Wrist pin bush fitting.

burgundyben · 10-21-2008, 06:10 AM

I removed my wrist pin bushes, the 'jigsaw' joint in the rolled strip was oriented such that the joint was along the bottom.

Let me try to explain that better. The top of the piston is at 12 o'clock, the oiling holes for the wrist pin are at 11 and 1 o'clock. The join in the bush is at 6 o'clock.

The new bushes I got from Pelican do not have oil holes in them, this is ok, I guess I press them into place and then drill the holes, carefull de-burr and I'm good.

I would have expected the jigsaw joint to be anywhere other than at 6 o'clock, 3 or 9 would have very little load and 12 would have less load than 6.

It strikes me having the joint at 6 o'clock is putting the weakest part of the bush in the highest stress area.

Any views on this?

Thanks

Ben

Henry Schmidt · Henry Schmidt's Garage

I understand your concern and you make a good point.
That said, most of the rods I see with bushings installed at the factory have the split joint at 6 o'clock. My guess is that it doesn't matter from an engineering stand point.
When analyzed closer, the load is not at 12 or 6 but distributed via an oil film along the circumference of the wrist pin at something slightly less than 50%.

afterburn 549 · 10-21-2008, 09:19 AM

Hip shot guess, Long stroke Hi torq eng do develop a pattern.. Back to the P eng.
There other forces at large and maybe not as great / or different
At high rpm ,unload the throttle and the wrist pin to piston load will change positions.
I would also postulate as the the RPM comes on with load it will change slightly through the whole range..........just guessing here ...as everything gets wants to get heavier with acceleration.... except maybe light ?

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by afterburn 549

Hip shot guess, Long stroke Hi torq eng do develop a pattern.. Back to the P eng.
There other forces at large and maybe not as great / or different
At high rpm ,unload the throttle and the wrist pin to piston load will change positions.
I would also postulate as the the RPM comes on with load it will change slightly through the whole range..........just guessing here ...as everything gets wants to get heavier with acceleration.... except maybe light ?

Now I'm feeling a little dense because I have no idea what this means. Can you explain?

burgundyben · 10-21-2008, 10:43 AM

You got me there too, I dont understand why the load would shift given the geometry of the parts does not change.

Henry - When I consider how little movement there is in the joint I think you are right, and also keeping in mind that there is nowhere for the material to go, its all trapped in place. I'll bash (gently press them in...) them in at 6 o'clock.

burgundyben · 10-22-2008, 12:40 PM

So I fitted the first bush into one of my rods with the join at 6pm.

The bore of the bush fitted is 21.6, the wrist pin is 22mm.

So I have to ream them. How best I wonder? Pretty sure I dont have a reamer that size so i guess I may well farm this out to a machine shop.

afterburn 549 · 10-22-2008, 07:26 PM

I am sure you know..the ream is EZ, but you have to be SQ. with the crank . That requires more tools then just a ream ..................