061 slug in the 3/4" range, leaving about 1/4" of good original metal on each side of the crank arm, and after installation of the slug, drill 3 or 4 dimples around the circumference top and bottom and plug weld them to keep it from spinning out. I'd still have enough room to cut threads for and use the 5/8-24 steel threadsert that has the 9/16-20 standard internal thread for the pedal. It's the long way around the corral, but might be worth it to get the most reliable outcome. 4943 is supposed to have nearly the thread strength of 5356 with the better 4043-like flow so maybe that would be the way to go.
Sound like what you'd suggest?
Yes, Ritchey's specification page said the crank was cold-forged, but didn't specify the alloy. I've asked Ritchey to check their files, but no word back yet.
The crank arm is about 1.25" across, so I'm thinking that I could use a 6061 slug in the 3/4" range, leaving about 1/4" of good original metal on each side of the crank arm, and after installation of the slug, drill 3 or 4 dimples around the circumference top and bottom and plug weld them to keep it from spinning out. I'd still have enough room to cut threads for and use the 5/8-24 steel threadsert that has the 9/16-20 standard internal thread for the pedal. It's the long way around the corral, but might be worth it to get the most reliable outcome. 4943 is supposed to have nearly the thread strength of 5356 with the better 4043-like flow so maybe that would be the way to go.
Sound like what you'd suggest?
Probably too late now, but on the note of holding that odd shaped part.
Lookup a fractal vise, designed to hold even weirder shapes than that slight taper.
My other thought for a quick one of fixturewould be to just makeup a couple softwood blocks cut to the right taper to hold the part in a standard vise.
Lookup a fractal vise, designed to hold even weirder shapes than that slight taper.
My other thought for a quick one of fixturewould be to just makeup a couple softwood blocks cut to the right taper to hold the part in a standard vise.
Toggatug, thanks for the fixturing ideas. The fractal vise is cool, but looks like the options are find an antique, or buy a new one for around $2,500.00. On the other hand, I think that the softwood blocks might be in my price range. 
I'll post up what worked as soon as I get the tools here to do the necessary threading.
I'll post up what worked as soon as I get the tools here to do the necessary threading.