CB77 Crankshaft repair

[G-Man]

I was searching through my pile of crankshaft spares and found a usable CB77 centre-section. I stripped and cleaned that before re-assembling with a nice pair of con-rods which I fitted with fresh bearings.

All was well until I needed to find an alternater-side crank web and shaft.



The only one I could find had a badly corroded section where the starter sprocket fits. Initially, I looked on eBay to try and buy a better part but the ones that turned up were worse than the part I had. Eager to get this one finished I decided to try out the little Dumore toolpost grinder I purchased last year.



Originally I hjad planned to make a sleeve to fit over the reduced dimater shaft but precision grinding is a very slow process so I had to settle for just cleaning up the corrosion and seeing where I ended up.

As it happened I was able to get a nice surface at 19mm diameter which is just 1mm down from the original shaft size. Of course the starter sprocket bush was now too slack so I had to press in a new one and bore it to size.



That fixed the bearing but I now needed a suitable oil seal to fit the new shaft. I discovered that I could get a replacement with a 19mm bore so all was well.

I am pretty pleased with how this turned out. The rotor taper is a little shorter than standard but I'm confident that it will be OK.



The starter clutch appears to work fine on its 'new' shaft, too. The main bearing journal also suffered from corrosion but I can fix that by pressing on a ball-race in place of the original roller bearing.

Another part rescued. :-)

G

[AlanW]

Superb work G, another one saved.
Al.

[Seadog]

Nice job.

[G-Man]

In the last week or so, I finally got around to an idea I've had for some time. I heard that this had been done before but wanted the challenge of seeing if I could work out a modification of my own to rescue cranks with big-end failure or corroded crank pins. Honda's design for these cranks means that if a big-end is damaged then the crank is, pretty much, headed for the scrap bin.

My method is to use a pair of con-rods which are cheaper and more available than the CB72/77 originals which are very hard to find, in good condition nowadays.

I found some Suzuki rods which are an exact match for the CB72 rods in centre to centre distance but are 20mm wide compared with the 18mm of the Honda originals. The rods would need thinning, somehow but they had a nice, convenient 38mm big end eye and a 16mm small end which could take a thin bronze bush to bring them back to 15mm for the Honda wrist (gudgeon) pin.

A nice, caged-roller bearing was available to fit the rod and had a 30mm bore. I picked up some 'needle roller' inner sleeves which, at 25mm inside diameter would need to be 'bored' to 26mm for the damaged pin and have holes put in them for lubrication. Not an easy task with hardened steel.

Here is a sleeve pushed on to an 'early' Dream crank which has a 25mm big end, compared with later ones which are 26mm.



Here with the needle bearing in place.



And with the rod attached.



All I had to do was make these parts fit a later style 26mm big-end pin.

G

[G-Man]

First job was to see if I could successfully thin the hardened connecting rods with my Dumore toolpost grinder in my little Myford lathe.

I clamped the rods to my trued faceplate and managed to work on two at a time with a rather praecarious set up.



The resulting rod weiged in at 224g versus the original CB72 rod at 210g so that shouldn't throw things too far out of balance.

Next up was to grind the bore of the sleeves for a nice close sliding fit on the pin.



G

[G-Man]

To get oil to my new bearing, the sleeve needed holes to transfer oil from teh big end pin out to the bearing. This is incredibly hard material and I went through a few optione before I could make the holes. The chosen method was to start the 'hole' with a dremel grinding bit and then drill with an Artu tungsten-carbide tipped 3mm drill. Without the initial divot the drill would not start. Once it starts you can put pressure on and the drill softens the metal to drill through it.

Just in case the sleeve slips in use, I ground a little channel inside the sleeve so, there would be a route for the oil-feed if the oil-holes become misaligned.



I found some 'donor' crank webs (which were a bit rusty) and cleaned up with th ebead-blaster and ultrasonic cleaner.



This crank fell into the 'turns smoothly with little play' category that you often see on eBay. The crank pin is pretty much toast and wouldn't last long at all.

The inner faces were badly rust pitted so they had a trip to the mill to clean up the surface.



The outer flywheels got treated in the lathe to clean up the 'tired' looking surface.



The original centre-pin and bearings for this crank were beyond repair so I dug out some better ones from the spares pile and assembled the inner part of the crank in my press fixture.



G

[G-Man]

The rods both got treated to some bronze bushes for the small ends.



And, with the centre-section assembled it was time to assemble the big ends.

My sleeves are a close sliding fit and are held in place by Loctite 620 which is an amazing 'glue'. I inadvertently tested teh strength when I put one sleeve on out of line and it locked on. It took some heat and a sturdy gear-puller to get it off. Thankfully it was undamaged....



With the sleeve locked in place, the caged-roller slips straight on to be followed by the outer flywheel.



The crank now needs truing up and the outer bearings and big end plugs will be fitted. Then I have to buid an engine and go out and test it.....



This crank will be used in my CYB72/77 so it has no hole for the starter clutch. If I change my mind later, I can always replace that flywheel or put in the oil hole.....



G