Motor guru's weigh in...

[Wasted wages]

Here's the deal,
NIB CMB green head motor, Broke it down before break in to clean it out, inspect for chips, break any sharp edges in the sleeve etc... did NOT pull the wrist pin from the piston/rod, in fact I never paid attention to how it was inserted.

After a brief break in period at the pond, I pulled it apart again to look for wear patterns etc.

This time I DID notice the wrist pin orientation in the piston...as it came from CMB.

I've always heard/ been told the open end of the pin should face the exhaust port. For a couple of reasons.

#1 The pin is threaded for removal. ( which doesn't make a difference overall because on this type .67 the piston/ rod falls out when the sleeve is removed) if it was a double counterweight crank, the pin needs to be pulled first thru the exhaust port.

#2 For wrist pin/rod bushing cooling.
I guess the theory is, it lets trapped heat escape ?
Which doesn't make sense to me as the searing exhaust gas being blown into the pin would not let much heat escape..unless the tuned pipe pressure wave provides some type of cooling effect?

So my question is, did CMB intentionally install the open end of the pin facing the cool intake charge?
Or did they just screw up when they built it?
Or does it make absolutely no difference and it can face either way ?

Picture for reference,,
Also, is the rod correctly indexed with the piston ?
Currently, there are two oil relief
cuts on the crank side of the rod and a single cut @ 12 o'clock on the drum rotor side...Is this the correct orientation?

What say all ye guru's of motor wisdom??

 

[K F Wheeler]

All excellent questions!!! I'll be watching this for future reference. I also have several smaller CMB's to go through...

Ken

 

[Wasted wages]

Typically on all of my old junk, I don't question things like this, but I know that sometimes manufacturers make tiny changes that can make a big difference.
And this being my first .67 Greenhead, I had to ask..

 

[Jeff Lutz]

Here's the deal,
NIB CMB green head motor, Broke it down before break in to clean it out, inspect for chips, break any sharp edges in the sleeve etc... did NOT pull the wrist pin from the piston/rod, in fact I never paid attention to how it was inserted.

After a brief break in period at the pond, I pulled it apart again to look for wear patterns etc.

This time I DID notice the wrist pin orientation in the piston...as it came from CMB.

I've always heard/ been told the open end of the pin should face the exhaust port. For a couple of reasons.

#1 The pin is threaded for removal. ( which doesn't make a difference overall because on this type .67 the piston/ rod falls out when the sleeve is removed) if it was a double counterweight crank, the pin needs to be pulled first thru the exhaust port.

#2 For wrist pin/rod bushing cooling.
I guess the theory is, it lets trapped heat escape ?
Which doesn't make sense to me as the searing exhaust gas being blown into the pin would not let much heat escape..unless the tuned pipe pressure wave provides some type of cooling effect?

So my question is, did CMB intentionally install the open end of the pin facing the cool intake charge?
Or did they just screw up when they built it?
Or does it make absolutely no difference and it can face either way ?

Picture for reference,,
Also, is the rod correctly indexed with the piston ?
Currently, there are two oil relief
cuts on the crank side of the rod and a single cut @ 12 o'clock on the drum rotor side...Is this the correct orientation?

What say all ye guru's of motor wisdom??

View attachment 314890

Everything is correct. The only reason the wrist pin hole faces out on the Evo style is because you have to remove it that way.

 

[John Beardslee]

Wrist pin is used in multiple applications.

 

[Danny King]

The closed end of the pin faces the exhaust except on RS/Evo style engines.
You want the closed end of the wrist pin facing the exhaust as it blocks exhaust heat from being inside the pin.
But either way is fine and it is really splitting hairs.

 

[Buckshot]

But wait, there’s more….I heard that having the closed end towards the intake helps reduce crankcase volume…..

 

[Timbo Whalen]

Here's the deal,
NIB CMB green head motor, Broke it down before break in to clean it out, inspect for chips, break any sharp edges in the sleeve etc... did NOT pull the wrist pin from the piston/rod, in fact I never paid attention to how it was inserted.

After a brief break in period at the pond, I pulled it apart again to look for wear patterns etc.

This time I DID notice the wrist pin orientation in the piston...as it came from CMB.

I've always heard/ been told the open end of the pin should face the exhaust port. For a couple of reasons.

#1 The pin is threaded for removal. ( which doesn't make a difference overall because on this type .67 the piston/ rod falls out when the sleeve is removed) if it was a double counterweight crank, the pin needs to be pulled first thru the exhaust port.

#2 For wrist pin/rod bushing cooling.
I guess the theory is, it lets trapped heat escape ?
Which doesn't make sense to me as the searing exhaust gas being blown into the pin would not let much heat escape..unless the tuned pipe pressure wave provides some type of cooling effect?

So my question is, did CMB intentionally install the open end of the pin facing the cool intake charge?
Or did they just screw up when they built it?
Or does it make absolutely no difference and it can face either way ?

Picture for reference,,
Also, is the rod correctly indexed with the piston ?
Currently, there are two oil relief
cuts on the crank side of the rod and a single cut @ 12 o'clock on the drum rotor side...Is this the correct orientation?

What say all ye guru's of motor wisdom??

View attachment 314890

First, rod orientation on crank pin- there is an internal chamfer on the big end that mates toward the counterweight...why?... there's a filet radius where crank pin intersects counterweight...why?...because it eliminates the possibility of stress risers developing and fracturing the pin/counterweight interface.

The wrist pin- I believe it facilitates installation/removal through exhaust port...a common theme on CMB's Evolution design. It would make sense from the standpoint of keeping manufacturing cost/inventory down.
The heat issue you mentioned is moot.

 

[Wasted wages]

Seems to be the general consensus that the pictured assembly is in fact correct.

Double relief cut on the rod towards the crank side,,,wrist pin with the closed end towards the exhaust port..

So I'm going to reassemble it as is, throw in the needle bearings and send it !!

 

[Anders_Martinelle]

I would say like this, IF you want to mount it in a special way use the opend end towards the exhaust.
Only one reson for that is to balance the piston better. Then if it makes a difference?? Feels better atleast.

//Anders