True crankshaft Ace 100

[Al Harpster]

What is a reasonable method to true the crank halves once re-assembled?

I can press them apart, replace the rod, press them together, put them in a lathe and indicate them between centers or indicate mounted on the crank bearing surfaces. I can measure the distance out-to-out of the two round discs that hold the crank pin.

I have not had success in aligning the two halves. I have a secured fixture plate with two short steel pins that locate in the two holes adjacent to the crank pin & hold the crank stationary. This secures one half of the crank. I have a long home made "wrench" with two steel pins that fit the two holes by the crank pin on the other half.

I can twist one half while the other is stationary with the crank pin as the pivot, but I get too much displacement. Displacement is announced by a "crack" sound when enough force is applied to get any movement. Seems like if there is no "crack" sound there is no movement.

I have not tried the "brass hammer" method. I assume the crank is held in one hand and the remaining crank is struck with the hammer by the other hand. Hitting this thing with a hammer sounds like a crazy way to do the job, but maybe it works?

There are lots of you tube videos showing how to indicate, but none showing how they got down to the .001 or .002" run-out in the videos.

If someone has a few tips on this I'd sure like to hear about them.

Thanks

[michael_perrett]

Yes the brass hammer and also using a wedge or a C-clamp will get them in line, but sometimes it takes time. I just spend a lot of hours on a Yamaha SR500 crankshaft. I thought I was going to wear out the floor between the lathe and the work bench (where the adjustment were made). I pressed together a 175SL crank and did not have to do a thing to it (a rare moment in history). Those are the two extremes. You can get them to with in .0005" run out at the bearing journals, which I do. Just take a moment to think which way you need to move the halves to either increase or decrease your run out.

Mike Perrett

[dcooke007]

I use a bench test center, surface plate gage and dial test indicator if the crank center points are in good condition. It is kind of difficult to explain this but I will try. There are two planes the crank halves need to align correctly to eliminate run out.

One, is how they clock on the crank pin to each other. Picture this by imagining two circles pinned together that are drawn on top of each other. When in perfect alignment only one circle will be seen. When out of alignment the out line of both circles will be seen over lapping in various amount depending on degree of misalignment. Clear as mud maybe?????

Second, although you would think when you press the crank halve on the pin it would press on true but that is not always the case. So the second area of misalignment is, are the crank halves perfectly 90 degrees to the crank pin. Almost always that is not the case.

Now to how to align the crank. With the set up pictured I measure the run out on BOTH crank bearing journals and mark the high and low spots with a Sharpie marker.

For condition one imagine a straight line drawn through the center of the crank 90 degrees to the crank pin center line. If this was the only area of misalignment you would find the low and high spots on the two crank bearing journals would be opposite each other. In this case I remove the crank from the bench test center and smack the high crank throw with a brass hammer.

For condition two you will find the high and low spots are on the same side of the crank assembly and you will have to wedge the crank apart a bit or squeeze it in the appropriate location in your vise.

The descriptions above are simplistic and in the real world you have to deal with a degree of misalignment in both planes. I usually try to get condition one as close as possible and then concentrate working both planes to less than .0008 inch run out.

I spent a whole day aligning my first crank but now have it down to about a half hour.....most of the time.

Danny

[Al Harpster]

Thanks for all the information about crank truing.

Just to be sure I understand:

Truing between centers by indicating on the bearing surfaces is a practical way to go? (Assume crank centers are OK)

Holding one half of the crank in one hand and striking the other half with a brass hammer will pivot one half around the crank pin?

I can adjust the parallelism of the crank halves by forcing a wedge between or squeezing them together approximately 180 degrees from the crank pin?

What weight brass hammer might I need? Harbor Freight has a 2 pound. That's a pretty big hammer to swing, but the crank pin fit is quite secure.

I've got plenty of time on my hands to work this through. I've got a ton of time in getting as far as I have.

Thanks again.

Al

[dcooke007]

Al,
From your comments it appears you understand my poor attempt to explain crank aligning. I have attached some hand drawn pics to try to illustrate for others. They are not to scale and are to demonstrate the concepts.

I believe truing between centers is the most accurate method, when the center points are in good condition, and you have an accurate method to measure run out between centers. If you look on the inside of the crank halves you can see it is center drilled also. So I am sure the crank halves were machined originally between centers. I have checked run out on individual crank halves with the bench center and, if not damaged, they have zero run out. The crank weights machined surfaces 90 degrees to the center line also had no run out. I was fortunate to purchase my bench center for a ridiculously low price, other wise I would have to support the crank with v blocks or other method to measure run out. And yes measuring on the bearing journals is how I do it.

I am sure you know all chucks and lathes have some degree of error. You could use your lathe to measure run out between centers if you can get an accurate and repeatable set up. One would be the tail stock must be properly aligned with the headstock. To eliminate run out in your lathe / chuck I would suggest chucking up a piece of steel and machining a 60 ??? degree center point. As long as you do not remove the machined center point from the chuck you should be able to remove and reinstall the crank between centers with out introducing mechanical error.

Holding one end of the crank and smacking the other with a brass hammer in the appropriate location is how I do it. I think my brass hammer is 16 ounces. You are only making small adjustments and the crank will pivot on the pin.

As for adjusting parallelism you can wedge the crank open or squeeze the crank as is appropriate. Out of parallel is not always EXACTLY 180 degrees opposite the crank pin but usually very close. You will have to measure and determine where it is out and adjust appropriately.

When you do get it aligned.....DO NOT DROP IT. When the crank is in the engine the bearings support the crank. Out side the engine all that hard work can be un done if dropped on a hard surface.

This is what the end view of the crank would look like when trued / aligned.

This is what the end view would look like with the crank halves not properly clocked / aligned.

And this is what the crank would look like when not 90 degree to the crank pin / parallel.

Man this is getting long......reminds me of some one else I know.

You can phone me if you need to...just pm and I will give you the number.

Danny

[michael_perrett]

Don't be afraid to do crankshaft alignment on your lathe.

Mike Perrett

[Al Harpster]

Thanks for that additional information, especially the tip about turning the center in the chuck to assure zero run out. I'd forgotten that one.

I also checked the halves between centers and concluded that the "shaft sections" had been ground between centers. Outside faces of the counter weights may have been side ground in the same set up.

I'm on the hunt for a cheap 16 oz brass hammer. Then I'll proceed.

Thanks again

Al