1961 CB72 Project

jensey · Post by **jensey** » Wed Mar 18, 2015 4:04 am

After talking to Jerry I may increase the thickness of my small end bushes or scrap the rods. It would be nice to build a proper test installation to learn about all these things. Perhaps when I retire....

Graham,

Boring out the conrod (and honing) and piston to fit a piston pin 1 mm thicker is also an option, especially when the crank is in one piece. I understand that you want to build to spec, so this is not an option for you. Next to that, it's very difficult to clock ?? the crank under the milling machine. I made a stand for that, which makes it possible to do this with the crank in one piece, making sure that the hole is perfect perpendicular to the conrod.

I've don that a view times, the only disadvantage is that when swapping pistons. you have to bore them out too.

I know, it's a work around, but one which is applicable and worth to consider,

Jensen

G-Man · Post by **G-Man** » Wed Mar 18, 2015 4:41 am

Hi Jensen

Yes - good points, there are a few 16mm pins out there. I was just comparing these rods. My bushed one is in the centre and the others are NOS.

The left one is a Dream rod with a much heavier small end and the small end is machined (ground?) after hardening as there is no evidence of copper plating in the eye. The right one is a CB72 rod with a lighter small end and machined before hardening (copper plating intact). The later rod would be cheaper to make as only the big end would need work after hardening.

They all weigh exactly the same and the Dream rod has slightly thinner flanges on the "I" section to compensate, I think.

Your rig for the milling machine must be quite a creation!

Graham

jensey · Post by **jensey** » Wed Mar 18, 2015 5:02 am

Hi Graham,

It takes a few hours to build the crank onto the milling machine (I'm aloud to use the milling machine as it iss not mine), once aligned, the boring and honing takes just a few minutes.

Very nice to see the differences in conrods in one picture...

btw, 450 rollers are prone to rust (especially main bearings), and looking at the conrod in the middle, it seems that the roller pins in the CB cranks are prone to rust too.

Jensen

G-Man · Post by **G-Man** » Wed Mar 18, 2015 5:37 am

Jensen

That is the beauty of making and using proper tools. The job itself becomes almost an anti-climax. My tools for shock absorber repair were like that. I am continually buying tools when there are interesting jobs to be done.

Yes the big end rollers in this bike were not good but the rod and the big end pin look OK to me in comparison with the new rods - which are also pretty old..... A while back I worked with a guy who had a PhD in "Elasto-Hydrodynamic Lubrication" and we discussed the phenomenon of rolling contact fatigue. It's quite scary but I'm keen to use this crank and monitor it closely to see what is possible. I have an old stereo microscope that I might put to use.

I bought a large quantity of rollers (too long as per my previous posts) for the big ends and now have rollers for all of the other bearings. At least it will have all new rollers.

I'm keen to look inside one of my 1960 Dream crankshafts next.

G

Tim Miller · Post by **Tim Miller** » Wed Mar 18, 2015 11:22 am

Yes guys just sharing information that I've learn with my cleaner. Even with 7200 peak watts and 3600 average it's challenging to remove thick stuff. I also try to use the ultrasonic for just final cleaning, my solution has anti-rust agents.

Rust pitting is a problem I have witnessed as well. It took about 30 dismantled cranks to have enough good parts for 10 cranks. Rods and cages were scraped the most. If you inspect the cages under 100x magnification it easy to see the hazards of running poor grade oils and no real filtration. The actual clearance is .0013 to .0018" between rollers, journal & rod ID. So, even small particles srubbed into soft cages and eventually starts allowing rollers to cock and that brakes the cage. Then game over....

G, I relize .020 wall thickness is thin for rod bushings but losing more material by going larger bore has some drawbacks. What are the concerns ?

Tim

jensey · Post by **jensey** » Wed Mar 18, 2015 11:46 am

Hi Tim,

I understand the concerns when it comes to cleaning the way I described it. It takes time, but looking to the amount of particles which come out of the oil passage's I think the method will save some cranks.

I use the US more then once, sometimes 6 or 7 times, sometimes even more, meaning, US, 3 or 4 days of pumping fluids around, US, 3 or 4 days pumping fluids around etc. I admit it's not the best method, but it comes close to it (only when bearings are within specification).

1 out of three, you're lucky ! I disassembled 4 C71 engines already, cranks were all thrash. Not beyond repairable, but it takes a lot of energy to get them right. I hope I can use 1 out of 6, and use the parts from the other cranks to build another 1 or maybe 2, welding and grinding involved (I can't do that myself, so it will cost me money).

Jensen

G-Man · Post by **G-Man** » Wed Mar 18, 2015 11:48 am

Tim

I think that Jerry was just uneasy about strength of that thin bearing layer. He would like to see .040 minimum.

I'm actually prepared to give it a go and inspect after 500 miles or so, I think. I'm not going racing, after all.

Time for some calculations to see where we are stress-wise. I'm not convinced that Honda took a lot of trouble getting the small ends central in the rod-end so that might give some clues as the factor of safety.

G

Tim Miller wrote:Yes guys just sharing information that I've learn with my cleaner. Even with

G, I realize .020 wall thickness is thin for rod bushings but losing more material by going larger bore has some drawbacks. What are the concerns ?

Tim