Why would someone spot weld the pin to the crank discs on a KT100 crankshaft assembly?

c2rower

c2rower

Member
I was given an old KT100 engine to "play with" and would like to do a little straightening of the crank assy before running it. Was this some old trick to try and make the crank "stronger"? It does not make sense to me. I was curious if anyone had ever come across this practice.


I have the V blocks set up under the bearing locations and indicators out on the mag side and clutch side. I am getting about 0.002" on one side and almost 0.004" on the other, which seems like quite a bit. As I understand it, there are two components to the truing process; 1: web needing to be spread or squeezed in (for runnout that is in phase) and 2: crank halves rotated relative to one another (for runnout that is out of phase). I also have a brass hammer to do the whacking. I am thinking the crank half phasing adjustment will not be possible because the pins will likely not rotate because of the weld.

If things won't budge I could try grinding out the spot weld on each side. The worst case would be a new assembly or find a workable used crank assy. Or just live with it.
 
If properly assembled with the correct (factory) interference on the crankpin to crankpin-bores, the crank will never move from running.

Once the halves are correctly "clocked", if there is still runout, then it's likely that the pin bore to centerline of crank half could be slightly different on each half.

Personally, I focus on getting the PTO side as near to perfect as possible, and I'm willing to live with a touch of runout on the ignition side.

PM
 
A few years ago, one of the engine builders on the west coast started putting a dab of weld on the crank pins to stop any potential movement as the reciprocating weight changed directions. He thought this slight movement could/would wallo the crank pin hole and make it untrue. Well, I'm not sure if there was any validity in that thought but, he won a bunch of races so no one disputed it. I personally have welded pins and also not welded them and never noticed any wear in the flywheels. If there was any run out while aligning, I always tried to balance it between both sides.
 
On the knife and fork/male female Harley cranks in Pro Fuel most people weld the pins. Another method I've seen is to drill/mill a hole right at the pin/flywheel junction and put a pin in thus creating a kind of keyway. And both methods can and do fail...96%+ Nitro has a mind of it's own.
 
Welding in crank pin = hack. If the pin is too loose, welding won't fix it. Also welding the pin usually makes it go out of wack.
 
I'm very fortunate. I know a hell-of-a-lot of 'Hacks' that have won a LOT of National events. Sundog, you don't have a clue about what you are talking about........
 
C2, if your V blocks are much wider than where the bearings race would set your probably getting some errors in your indication. What will happen is if you have deflection with that block half way to the crank end it's gonna pick it up at the bearing. Easy enough to check , put one indicator out on the end, the other on the bearings spot on the same side, see if you have 0 deflection at the bearing when spun. I had a truing stand made for that way of checking them, it had narrow blades the crank set on.
 
C2rower, Is your engine stock? The reason I ask is that I have a KT crank that has been welded & stroked. At first glance, it would appear that someone welded the crank pin to the flywheel. But, if you look closer, the weld was only on one side as filler & then the pin bores were moved by re-boring them.

Some pics might help.

Brian #89
 
Many mx engine builders tig weld the crank pin and have for many moons. Some don't, Many do. You see it more in four cycle cranks.
 
Jon Brogden said:
C2, if your V blocks are much wider than where the bearings race would set your probably getting some errors in your indication. What will happen is if you have deflection with that block half way to the crank end it's gonna pick it up at the bearing. Easy enough to check , put one indicator out on the end, the other on the bearings spot on the same side, see if you have 0 deflection at the bearing when spun. I had a truing stand made for that way of checking them, it had narrow blades the crank set on.
Click to expand...
Jon, I think I understand your point. In this case the crank saver sleeves are sitting in the v blocks. It is not perfect, but the error is reduced.
 
phantomkartracer said:
C2rower, Is your engine stock? The reason I ask is that I have a KT crank that has been welded & stroked. At first glance, it would appear that someone welded the crank pin to the flywheel. But, if you look closer, the weld was only on one side as filler & then the pin bores were moved by re-boring them.

Some pics might help.

Brian #89
Click to expand...
Hi Brian. I am pretty certain the cranks were not modified to increase the stroke. I will try and add some pictures if i can figure out how.
 
Well, here is what i did. I figure I have nothing to loose by trying to straighten them. I know I will eventually have to buy a new crank when the rod and bearing need replacement. I bought a 2 lb brass hammer at HF, and created a brass wedge from some rod stock. I made a sketch to understand the error component with the two indicators in phase. The out of phase component is more difficult for my simple mind so I made a little mock up of a crank assy. I got the idea from a guy on YT. That enabled me to figure out where to whack the crank half to correct the out of phase error. I approached it slowly, as it is kind of weird smacking something to straighten it. Having two indicators set up makes it easier to keep track of the relationship between the halves. I indicated as far out on the tapers as I could to maximize the error. The crank saver sleeves are in the Vees and I put some oil on on them to help make rotation easier. After about 45 minutes I got the indicated runout on each side within 0.0005 inches, which is good enough for me!

So I feel much better about running this crank and I got a little experience tweaking the assembly. Hopefully the pictures will show up in the post.

v blocks.jpg
crank sketch.jpg
 
Pete_Muller said:
If properly assembled with the correct (factory) interference on the crankpin to crankpin-bores, the crank will never move from running.

Once the halves are correctly "clocked", if there is still runout, then it's likely that the pin bore to centerline of crank half could be slightly different on each half.

Personally, I focus on getting the PTO side as near to perfect as possible, and I'm willing to live with a touch of runout on the ignition side.

PM
Click to expand...
Peter, are you the gentleman that was making the brass threaded inserts for the KT 100 cylinder? Are they available? I recently did a 8mm TimeSert - I did not think those brass versions were available. I just discovered this forum.
 
c2rower said:
Well, here is what i did. I figure I have nothing to loose by trying to straighten them. I know I will eventually have to buy a new crank when the rod and bearing need replacement. I bought a 2 lb brass hammer at HF, and created a brass wedge from some rod stock. I made a sketch to understand the error component with the two indicators in phase. The out of phase component is more difficult for my simple mind so I made a little mock up of a crank assy. I got the idea from a guy on YT. That enabled me to figure out where to whack the crank half to correct the out of phase error. I approached it slowly, as it is kind of weird smacking something to straighten it. Having two indicators set up makes it easier to keep track of the relationship between the halves. I indicated as far out on the tapers as I could to maximize the error. The crank saver sleeves are in the Vees and I put some oil on on them to help make rotation easier. After about 45 minutes I got the indicated runout on each side within 0.0005 inches, which is good enough for me!

So I feel much better about running this crank and I got a little experience tweaking the assembly. Hopefully the pictures will show up in the post.

View attachment 22806View attachment 22807
Click to expand...

Well done with the research and planning !
 
flattop1 said:
Sweet . Did the weld crack or had you ground it out ?
Click to expand...
I see no signs of cracking. Prior to set up I did smooth out the welds so they would more or less flush with the plates as they were touching the sides of the vee blocks as I rotated the crank. Any axial movement would cause some error when indicating on the tapered surfaces.
606B06BA-BC82-4965-882E-22C3B704C279.jpeg
A9EE5281-9FC5-49E1-8376-772A2515C0C7.jpeg
 
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