Wrist pin bushing material

[DavidMartin]

Is C93200 bearing bronze the only suitable material for a wrist pin bushing? The specific application is for a 450cc motorcycle engine. The original design never included a bushing which will be the driving design constraint due to limited material removal from the connecting rod. The purpose of adding a bushing is to avoid splitting the case and replacing the rod. What is the typical range of a wrist pin bushing wall thickness?

Thank you

 

[ivymike]

it's not the only one... it has been a while since I did any wrist pin bushing design, but as I recall there were 2 or 3 different bronzes that were used, with different levels of gross pressure capability (55 MPa up to 130 MPA or so).

Glacier is a supplier I remember

http://glacier.de/products.php

 

[ivymike]

might have implied something I didn't intend - the discussion above was w/r/t bronze only. I've also seen aluminum bushings for light load applications, and bushingless rod designs for very high load applications (not sure if any of those are in production).

 

[Tmoose]

Will you be tipping the engine sideways and boring the rod using a milling machine?
The original Honda four rods were bushing-less, and a misery to machine even with carbide tooling using a dedicated Tobin-Arp rod boring machine.

http://www.rtsalesinc.com/web_quote..._PM-1900_Rod_Boring_Machine_Stock_48_w300.JPG

 

[SMOKEY44211]

The smallest wall I've seen is .030". Consider going to a press fit. Unless you are going to assemble and disassemble it routinely much greater load carrying capability. -----Phil

 

[DavidMartin]

ivymike - Thank you for the link, it will answer alot of questions. As Tmoose points out, this is a bushingless design.

Tmoose - I have considered using a brake cylinder hone and slowly increase the bore diameter. This isn't a practical solution anymore considering I will need to remove .050"+ of material. By any chance do you know what alloy the Honda rods are cast from?

Smokey44211 - Why would the classification of fit effect load capacity.

After what I read today, I'm realizing adding a bushing is adding a potential failure. I think the best approach is to replace the rod or recoat the bore and then hone to size. Thank you for all the help.

 

[patprimmer]

The Honda Four rods are almost certainly forged, not cast.

The fit influences the level of support for the pin. If it is tight it can't flex and is less subject to fatigue as a result. Of course it is still subject to about the same shear at the edge of the rod bore.


Regards
Pat
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[DavidMartin]

I know it has been awhile since the original post, but figured it was better than starting a new thread. I have eliminated the bushing idea in favor of electro plating and then honing to size. Based on my one semester of materials engineering, nickels properties seem more suited than copper, but I have zero experience. Nickel has a lower thermal expansion rate, twice the modulus of elasticity and is much harder (40HRC).

Due to the fact I'm adding a new variable into the system I have considered drilling a small hole on the top of the connecting rod to allow oil to better lubricate the wrist bin and bore. I'm fairly convinced the hole will have very little if any affect on stress due to a decreasing non linear acceleartion as the piston travels from 270 degress to top dead center. I have thought about how to calculate the acceleration, but since I believe it to be non linear I haven't any idea how to start.

 

[NormPeterson]

Write the equation for piston pin position in terms of the crank angle. Differentiate twice.


Norm

 

[ivymike]

..or simply look up the equation for piston pin acceleration. it's on the internet

 

[evelrod]

Drilling and chamfering a small hole as you describe has been SOP for me for many years. My Cosworth rods came so modified and I have continued the mod for over 40 years with no problems. Currently I use rods that have no bush at the small end, racing only, and have seen no problems or excess (???) wear between rebuilds. I have a set of Carillo H beam rods that have the hole at the bottom where the beam section meets the small end. We have only used the rods in one engine...no problems so far.

Rod

 

[Tmoose]

Now's a chance to magnaflux the con rod to be relatively sure there are no fatigue cracks a-formin'. Trickier with the rod assembled, but I'd give it a try if mine. Probably easier to clean off than the honing grit will be.

I worry about hydrogen embrittlement on plated high stress parts.

http://www.finishing.com/82/54.shtml

I'd check the rod for straightness (2D). One method is against the crankcase base with a LONG wrist pin. It's pretty good if the cylinder bore is proven perpendicular the base gasket surface, and the BGS is parallel to the crank CL.

 

[patprimmer]

I would not nickle plate a little end as that gives hard against hard bearing and a risk of the nickle chipping off and creating havoc. A very hard tin skin over a relatively softer matrix is not really all that hard as the matrix under it yields.

If I where to electroplate, I would hone to slightly over sized then plate with silver to required size. I am thinking in terms of microns or tenths of a thou. Hydrogen embrittlement is a real problem that would need to be addressed.


Regards
Pat
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[DavidMartin]

Thank you for everyones responses. I learned of hydrogen embrittlement this past semester and would never have thought it was a concern due to my assumption hydrogen was not present in the reaction. I'm embarrassed to say I was unaware of the difference between electro and electroless plating. Thank you for the enlightenment.

I was concerned nickel might not have ideal yield properties due to such a high modulus of elasticity. Thank you for the conformation. Back to research.

 

[SMOKEY44211]

If I may digress to my original response. My suggestion is to install a steel bushing in the small end if the rod. The final sizing would be smaller then the wrist pin thus the term "press fit". This method is used by most engine manufacturers. The practice when rebuilding such an engine is to heat the small end of the rod allowing the pin to simply slide in. Once cooled the pin can not move about, eliminating the need to use clips to contain the pin. Also eliminates the need to for lubrication. This method essentially doubles the load carrying capability of the small end by virtue that the force is now dispersed through the two bosses machined in the piston. The only drawback to this method is that is a little more time consuming to assemble vs. a full floating pin. -----Phil

 

[RossABQ]

David, you've kind of jumped straight to bushing the small end as a requirement; can you give us the history here? Why does the stock pin not work; did it run dry, overheat, ?? Is the small end of the rod damaged, and how? Scored, out of round, ?? Is there any reason a minor hone and an O/S pin wouldn't work? Some manufacturers offer an O/S pin and a piston to match for just this kind of situation. The pin is maybe .002" O/S.

Out of curiosity, is this the old '60's Honda CL/CB450 450cc DOHC engine?