SOS ... Some crank truing help required please!


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Auckland, New Zealand
Hi guys,

So I'm hoping that someone has seen this issue before can help me out as I'm scratching my head a little and would be keen to hear what people have to say before I tear my rebuilt crank apart again....

So a bit of background, I'm building a new engine to go into a project bike I'm building at the moment, nothing too wild - 9:1 700cc JE pistons, hotter cam, cleaned up head, flat slide carbs etc. but staying with a 360°crank. So I wanted to make sure the bottom end was nice and strong and would be more than capable of handling it.

On inspection the little ends were worn so I decided to go all in and rebuild the crank including new main bearings.

I split the crank using the heavy wall pipe jig with some 1/2" press plates, it all came apart a lot easier than I thought it should but I was always going to weld it back together so I wasnt too concerned. Everything came apart (got thoroughly cleaned with acetone) and went back together as it should and I set all the big end side clearances close to the center of the spec.

Set it up in a V block as per the factory manual for truing it with a dial gauge etc. Got the drive side runout to about 1 thou without too much trouble, will try get it better but thought I'd better get the alternator side close first. This is where my issue starts. I have got it down to around 7 thou runout, but......the high point at the end of the taper keeps coming out in line with the big end pin (see badly drawn diagrams attached). Obviously I can't just tap this out as it is effectively trying to bend the pin. I've measured the parallel of the flywheel pair and they seem ok, but across the 2 pairs of flywheels I have about a 0.1mm difference from in line with the pin to directly opposite it. At first I thought it could be a bent center shaft but then surely i wouldn't be able to get the drive side in spec? Could the inner flywheel have pulled on the shaft when I welded it? I followed the "Hugh's Handbuilt" how-to for the welding procedure, with the addition of tacking in a star pattern around the shaft first to try minimize any chance of pulling.

Has anyone had this happen to them before, any suggestions or insights are greatly appreciated as I'm starting to run up against a brick wall as far as ideas go. The next thing I'm looking at is grinding out the weld and starting again ...........

Thanks in advance!


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I admire your welding skill.

Crank run out is caused by the crank wheels being not parallel or concentric.

There are two ways to check run out either with V blocks as you are doing or on centres at each end.

Depending on if V blocks or centres are used changes the way you correct the run out. Beware of this if you are researching how to true a crank.

I will only talk about V blocks.

When you have established the point of maximum run out look at the position of the crank pin.

If the max run out is 90 degrees to the crank pin the wheels are not conenctric.

If the max run out is inline with the crank pin the wheels are not parallel.

Non concentric wheels are corrected by hitting the wheel on the high point (at 90degrees to the crank pin). Use a big copper hammer it takes a bit of force to move the wheels.

Non parallel wheels are corrected by squeezing the wheels together with a clamp or vice or driving them apart with a wedge. The squeeze or wedge is applied in line but opposite of the crank pin.

In practice the run out is often a combination of non concentric and parallel.

If you go the wrong way the run out gets worse to correct just do the opposite of what you did previously.

If the centre is not true when you place the crank in the engine half the bearings will rise and fall as the crank is rotated. I have had this issue before and made a tool to straddle the cam sprocket to allow me to press the crank true. Actually I have this issue again with another welded crank for a motor that is on the back burner at present.

Truing a crank takes patience and I often take a break for a while an come back to it. You will get faster at it.

On this site somewhere under a crank rebuild thread is a PDF on crank rebuilding by an English firm I found it helpful. I hope this helps and doesn’t add to the confusion.
Not sure I can add something .But I had a Bike that had taken a Crash on the left side .I believe it had fallen and sliding .
Cover was weld repaired and the alternator shorted.
I repaired the alternator and had a bit of vibrations on the bike not thinking much over it.
A cheap bike and vibrations do happen on twins.
So after about 50 - 60000 miles or so there was noise coming out so I parked it .not wanting to ride it to the final curtain
and found another cheap one
Then when the second one started to give problems I decided to do a rebuild of the first
and took the crank to an experienced mechanics at the Yamaha dealer for inspection.
He said it was " Twisted " as I recall it .And used the words " Hammered " on it
I don't know if that is a possibility if it can get twisted. Upon rebuild it most definitely ran better .Less vibrations
Cant tell the reason for why.
But I am positive that the Word " Hammered " was used and the mechanic is the most senior at the shop now retired only do a few things now and then
when it comes I think he is the man for more vintage machines also.

So looking at your sketch there first picture it looks as if the alternator pin is pointing out off center line
And as a first thought than can most likely happen and as in post # 2 it can be a thing of patience and effort
As with truing spokes and wheels.
He did not speak of any bent or twisted metal part not the Alternator pin which could have been a possibility if it had taken a hit against
the gutter or so
But welding do sometimes cause problems. These looks fine :But there is always the material and shrinking.
It takes skills to weld many things and not moving things around. Procedures and so .Are we talking tolerances as here I would not have the Skills.
To do it

Finally the workbench background in picture 4 is something that puts a smile on my face.
The Holy Grail there somewhere ??? ..????
Hi Signal,
Thanks very much for your comprehensive reply!

I did use V blocks for determining runout - see attached pic below, only difference is I clamped the bearings into the V block for a bit more rigidity.

I totally understand when you refer to concentricity and parallel. The issue I am having is that the max runout on one side only occurs in line with the big end pin (implying parallel issues). However when i check the parallel between the flywheel pair it is pretty well spot on (measured across the flywheel machined surfaces with a micrometer). For ease of explanation I've attached a quick diagram below. Flywheels 1 & 2 are parallel, flywheels 3 & 4 are parallel, however when I measure flywheels 1 & 4 they are out of parallel.

Now this is where I get a little confused, at first I thought this pointed to a bent main shaft, however I was able to easily get runout of the the drive side of the crank in spec. Surely a bent main shaft would make it equally difficult to get runout on either side of the crank in spec (with high points always in line with the pin)? I'm only having trouble with one side. Which then points to the #2 flywheel somehow not being square to the main shaft, is this likely? Or am I overlooking something and my main shaft is indeed bent. When these shafts bend do they tend to bend at the connection to the flywheel on either side or do they tend to bend uniformly across the whole shaft?

I guess I'm keen to see whether anyone has had that main shaft pull when welded (I'm dubious of this given the length of the press fit shaft to housing on the inner flywheel) or whether anyone has had the same issue where one side tries up perfectly and the other has a persistent high point at the pin.

I should add that this crank came out of a running bike that I purchased, got going and stripped to rebuild so I dont know exactly how good it was running but all the roller main bearings were fine - so I'm guessing it can't have been terrible.

Thanks again guys, I'm still scratching my head but hopefully getting closer through a process of elimination.


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Jan P

Thanks for the reply,

You are exactly right- hammering (carefully) is how you remove twist from a crank to bring it into true. However unfortunately this isn't the issue I'm having - mine is suffering from one (or both) of the inner flywheels not sitting parallel which is a little more difficult to rectify. As if you "hammer" at the high point as you would for a concentricity issue you are effectively trying to bend the big end pin (which isn't going to go anywhere!).
Thinking out loud here .. Not certain that it is Physically possible Or I got this clear.-
But as I read it
pairwise 1,2 and 3.4 are parallel
1 and 4 are not
that means that 2 and 3 are not if I think right here
The suspicion is that 2 is not a 90 degrees at the axle ,,
Cant that be measured at the protruding axle end left on the picture
Measuring at the inside axially on 2 flywheel gives axial runout ( Wobbles )
that is not there at 3 axially
And if 1 , 2 parallel and axle out at 90 degrees to 1 ( correctly )
The the axle end would make a radial runout making a small circle
Please disregard if this is already measured.
This is my take on your situation, others may have different opinions.

I think you are getting hung up on the wording. The terms concentric and parallel are used to help visualise what is going on and what to do to adjust it.

Crank wheels apart from the areas that need to be precise are pretty crudely made and measuring the distance between the wheels I think would lead to variable results.

The end game is to get the get the ends and centre of the crank running as true as pracitable. Aside from going to extremes it does not really matter how the individual wheels relate to each other as long as the end product is a true crank.

I suggest you adjust the end wheel as necessary to get a runout that suits your intended purpose for the motor. Then place the crank in an engine case and check that the bearings are not lifting.

If its all good then job done, although to be thorough you may want to recheck the conrod side clearance.

Out of curiosity I checked a crank for trueness before it was taken apart and it was considerably out of tolerances given.
G/day Mike
I did a couple of cranks at work a few years ago and i had the opposite of what u have. i opened the flywheels by wedging them open opposite to the crankpin. I actually used a piece of solid round bar and not a wedge but the effect is the same. Now bear in mind this was NOT a welded crank and I was suprised by how flexible it was.
You need approx. 3 thou to get yours in spec. so try closing the flywheels a little in a vice. Go very slowly to see what happens. Now i want to conclude with a bit of a story, always time for a story.

Now when i did my mechanical apprentiship in 1970/71 at Oswestry Tech Collage. I was riding a BSA B33 on which i broke the con rod (yes i know, how the hell did i do that) I got permmsion from the collage to press in a new crankpin and Rod. Now while me and my mate John Jones were busy doing the crank the Automotive instructor (Wally Sager) came along. I'll show you a trick he said and he took the newly pressed crank by the the small end and swung the crank from about shoulder height down hard onto a heavy hardwood block keeping the the flywheels parallel. We could hardly beleve what we had just seen. Wally said "now check it for truth, I bet u it will be close. It takes the stresses out of it as well but that u will find out after you've done a few yourself "
Guess what, it was spot on for truth. Never seen or heard of that method since. I don't know what Wally's history was but that Collage had some incredibly good instructors, all you had to do was show an interest and they would show all the skills and tricks.
Oswestry is in Shropshire which is not too far away from the Black Country which once was the Hub of Automotive production in the Midlands, Birmingham, Wolverhampton etc, etc. I think Wally had picked up the black art from some of these Factories, just my guess.
He was right, once you've done a few u can sense the stress in the assembly from the pressing process.
Bit of a ramble, I know but it may help. Cheers Mike.
Hey Signal and Jonesey,

Thanks again to both of you for the replies!

Maybe you are right and I'm overthinking this all. We had a parallel issue with the drive side of the flywheels and we rectified this exactly as you both mentioned. However before we did that we measured across the machined surfaces (not the cast faces) on the outer sides of the flywheel pair with a micrometer and it was indeed out of parallel. We popped it in the press, gave it a small tweak and everything came out spot on. Our high spot was gone and the 2 flywheels were parallel.

I guess where my reluctance comes in just squashing the flywheels together opposite the pin is those 2 flywheels already are parallel (measured in the same method as the drive side), and if we are pressing these noticeably out of parallel the machined surfaces the thrust washers run against will be tapered which is obviously not ideal.

Or again am I thinking too hard about this all?

How does a bent main (center) shaft generally manifest itself when trying to true the crank? Is it possible to only have an issue like I am having on one side of the crank? Or generally will you see the high points at or opposite the pin on both sides of the crank? Am just trying to do my research before I jump back in and potentially damage something.

Thanks again
G/day mike
I'm an insomniac, its 1 am here.
I think Signal has kind of answered your question, but since your on a center V block without the outer mains fitted you could run two DTI's on the bearing journals and see if they rise and fall. it may give a picture of whats going on there. To be honest I've done very little measuring on the flywheels, usually just bashed and coerced the cranks in line.
Just one point, I use a big lead Hammer not a copper one, coz if u pick up a work hardened Copper hammer it will dent the flywheels. Don't ask me how i know. Cheers and good luck Mate.
Someone already said both these things but...
Measure the runout at the outer bearing journals, the farther out the better. The flywheels are too rough to measure from.
If it seems like you need to bend big pin either squeeze the offending flywheel pair in the press or drive a wedge in between them.
Worst case would be to press the crank back apart and then try again.
We hit them with a 3# copper hammer and it can take several hard hits to move a flywheel around. If you already welded it I'm not sure it will move much.
Are you tryin to measure runout on that block, on that bench with that dial gauge?
Hi Bikefreakjon,
Thanks for the reply. I'm measuring runout at the specified locations as per the factory manual which is slightly outboard of the outer bearing journals. I haven't measured runout on the flywheels at all. I have only measured across the machined surfaces on the flywheel to check for parallel. I have already pressed the crank apart and back together again - exactly the same result. What I was really trying to find out was whether anyone had dealt with a bent main shaft and how it manifests itself when measuring runout. My next step will be to squeeze flywheels together to rectify the issue but just wanted to make sure I wasn't missing something obvious before I did so.

WideAWAKE, yes that is the setup I am using with the addition of a clamp to hold the bearings down into the V block and minimize any additional movement.
What I was really trying to find out was whether anyone had dealt with a bent main shaft and how it manifests itself when measuring runout.
How does a bent main (center) shaft generally manifest itself when trying to true the crank? Is it possible to only have an issue like I am having on one side of the crank? Or generally will you see the high points at or opposite the pin on both sides of the crank? Am just trying to do my research before I jump back in and potentially damage something.
I have explained how a crank with a bent centre pin behaves in post #2 and post #7.
It is easy to spot when you check the cases using an engine case to hold the bearings.
How are you maintaining a zero point when you move that from side to side with the unevenness of that work bench??
WideAWAKE, What is being checked is the deflection of the shaft. It does not mater where the movement is on the scale as long as the start is greater than the expected deflection so a negative reading can be obtained.
So only measuring run out at each side and not how they relate??

I’ve only built a few cranks but I always measured them for run out side on each side and concentricity (how they spin in relationship to one another)

Adjustment done with brass and an adjustable wedge.

Seeing as how these are already welded, I’d think no adjustment is possible (at least it shouldn’t be, that’s the whole point of welding it)
I'm not trying to be a jerk but, you stated that the sides were within .001 thou. And the center was was at .1mm. That's apples to oranges, .1 mm comes in at about .003" Not what you want, but could be a source of confusion. Try using just one measurement standard.
That said, .003 runout between the centers, could very well result in .007 thou runout at the outboard end. Maybe your bent pin theory isn't that far off, after all. Or just needs a tweak in the right direction with a press.
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Hey guys,
A very delayed response but I finally got a chance to finish this thing off!

Signal - you were 100% right and I was overthinking things, I pressed the flywheels together slightly opposite the pin and managed to get the high point out. Managed to get one side down to about 0.015mm and the other to a hair over 0.020mm, so both well in spec now. Popped the whole crank into the lower case and ran a dial gauge over the centre bearings to check for centre pin bend, only got about 0.02mm of movement (which I would imagine is to be expected as it matches the runout on either end?).

So all looks to be sitting in spec, just need to weld the outer flywheels and start buttoning this thing up!

Are there any pitfalls to be wary of with welding the outer flywheels with pulling out of true or anything? I was just going to tack it solidly before welding completely.

Thanks again for all the advice guys, have learned a bunch along the way!
Glad you got it sorted.
As to welding the outer wheels Get them running true and tack them, then recheck. If all ok you may decide to leave it at that or finish the welding.
Are you building a race motor or did the crank press apart very easily?
If you are not racing welding the crank is not always needed. I have a motor that had done a lot of races and still going well without a welded crank.