wrist pin design

[petertha]

I'm working on the wrist pins for my 5-cyl radial radial (24mm bore x 22mm stroke). The plans call for 5mm diameter 'hardened' steel & solid cross section through the length. No provisions were made for end pads which has me a bit concerned. The liner is cast iron & the link rods are 7075 aluminum, no bronze bushings but does have the classic angled oil weep holes feeding the wrist pin.

I had access to an older gen OS-FS-61, about 9% larger displacement. It has a 6mm diameter wrist pin, nylon end pads & a 3.5mm hole through the entire length. The end pads have a tiny weep hole.

- any ideas as to why the holes in the pads? Its not like lubrication on one side should be any different than the other? I thought maybe to drift a screw in there to remove but they popped out pretty easy.

- if my wrist pin is running in the aluminum link rod, does it make sense to harden? I've made some testers out of O1 so I'm covered, although I suspect the post-hardened dimensions will vary a bit which is a PITA trying to keep a snug fit on the piston & proper sliding fit on the link rod.

- I'm considering just partial drilling the ends of my wristpin with 0.118" dia hole x 0.100" deep & then permanent Loctite a brass end pad which has a matching 0.118" boss for the hole. See anything wrong with this approach?

Pic of my piston work-in progress alongside an OS-56-FS which is identical displacement. It has 5.5mm diameter wristpin, but I'm not sure if its drilled through or not.

 

[Cogsy]

For the holes in the pads - I would expect they're a snug fit into the rod, which is hollow? So the holes are just to vent the air inside the rod so they (a) don't build up pressure when the pads are installed and (b) don't build up more pressure in a sealed volume as the engine heats up.

For the hardening, if it makes sense to harden the pin in a bronze bush which is much softer than steel then it makes sense to harden it in aluminium as well. For a display model I most-likely wouldn't bother (I never have so far) but a flying model should get a lot of hours on it so I probably would.

 

[tornitore45]

On my Edward I placed a set screw in the underside of the piston. The screw is tangential to the wrist pin and the pin has a slight trough for positive detention.
Just another approach.

 

[WOB]

The Hodgson radial uses full floating drill rod wrist pins running in 2024 pistons and 6061 rods. The pin is drilled thru with Mil Spec. 20470 dome headed soft aluminum rivets inserted in the ends to provide end cushions. No complaints but I will never run it enough to test the scheme.

WOB

 

[petertha]

tornitore45, I had a quick look at the Edwards drawings prior to pondering my decision. Maybe I missed but what did the stock design call for? I think I saw hardened dowel pins but maybe that was for the bottom ends in the master rod? Are you going to harden yours?

WOB, thanks for another data point. This aspect has always been mysterious to me. Does 'full floating' mean the wristpin can float laterally inside the piston hole with about the same clearance to the wristpin as would be to the link rod ID hole? Or would the link rod end hold the wristpin a bit firmer in this case?

 

[tornitore45]

I searched the PDF plans for the word "hard" -not found,
I did harden the pins, nor because harder wear better but because hard drill rod can be polished to mirror finish and that reduce the friction contributing to wear.
Is a roundabout way to look at it, but that the way I see it.

 

[WOB]

delete post

 

[WOB]

tornitore45, I had a quick look at the Edwards drawings prior to pondering my decision. Maybe I missed but what did the stock design call for? I think I saw hardened dowel pins but maybe that was for the bottom ends in the master rod? Are you going to harden yours?

WOB, thanks for another data point. This aspect has always been mysterious to me. Does 'full floating' mean the wristpin can float laterally inside the piston hole with about the same clearance to the wristpin as would be to the link rod ID hole? Or would the link rod end hold the wristpin a bit firmer in this case?

To me it means same size hole in piston and rod(about .001" bigger than pin)

WOB

 

[petertha]

I searched the PDF plans for the word "hard" -not found

There are a few instances of 'dowel' for various similar shafts, but nothing I see for the wrist pins. Assumed it was just me. I thought those Edwards drawings were bulletproof by now.

 

[josodl1953]

Peter,
The Edwards design specifies 6 mm OS Max wrist pins, part no.10893, see sheet "Part list and notes" .I made mine myself from 5 mm drill rod because my Edwards is smaller ,see attached drawing.

I did not drill relief holes in the buttons. I suppose manufacturers who drill them are worried about pressure build-up when the air inside heats up during running, but I think the volume of this included air is so small that it does not present a problem.

Jos