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Crankshaft Rebuild - How to Split the Crank?

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I want to build my expertise on crank rebuilds so I can do the whole job myself. I've seen plenty on how to balance the crank, I've got a nice 20T hydraulic press and all.

But now, how do I split the crank without some kind of cradle jig? If the pin clears the crank webs on both sides, is it okay to push it through?

I have two cranks to work on, one from a KX100 that I think I could push through, the other is from a CRF450 where the pin is blocked by the shaft on either side. I suspect the CRF crank would require a special jig puposely milled out of a big steel plate. That's kind of a turn off for me.

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I've already evaluated the cost and decided it was worth the effort. You can get a OEM quality rod for 1/3 the price of an aftermarket crank assy... 

Anyway, cost is not the issue here. How do you split a crank?

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Proper jigs. Then you'll need a truing stand with dial indicators. Lead mallet and brass wedges for truing. A skilled guy can do a crank in thirty minutes, unskilled, several days.

To redo a crank, you may need to replace the big end pin too.

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One local shop wanted $125 to install my new rod.  Then, I found an independent local mechanic who did it for $35.

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I want to build my expertise on crank rebuilds so I can do the whole job myself. I've seen plenty on how to balance the crank, I've got a nice 20T hydraulic press and all.

But now, how do I split the crank without some kind of cradle jig? If the pin clears the crank webs on both sides, is it okay to push it through?

I have two cranks to work on, one from a KX100 that I think I could push through, the other is from a CRF450 where the pin is blocked by the shaft on either side. I suspect the CRF crank would require a special jig puposely milled out of a big steel plate. That's kind of a turn off for me.

I have always used a set of bearing splitters to place on the press and push the pin from the outside in.  Obviously you put something under the press to do a soft catch on the crankshaft and rod as it falls away.  The 20 ton presses are not always what they seem,  and the frames tend to flex more on them than say a 40-50 ton will.  A way that I would try if I ever decided to do one of the Honda sheetmetal cranks would be to cut the pin through the oil slots on the rod,  and then press it flat on the press table.  Don't ever push the pin through both sides at once,  and don't ever press the surface that was run on by the bearing through the flywheel of a crankshaft.  Knock wood,  probably 50 single cylinder crankshafts and only one needed anything from wedge.  That is a crude kind of machine work to begin with,  so large chisels are probably about as good as you need,  and that is what I used.  My truing stand is usually a lathe.   (I don't strike the work while it is in the stand.)

Everyone works differently,  but if you put a similar crankshaft on whatever you decide to use as a truing stand,  just so you can see what it looks like eyeballed (the people who taught me used to hold the crankshaft up similar to a violin and you look across  the flywheels.  When I saw them site across those I couldnt' believe it would showup as out of phase like it will.  try it a few times and you will see.   

I used to put the crankshaft in the stand and use a square to scribe a mark for initial lineup on the press.  It is a way to start.  Most abandon  that before long. If you use that method,  I suggest you place it in a position that won't draw you line of sight when you are doing the "violin" phase of the alignment.  I like brass rather than lead as a hammer.  12-16 oz.  I usually weld them with a bit of wire feed.  (If you get all teary eyed about balance, mark your wire off and then put the same amount on the other side.  Again,  hard to do with the sheetmetal hondas.  God Honda has built some screwball crankshafts.) 

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Proper jigs. Then you'll need a truing stand with dial indicators. Lead mallet and brass wedges for truing. A skilled guy can do a crank in thirty minutes, unskilled, several days.

To redo a crank, you may need to replace the big end pin too.

The only big end pin I ever reused was the one that is cast into the early (79-82) honda xr 500's.   Enter the first really stupid design that I saw from Honda.  They are my favorite of all times brands, but that was  a test of loyalty.  

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2 hours ago, ossagp said:

Don't ever push the pin through both sides at once

I kind of caught that, but what's the reason behind?

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Us to do them back in school but now just get new or have bike shop so them. You'll need a trueing table, dial gauges, brass gamer and some heat if I remember right lol goodluck :thumbsup:

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3 hours ago, Still Bill said:

One local shop wanted $125 to install my new rod.  Then, I found an independent local mechanic who did it for $35.

If the local guy doesn't know what he is doing and your crank comes apart, the $90 you saved will look like chump change compared to the cost of replacing all the bits that will get destroyed.   Sometimes it is worthwhile paying extra to ensure the job is done properly.

 

 

10 hours ago, 975 said:

I want to build my expertise on crank rebuilds so I can do the whole job myself. I've seen plenty on how to balance the crank, I've got a nice 20T hydraulic press and all.

But now, how do I split the crank without some kind of cradle jig? If the pin clears the crank webs on both sides, is it okay to push it through?

I have two cranks to work on, one from a KX100 that I think I could push through, the other is from a CRF450 where the pin is blocked by the shaft on either side. I suspect the CRF crank would require a special jig puposely milled out of a big steel plate. That's kind of a turn off for me.

Without being a jerk, I might gently suggest that if you have to ask these questions, you may not be ready to start rebuilding cranks?

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Thanks all for the replies. I wanted to test it out for myself, I went ahead and pushed the pin through on the kx100 crank. That worked relatively well!

Crank is split and I already pushed the new pin in so I'm pretty much halfway through.

Now the problem is the pin doesn't go in smoothly; pressure builds and then it gives at once by jumping a couple mm. That's pretty scary actually and I bet it'll be impossible to have any precision for pressing the other web on.

Feels like I may have reached the limit of the press and it sticks because the base plate flexes under the pressure. Is that show stopper or is there a workaround?

Would heating up the webs help?

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6 hours ago, 975 said:

I kind of caught that, but what's the reason behind?

You pick up tiny little fragments,  then you push them through the next half,  as well as those same fragments hitting the surface your bearing is going to run on.  Ditto about pushing the surface the bearing was running on through a crank half.  I found doing work like that satisfying.  

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4 hours ago, 975 said:

Thanks all for the replies. I wanted to test it out for myself, I went ahead and pushed the pin through on the kx100 crank. That worked relatively well!

Crank is split and I already pushed the new pin in so I'm pretty much halfway through.

Now the problem is the pin doesn't go in smoothly; pressure builds and then it gives at once by jumping a couple mm. That's pretty scary actually and I bet it'll be impossible to have any precision for pressing the other web on.

Feels like I may have reached the limit of the press and it sticks because the base plate flexes under the pressure. Is that show stopper or is there a workaround?

Would heating up the webs help?

That is a never for me.  I think that one will have trouble staying true if you don't weld it.  the top half will be fine.  heating may help.    I think if you are going to try the 450 crank you are better off cutting the pin,  then pushing outward.   All in learning.

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4 hours ago, 975 said:

Thanks all for the replies. I wanted to test it out for myself, I went ahead and pushed the pin through on the kx100 crank. That worked relatively well!

Crank is split and I already pushed the new pin in so I'm pretty much halfway through.

Now the problem is the pin doesn't go in smoothly; pressure builds and then it gives at once by jumping a couple mm. That's pretty scary actually and I bet it'll be impossible to have any precision for pressing the other web on.

Feels like I may have reached the limit of the press and it sticks because the base plate flexes under the pressure. Is that show stopper or is there a workaround?

Would heating up the webs help?

When I said 20 tons can be light,  things can flex and stutter.   

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16 hours ago, CDNSXV said:

If the local guy doesn't know what he is doing and your crank comes apart, the $90 you saved will look like chump change compared to the cost of replacing all the bits that will get destroyed.   Sometimes it is worthwhile paying extra to ensure the job is done properly.

 

 

Without being a jerk, I might gently suggest that if you have to ask these questions, you may not be ready to start rebuilding cranks?

No worries - Fast Frank is a retired mechanic who worked 20 years at local Jap bike dealer.  His son raced AMA SX.  Just a nice guy who does great work at reasonable price.

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Press job complete! Just have to do the balancing, as soon as my eBay V blocks and dial supports come in ;-) Yes I already have a good 5lbs brass mallet...

I've inspected the races, there was no damage from pressing the pin through, only slight wear from the thrust washers.
Note that the original rod was still in pretty good shape, this is a preventive rebuild.

Even with heat, there was stutter and scary noises, but I figure if the pin it blocked on both sides, it can't overshoot by much.
Final clearance is .5 mm which is borderline on the standard specs, but good enough for me on this fist experiment.

Horizontal alignment looks pretty good already, vertical needs slight correction. I'm eager to see what the dials say.

 

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