Author Topic: Welding a crankshaft?  (Read 1634 times)

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Welding a crankshaft?
« on: December 04, 2022, 07:58:05 PM »
Has anyone have experience welding up the end of a crankshaft? A Benelli single motor I recently picked up has a damaged threaded tip on the shaft end that secures a magneto. Looks like someone tried to get a very seized magneto off the taper and whatever puller was used damaged the shaft. About a 1/2 inch of the shaft is missing.



This crank is a spare but I would like to save it if I can

My thoughts are to use a 4 jaw independent Chuck and true it in the lathe, shave off the damage and using MIG weld it back up then turn the tip back to dimension and thread it in the lathe.

Itís not a load bearing part other than it holds the big nut that compresses the magneto onto the shaft.

Thoughts?

To get this magneto off I ended up grinding the brass magneto body off the steel center section so as to get oxy/acet onto the magneto sleeve, red hot, then a big bearing puller. A typical rotor extractor did nothing but damage the crank tip even more.



« Last Edit: December 04, 2022, 08:32:34 PM by Canuck750 »
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Re: Welding a crankshaft?
« Reply #1 on: December 04, 2022, 09:38:51 PM »
With that much damage, I think you would be better off finding find someone who could make a new crank half.

<edit> Is the main shaft pressed into the crank cheek?
« Last Edit: December 04, 2022, 09:39:48 PM by nc43bsa »
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Re: Welding a crankshaft?
« Reply #2 on: December 04, 2022, 09:44:30 PM »
OR . . .

You could have the threaded portion cut off the main shaft, drill and tap the shaft, and fabricate a threaded piece to replace the damaged portion.

Triumph twins retained the alternator rotor with a threaded stud and nut for decades.
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Re: Welding a crankshaft?
« Reply #3 on: December 04, 2022, 10:02:04 PM »
OR . . .

You could have the threaded portion cut off the main shaft, drill and tap the shaft, and fabricate a threaded piece to replace the damaged portion.

Triumph twins retained the alternator rotor with a threaded stud and nut for decades.

Now thatís an idea I hadnít thought of, interesting.
The shaft and flywheel are one piece.
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Re: Welding a crankshaft?
« Reply #3 on: December 04, 2022, 10:02:04 PM »

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Re: Welding a crankshaft?
« Reply #4 on: December 04, 2022, 11:22:00 PM »
Now thatís an idea I hadnít thought of, interesting.
The shaft and flywheel are one piece.

If you go that route, I'd red loctite the new piece into the crank.  If you ever need to remove it, you can heat it with a Mapp gas torch.
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Offline moto-uno

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Re: Welding a crankshaft?
« Reply #5 on: December 05, 2022, 12:27:46 AM »
  I might be able to help , what is the diameter of the flywheel ?  Peter ( in Burnaby, B.C)

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Re: Welding a crankshaft?
« Reply #6 on: December 05, 2022, 02:40:01 AM »
  I might be able to help , what is the diameter of the flywheel ?  Peter ( in Burnaby, B.C)

A couple of sketches





Thanks!

Jim
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Offline Mike Tashjian

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Re: Welding a crankshaft?
« Reply #7 on: December 05, 2022, 07:52:05 AM »
I would probably weld up the end and then turn it back to size. My mig has 70 series wire which is not too hard to turn.  Holding the large end I would face the small end and put a center hole back in. Then do the job between centers or holding the large end(chuck) and center in the small end.  Single point threading the end should not be hard at all with your lathe.  As to drilling and adding a piece that would be a second option if you are not confident threading a new end.  Too bad you had to destroy the flywheel.  I have used my cutting torch head to quickly heat parts like that for removal. You have to be careful and fast but usually I can do it with no damage.  I have also had good luck drilling rivets out and then just making new ones in situations where I was trying to save a part. Rivets expand when set but, really with your lathe making new ones any size or shape is pretty easy.   

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Re: Welding a crankshaft?
« Reply #8 on: December 05, 2022, 09:28:39 AM »
Would it be a viable option to clean up the damaged portion in the lathe by turning away the affected bits, then turn the recess a bit deeper where the nut and washer beds down and put a bit more thread on the shaft ?

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Re: Welding a crankshaft?
« Reply #9 on: December 05, 2022, 10:27:44 AM »
I'd try what Huzo is suggesting. The tapered shaft part and the woodroof key slot is clean so once the nut is tightened enough, it should all lock in nicely (obviously, they couldn't get it back out). If the tapered shaft were scuffed from stripped woodroof keys over the years, then you'd have to rely on a lot more torque on the nut to lock it all down.
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Re: Welding a crankshaft?
« Reply #10 on: December 05, 2022, 10:38:51 AM »
Drill the end and thread the hole. Works for Moto Guzzi and BMW.
Loctite (RC680) in a quality stud and you have an easy lifetime repair.
If it ever gets messed up just torch out the stud and install a new one.
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Re: Welding a crankshaft?
« Reply #11 on: December 05, 2022, 11:16:07 AM »
Drill the end and thread the hole. Works for Moto Guzzi and BMW.
Loctite (RC680) in a quality stud and you have an easy lifetime repair.
If it ever gets messed up just torch out the stud and install a new one.

I think this is the safer route for me, I trust my ability to weld up the shaft tip but my thread turning skills on a lathe are crap. I can drill a straight hole with the lathe, just have to be very careful to not snap a tap cutting threads. I need to decide upon a stud diameter, I think the threads are M12 on the tip, could probably get away with an M8 or M10 stud.
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Re: Welding a crankshaft?
« Reply #12 on: December 05, 2022, 12:59:06 PM »
I'd try what Huzo is suggesting. The tapered shaft part and the woodroof key slot is clean so once the nut is tightened enough, it should all lock in nicely (obviously, they couldn't get it back out). If the tapered shaft were scuffed from stripped woodroof keys over the years, then you'd have to rely on a lot more torque on the nut to lock it all down.
Iíve had good success with old BSAís and such on the clutch shaft, by lapping them with valve grinding paste then talcum powder.
All my Beezers had well fitted clutch bodies and all the kids had chaffed arsesÖ..! (We ran out of powderÖ) :embarrassed: :rolleyes:

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Re: Welding a crankshaft?
« Reply #13 on: December 05, 2022, 01:00:43 PM »
Drill the end and thread the hole. Works for Moto Guzzi and BMW.
Loctite (RC680) in a quality stud and you have an easy lifetime repair.
If it ever gets messed up just torch out the stud and install a new one.
I reckon thatís what Iíd do. Seems the best way to save the whole damn thing.

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Re: Welding a crankshaft?
« Reply #14 on: December 05, 2022, 01:03:14 PM »
I think this is the safer route for me, I trust my ability to weld up the shaft tip but my thread turning skills on a lathe are crap. I can drill a straight hole with the lathe, just have to be very careful to not snap a tap cutting threads. I need to decide upon a stud diameter, I think the threads are M12 on the tip, could probably get away with an M8 or M10 stud.
Itís possible your lathe has a metric pitch leadscrew.
If it has then the TPI will almost certainly not be obtainable if attempting to cut the thread.

Offline Mike Tashjian

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Re: Welding a crankshaft?
« Reply #15 on: December 05, 2022, 01:29:05 PM »
Now that you have picked a repair option let's explore the stud size.  Metric 8.8 bolt torque for m8 x 1.75  18.8 ftlbs, M10 x1.5  37.2 ftlbs,  M12 x 1.75 64.9 ftlbs.  Looks like you should be aiming for the M12 if you go this route.

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Re: Welding a crankshaft?
« Reply #16 on: December 05, 2022, 01:49:41 PM »
Agreed, I'd match what is there already, but drilled as you plan (which I think is a great idea). Also, make note of the thread direction and match that.
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Re: Welding a crankshaft?
« Reply #17 on: December 05, 2022, 06:54:17 PM »
OR . . .

You could have the threaded portion cut off the main shaft, drill and tap the shaft, and fabricate a threaded piece to replace the damaged portion.

Triumph twins retained the alternator rotor with a threaded stud and nut for decades.

I would go this route!  ^^^^

Hard to tell from the picture, but it looks like the crank end is tapered.  The taper hold the flywheel in place, the keyway controls timing and the threaded portion just keeps a bit of compression on the flywheel to keep it from vibrating loose.

Male thread on crank end, or female thread inside crank end.  Not a dimes worth of difference either way.

Just be mindful that your final torquing values should always consider thread stress area.  Smaller thread, lower torque.

If you decide to go the other route of rebuilding crank end with weld metal, and re-machining the thread, it would be a very easy fix for almost any machine shop I have ever encountered.
« Last Edit: December 05, 2022, 06:57:22 PM by SIR REAL ED »
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Re: Welding a crankshaft?
« Reply #18 on: December 05, 2022, 07:02:11 PM »
I think this is the safer route for me, I trust my ability to weld up the shaft tip but my thread turning skills on a lathe are crap. I can drill a straight hole with the lathe, just have to be very careful to not snap a tap cutting threads. I need to decide upon a stud diameter, I think the threads are M12 on the tip, could probably get away with an M8 or M10 stud.

If in doubt, load the tap into the tailstock, and just turn the chuck by hand or with a strap wrench.

Just for giggles, check Mcmaster.com for shoulder bolts.  You might get lucky and based on the thread size you pick, end of with a should diameter that matches the flywheel inside diameter.
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Offline n3303j

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Re: Welding a crankshaft?
« Reply #19 on: December 05, 2022, 08:52:08 PM »
If that is a flywheel on the crank I'm pretty sure the key is used for timing purposes, not flywheel retention. Flywheels are often located with keys but retained by being bolted firmly on a locking taper. They are fit dry on the taper so they lock. The BMW R60/2 locks its flywheel on to the tapered crank with a single bolt through the center of the flywheel. This bolt is tightened to 175 ft/lb. It is also keyed to put the timing marks in the correct place. If the flywheel is not tightened fully it shears the key then damages the taper.

If your crank is similarly configured. I would suggest you make the stud out of a 12.9 bolt using the threaded portion as a stud (cutting off the rest)  Drill and tap the crank the diameter and pitch of the existing stub. Make the working threaded hole depth 2 times the diameter of the existing stud. That way it won't strip out. Then torque to original specs, if available. Otherwise use a good reference chart for the materials  used.

« Last Edit: December 05, 2022, 08:55:10 PM by n3303j »
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Re: Welding a crankshaft?
« Reply #20 on: December 05, 2022, 08:55:44 PM »
If in doubt, load the tap into the tailstock, and just turn the chuck by hand or with a strap wrench.

Just for giggles, check Mcmaster.com for shoulder bolts.  You might get lucky and based on the thread size you pick, end of with a should diameter that matches the flywheel inside diameter.

I see where you going with the shoulder bolt, do you know if anyone makes a stud that is 12mm on one side and 10mm on the other?

Based on the measurements I took off a good 200 cc crank, the threaded end of the crank shaft that takes the rotor nut only projects 13mm (1/2") and is 12mm diameter. I am a little reluctant to use a 12 mm stud into the end of the crank, not much meat there at the end of the taper. I think a 10mm could be drilled and threaded into the crank shaft. Alternately I could just machine up a 10 mm deep nut for the rotor securing fastener. Pretty sure the thread pitch is 1.25 fine (maybe 1.5 - will check tomorrow), will probably need to order a stud, should I use a carbon steel stud or a stainless steel stud?

This is the deep nut used on the rotor, off another engine I have



I usually tap threads on parts I can chuck in the lathe and use a spring die holder tool in the tail stock to keep the tap straight and under light pressure, turning the chuck by hand while holding the tap wrench stationary, rocking the chuck back and forth to clear chips. I can chuck up the crank flywheel in my 4 jaw, center it and if I am real picky set up a steady near the tip before drilling and tapping.
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Re: Welding a crankshaft?
« Reply #21 on: December 05, 2022, 09:02:11 PM »
If that is a flywheel on the crank I'm pretty sure the key is used for timing purposes, not flywheel retention. Flywheels are often located with keys but retained by being bolted firmly on a locking taper. They are fit dry on the taper so they lock. The BMW R60/2 locks its flywheel on to the tapered crank with a single bolt through the center of the flywheel. This bolt is tightened to 175 ft/lb. It is also keyed to put the timing marks in the correct place. If the flywheel is not tightened fully it shears the key then damages the taper.

If your crank is similarly configured. I would suggest you make the stud out of a 12.9 bolt using the threaded portion as a stud (cutting off the rest)  Drill and tap the crank the diameter and pitch of the existing stub. Make the working threaded hole depth 2 times the diameter of the existing stud. That way it won't strip out. Then torque to original specs, if available. Otherwise use a good reference chart for the materials  used.


Yes, the woodruff key is set the rotor to the timing, the points cam is on a sleeve over the steel spindle riveted to the brass flywheel. The sleeve rotates slightly to form the advance mechanism, a single weight pivoting on a fixed pin and spring retained, causes the sleeve to rotate with rpm.



Good tip on the stud, I can get a high strength bolt to make the stud
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Offline n3303j

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Re: Welding a crankshaft?
« Reply #22 on: December 05, 2022, 09:06:28 PM »
Not a lot of meat at the smallest of the taper but it is supported by the 12 mm stud inside and compression from the flywheel outside. Alloy steel stud only. Either cut it out of a 12.9 bolt or socket head cap screw if you can't find the equivalent stud material. Stainless is much weaker. There's a reason they put a 12 mm thread there.
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Re: Welding a crankshaft?
« Reply #23 on: December 05, 2022, 09:12:06 PM »
Not a lot of meat at the smallest of the taper but it is supported by the 12 mm stud inside and compression from the flywheel outside. Alloy steel stud only. Either cut it out of a 12.9 bolt or socket head cap screw if you can't find the equivalent stud material. Stainless is much weaker. There's a reason they put a 12 mm thread there.

OK thanks, I will look for high strength M12 bolt to use.
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Re: Welding a crankshaft?
« Reply #24 on: December 05, 2022, 09:40:21 PM »
McMaster Carr has 10.9 12mm threaded studs 150,000 PSI Yield.
They also have alloy steel socket cap screws 12 mm 170,000 PSI Yield.
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Re: Welding a crankshaft?
« Reply #25 on: December 05, 2022, 10:36:37 PM »
I see where you going with the shoulder bolt, do you know if anyone makes a stud that is 12mm on one side and 10mm on the other?

Based on the measurements I took off a good 200 cc crank, the threaded end of the crank shaft that takes the rotor nut only projects 13mm (1/2") and is 12mm diameter. I am a little reluctant to use a 12 mm stud into the end of the crank, not much meat there at the end of the taper. I think a 10mm could be drilled and threaded into the crank shaft. Alternately I could just machine up a 10 mm deep nut for the rotor securing fastener. Pretty sure the thread pitch is 1.25 fine (maybe 1.5 - will check tomorrow), will probably need to order a stud, should I use a carbon steel stud or a stainless steel stud?

This is the deep nut used on the rotor, off another engine I have



I usually tap threads on parts I can chuck in the lathe and use a spring die holder tool in the tail stock to keep the tap straight and under light pressure, turning the chuck by hand while holding the tap wrench stationary, rocking the chuck back and forth to clear chips. I can chuck up the crank flywheel in my 4 jaw, center it and if I am real picky set up a steady near the tip before drilling and tapping.

I'm sure there are lots of double sized studs out there to buy off the shelf, but off the top of my head, I can't think of a vendor.  Any local machine shop could probably fab one for you.

I would stay away from stainless in this application and use a good high quality steel stud or bolt.  Grade 8 in the US, I forget the DIN equivalent.  IIRC, L10 is even stronger than grade 8.  Carbon steel shoulder bolts are usually very high strength.  McMaster-Carr will have all the specs you need.

I'd bet good money, that a high strength bolt will be much stronger than the steel in the crankshaft.  Occasionaly I might lose a bet, but not often.

My thought process was to use the shoulder bolt instead of a double threaded stud.  I was thinking you might be able to match the shoulder diameter to the diameter of the thru hole in the flywheel.

If you think about, the primary use of the nut on the end of the crank is to press the flywheel on to the crankshaft taper.  Once that is done, it only serves to keep the flywheel from vibrating loose on the taper.

Some of the engineering reference materials will state that the first 3 threads that get stretched carry 90% of the bolt tensile load.  I would be reluctant to put a lot of confidence in that with my own equipment.  One thread diameter of thread depth is usually plenty, two is kind of a "rule of thumb" upper limit, and when you are designing, a thread depth of 3 diameters often gets overly expensive to machine.

So if you use a bolt instead of a stud, I would shoot for 1-2 thread diameter of bolt engagement in the crankshaft.

Of course, make sure you torque is not attained by bottoming the bolt in the blind hole.

So far, it looks like you are thinking this thru properly and getting plenty of good advise from the posters on this thread.  Should work out fine.
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Re: Welding a crankshaft?
« Reply #26 on: December 06, 2022, 03:12:45 AM »
I confess to being interested in this thread and out of my depth but I've had a though. Could you not buy an M12 stud and cut an M10 thread in one end, or is there not enough metal?

Offline Mike Tashjian

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Re: Welding a crankshaft?
« Reply #27 on: December 06, 2022, 09:50:39 AM »
I am missing something here.  I thought I saw pictures of your lathe in a previous post and it looked nice.   You all know lathes are great at making round parts and round parts with threads right?.  Why are we not thinking of making parts exactly as we want instead of buying one that is pretty close. I think a little more time to learn how to use a lathe is in order.  I have been making parts for quite a while and these are some of the most basic parts one might make with a lathe.  I have seen your work on more than a few bikes and see no reason lathe work should not be an easy step forward.

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Re: Welding a crankshaft?
« Reply #28 on: December 06, 2022, 10:13:07 AM »
https://youtu.be/AuS6HDHc7XE
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Re: Welding a crankshaft?
« Reply #29 on: December 06, 2022, 10:52:46 AM »
I am missing something here.  I thought I saw pictures of your lathe in a previous post and it looked nice.   You all know lathes are great at making round parts and round parts with threads right?.  Why are we not thinking of making parts exactly as we want instead of buying one that is pretty close. I think a little more time to learn how to use a lathe is in order.  I have been making parts for quite a while and these are some of the most basic parts one might make with a lathe.  I have seen your work on more than a few bikes and see no reason lathe work should not be an easy step forward.

My lathe skills are very basic at best, agreed I need a lot more practice before I could replicate the entire left hand crank piece. Welding an extension on the broken tip and machining it back with threads is doable but I think the threaded stud insert is a good option. I am still learning to make decent nuts and specialty bolts.
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