72 crank replacement

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My 72 Commando had the timing pinion key pinned and permanent Loctite applied...NOT GOOD. I ruined the crank getting it apart since I had to use a die grinder to remove the top of the half moon key. I'm out looking for a used crank and found a 69 750 (at least that is what it is reported as)..... seems like many years can be used but I'd like to be sure. Ya never kn ow what you are buying until you get inside....that said I've made many bonehead mistakes on this rebuild.....thanks for the help
 

illf8ed

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Couldn’t you get just the timing side crank cheek? This is a bolt together crankshaft. You will need to weight both cheeks and grind if needed to balance them.
 

johnm

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I can't be certain it will fit but whatever you replace it with ask for the bigend to be measured and count on probably needing to regrind it. Also check for the proper radius grind on the big end journal's.

I would also recomend a crack test especially at the drive shaft to pork chop change of section. Many are cracked there.

Look at this.

 

johnm

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Couldn’t you get just the timing side crank cheek? This is a bolt together crankshaft. You will need to weight both cheeks and grind if needed to balance them.
You could do this but in my experience it's easier to find a whole crankshaft. But maybe since it's the drive side that cracks you might find one. I'd get the crank dynamically balanced if I was mixing parts. I got mine dynamically balanced and it did seem to smooth things out.
 

illf8ed

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You could do this but in my experience it's easier to find a whole crankshaft. But maybe since it's the drive side that cracks you might find one. I'd get the crank dynamically balanced if I was mixing parts. I got mine dynamically balanced and it did seem to smooth things out.
I agree. I mentioned weighing and balancing the two cheeks, but even better to dynamically balance the crankshaft and all moving parts…pistons and rods….etc.
 

johnm

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I agree. I mentioned weighing and balancing the two cheeks, but even better to dynamically balance the crankshaft and all moving parts…pistons and rods….etc.
Yes mine were a surprising amount out. Side to side. And now my bike is extraordinarily smooth above 2400 rpm.
 

RoadScholar

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New cranks are as scarce as hens teeth. After an unnamed service turned the crank cheeks and made it unsuitable for a belt drive I went looking for a new or serviceable crank. None of the major parts suppliers had any. I could have purchased a billet crank, but didn't need the strength and didn't want to spend $2K; I kept looking. I found one, in the original Berliner box, that had been sold to a dealer in Pennsylvania, was resold twice and ended up in New Orleans. I bought it for $550 delivered.

I found several alternatives, to use the word extremely loosely; the majority of these looked like they are done duty as anchors, all were rusty, part number references were equally loose, hard to say what they came out of, and the prices were totally outrageous (Ebay--where else?), the least expensive of which in the $700 range.

I should have started calling second tier vendors, but I wanted it NOW. If you have the time to be patient, make the calls and ask the right questions you will find a serviceable crank. Beware of '72 and '73 750 cranks as they have a reputation for not having much of a radius where the pins are joined to the cheeks. A radius can be machined, but get the pieces magnafluxed to insure that cracking hasn't already started; I suspect that the cracking may be due to the factory use, initially, of non C3 spec main bearings; others will have greater knowledge of this.

My first and last Combat crank had these cracks and I was dumb enough to think that I needed a bullet crank, 'cause, of course, I was planning on at least 500 RWHP at 10K RPM...

There are enough builders that install billet cranks for power seekers that they should have a good selection of cast-offs (no pun intended).

There are plenty of members that, hopefully, chime in and help identify such builders. Think positive thoughts, want you want is out there.

Best.
 
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Thanks all...I bought a 69 crank on ebay that looks good. I do plan to have it magnafluxed (nondestructive tested), balanced and journals ground (with the proper big end bearings)and a new set of roller bearings. I was in the bearing biz for 40 plus years and noticed the upgrade to a bronze retainer on replacements (also the change to C3 internal fit). Hope it all works. I did break case at the "seal stop" on the drive side (while removing the bearing). The Rooke book showed them driving out the bearing with a properly sized socket....I thought I was hitting a thrust washer!! Not good! I'm taking the cases to the machine shop today to investigate if this bone head error can be repaired. this is a non-structural case flange that simply limits the amount the drive side seal can be driven in. Looks like a new case is available for approximately 1,000 pounds but I'm not sure about the proper case numbers. I'll ind out and move forward when I get the replacement crank.

THANKS ALL
 
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