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Author Topic: Cracked Lower Crankcase  (Read 2418 times)

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Offline 74CB750Chopper

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Cracked Lower Crankcase
« on: January 13, 2015, 10:16:40 PM »
So today was the day I was going to put my engine back together after being in pieces for the last two months for cleaning, painting, inspection. When I was tightening the bolts I failed to check that everything was properly seated and I cracked the case pretty bad.

 In my defense this was the second time I closed a CB750 case back up and I was a bit distracted by my 18 month old boy running all over my garage trying to keep him from hitting things with the hammer and going for a swim in a pan of engine degreaser.

Anyways my question is when I get a new case (I am pretty sure based on what I have read on the forum that they have to match and I can't just replace the bottom half ) I have a K4 so do I have to use a K4 case or are they identical for a certain range of years. I know alot of things are interchangeable for pretty much everything 70-76 but the case seems like it might have minor yet significant differences. I need all the guts to fit back in the new case.

Once I get the new case do I need to replace the journals? I am worried about that because I have no idea how to do that stuff, I am pretty sure I can figure it out if I can find some guidance. Anyways I am just looking for any pointers on how to deal with this situation from someone who may have had a damaged crankcase that they replaced. I will most likely start a build thread since I have had so many complications I figure that maybe someone can learn from my many mistakes.
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Offline calj737

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Re: Cracked Lower Crankcase
« Reply #1 on: January 14, 2015, 04:30:20 AM »
I can not answer with certainty your question about compatibility between the case years, other than to say, from all I've read, they are within the range you listed. And yes, the top and bottom must be mates.

As for the big end bearings, if you swap your crank over, then you must confirm the bearings in the case are the proper size for your crank. This must be measured, and at worst, new beating shells have to be ordered and installed.

There's a member here, Bill Benton that may be able to help with your search for cases. Send him a personal message, billbentonracing. You can find his "For Sale" threads and he's quite active in the Hi-PO forum.

Picture of the crack? It might be possible to have it repaired by a skilled welder
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Offline Cqyqte

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Re: Cracked Lower Crankcase
« Reply #2 on: January 14, 2015, 05:03:03 AM »
Can't say for sure but I would try to find a replacement case part that you cracked.  I had a CB450 and found on reassembly that the lowercase was cracked and I sourced a year correct replacement for $40 and simply switched  the broken one out.  Being the lowercase nothing was mounted in it as everything is held in the uppercase when you dismantle.  When I button everything up everything was fine, the important thing to remember is that Honda has incredibly high levels of accuracy so the tolerances should be the same for each case.

For the cost I would try it first before doing a complete teardown and replace both top and bottom cases.  ;)
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Offline MCRider

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Re: Cracked Lower Crankcase
« Reply #3 on: January 14, 2015, 07:00:31 AM »
Quote from: Cqyqte on January 14, 2015, 05:03:03 AM
Can't say for sure but I would try to find a replacement case part that you cracked.  I had a CB450 and found on reassembly that the lowercase was cracked and I sourced a year correct replacement for $40 and simply switched  the broken one out.  Being the lowercase nothing was mounted in it as everything is held in the uppercase when you dismantle.  When I button everything up everything was fine, the important thing to remember is that Honda has incredibly high levels of accuracy so the tolerances should be the same for each case.

For the cost I would try it first before doing a complete teardown and replace both top and bottom cases.  ;)
The CB450 has a roller bearing crank, so you can switch just the bottom half. The CB750 has a plane (plain) bearing crank and the cases must travel as a set.  They are line bored at the factory for the plane bearings, and that requires they be a set only.
« Last Edit: January 14, 2015, 07:02:14 AM by MCRider »
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Offline 74CB750Chopper

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Re: Cracked Lower Crankcase
« Reply #4 on: January 15, 2015, 02:19:54 PM »
Here are some pictures of the crack,
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Offline calj737

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Re: Cracked Lower Crankcase
« Reply #5 on: January 15, 2015, 02:26:23 PM »
Replacement cases.
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Online Don R

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Re: Cracked Lower Crankcase
« Reply #6 on: January 15, 2015, 02:28:34 PM »
Was a shift fork out of place? It may be bent if so.
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Offline 74CB750Chopper

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Re: Cracked Lower Crankcase
« Reply #7 on: January 15, 2015, 08:16:51 PM »
Replacement is on the way lol. Ill check out the forks, thanks for the tip! Hopefully they are all good.
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