1961 CB72 Project

[G-Man]

LM

There are two sources for oil to get up from the crank to the piston / small end. The main bearings and the big end. The leakage from the cam chain sides of the mains goes either up the cam chain tunnel or down to the oil pump.

Anything on the big end side of the main bearings gets flung up to the big end (on the hollow inside of the pin) so that leave only one source.

Doing a little math on the possible escape routes, which all have the same pressure oil behind them.

a) the side of the big end
b) the slots cut into the big end

Taking the sides first. The bore of the con-rod is 31 mm so the curcumference is PI x D which is about 97.5 mm. To get the area of the escape path we need the side clearance of the rod. I set mine to around 3-4 thou (0.003-0.004 inch). I know that Jerry uses around 9 to get more oil flowing out but I think 4 is about right for a road engine.

Consider 4 thou or 0.101mm clearance. That gives an escape area of 9.7 square mm. You can share that between the two faces of the rod but it is the same figure.

Now, taking the 12 O'clock slots in the later rod (right picture) which are around 5.5mm wide by 2mm deep, that gives an area of 11 sq mm each but there are two vertical slots per rod so that is 22 square mm.



That means that 2 1/4 times as much oil is directed upwards toward the small end than escapes randomly past the rod faces. Closer clearances at the sides of the big-end means that even more goes up top where it can do something useful. If you could run with 1 thou side clearance, more would go to teh small end.

Is that a significant amount?

When I have a moment, I'll build a test rig and measure it and the flow from good / bad pumps.

Regardless of all this, the cleaner you can get the insides of a used crank, the longer it will last..... :-)

Have fun at the Barber Vintage Festival. I'll be on a plane to New York but I shall be back home for the Stafford Classic Bike Show.

G

I'm on my way to BARBER and will read your reply when I return.
A quick look still didn't seem to explain your idea that any real amount of oil is sent to the bottom of the piston from the design of the big end of the rod.
I didn't include the oil path to the center bearings as that is very visible as one looks at the cases and outer race of the center bearings.
I'm respond more when I return. ...............lm

[G-Man]

Take the time to show any data about the design of the BIG END that actually makes a difference as I see that area as nothing other than a place for the crank to rotate and the oil under no pressure other than SPLASH OIL to lube the BIG END.

See previous posts. The big end is lubricated by oil pumped directly through the filter and into the gallery at the front of the upper crankcase where it is then passed to the big ends via the center main bearings.

(rod pin doesn't have a hole through the center) Same for the LITTLE end except some have round holes and early issue have cut slots..

The big end is hollow and has oil flowing through it. There are two small holes that feed oil out from the hollow center to the rollers. You can just see one of the feed holes in teh picture. The other is about 120 degrees around the pin.

As I know the crank, the part which gets oil by Pressure is the right crank bearing and the centers get oil from the top as the engine splashes oil.

Post '62 engines had pressure feed to the right hand bearings. Early engines do not.
All engines have direct pumped feed to the mains and big ends.

Of course the cam followers/rockers and other parts in the top area get oil under pressure and splash.

Indeed


G

[G-Man]

Someone asked recently on how to distinguish an early '61 style rear hub. These are smaller diameter than the later type so fitting an early brake plate to a later hub leaves an unsightly gap.

The diference is due to the fact that the early hub / brake did not have the brake liner engaging in a slot in the brake plate to exclude water /dust.



The double flange on the later brake plate actually sits over the protruding drum liner.



The early drum has the liner sutting flush with the hub casting.




G

[jerry]

Dear G Man, I checked on Jerry Kooistra Facebook and found on the July 1 Post photos that conrod side clearance is 0.012" and centre main bearing cage clearance is also 0.012". Oil used is Monograde 60. All the best Jerry

[G-Man]

Jerry

Many thanks! I stand corrected. :-)

I'll check out the page.

Graham

[sixtiescycle]

Graham,
Seems like everytime you split a crank it scares the hell out of me with the amount of sludge you are finding! The last CB I bought came with a newly rebuilt, never fired engine, built by a 34 yr American Honda tech who has since passed on so there is no way of knowing what was done to the crank. The work I can see done is perfect in every way (I don't trust anyone's work who I don't know so had to explore his work!). Guess I'll cross my fingers on that one. I've been saving an nos CL crank for a "keeper" to use and have one new set of rods which I want to use in another "keeper" at some point. Not sure how many used cranks I have lying around but I sure want to experiment some time soon. I sure do enjoy your posts! Definately makes one think about regular oil changes.

[G-Man]

Dick

I'm glad you find this stuff interesting, because I do. I think that the positive message one can draw is that the design of the crank and oiling method is pretty thoughtfully designed. Yes there is dirt in those places but the design of the centrifugal slingers in the crank webs and the positioning of the oil holes in the big end pin means that the heavy dirt particles are spun out into relatively harmless positions.

The force generated in the slinger is proportional to the diameter (larger than the oil filter) and proportional to the square of the engine speed. No wonder the stuff gets compacted in there. The danger, I think, is engines that have not run for a long time and the risk of it getting washed through by modern oils.

It is fascinating for me to look at the various parts and work out what they are designed to do and how well they do it. Early Dream big-end pins were 25mm with 3mm rollers to fill the space in the rod end which is 32mm diameter. Pretty early on they enlarged the pin to 26mm but I think they were stuck with the design of the rod because it was an expensive part to re-tool. They had to go down to 2.5mm rollers to make everything work. Looking at similar designs I thing that was a bit limiting. Later Hondas, and the roller bearing crank Suzukis of similar size, have a bigger big end assembly with larger rollers.



A number of the rods I have looked at show pitting at the edges of the bearing at high mileage. That suggests to me that the crank is distorting at high revs / load. If the bearing contact was even and parallel the pitting should be spread evenly across the bearing.



The other unknowns for me are : What would that rod bearing sound like and how long would it last? It might be possible to swap the rods from left to right or turn them to get a bit more mileage. In our quest for perfection we probably throw away a lot of usable parts.

I quite fancy putting a crank together out of worn parts to find out what would happen. It would be interesting to see the cranks that Jerry throws away after they have been run to their limits in his racers.

G