Fox 35 wrist pin

Hey guys,-first post--Anyways I took apart my 50th anni 35 and noticed how tight the wrist was on the piston. I had to use a punch to tap it out.So anyhow after i desplooged it in the crock pot antifreez, and cleaned it well, I still had to punch tap the pin to get it back in. SO my question is, does it really matter? Is this common? My other fox 35 wrist pins are better fit and slip right in.The motor runs great,---Just like my others,-I just hope it doesnt ware out the conrod. Thanks-Paul

I'd be a little concerned, but not as much as if the tight bearing area were in little end of the rod.

If you've got a fair amount of run-in time on it and all seems okay, I would hesitate to take it apart, what with the Fox 35 being time consuming to assemble.

But if it's still fresh, I'd found out what the problem is and fix it.

Hint: As Fox no doubt buys the pin material from an outside supplier, chances are it is to spec. But as they drill the pistons in-house, well, that's where I would look first.

Thanks Dan--I think I,ll just leave it as is and not worry so much about it.The rod and pin fit are normal,so i guess that should be good enough for even a worry wart like me.The motor is well broken in and runs great. -Paul

The one concern may be in the expansion of the piston if the wrist pin is not free. I would check on it again. Run a little 0000 steel wool around the wrist pin and in the hole of the piston. You may have some castor oil in there that has gummed up and not know it.

HI leonard--I think your right, I think the piston grew! I dont think the fox people would over look something like this. I used steel wool like you said, and it was still tighter than a rats%##@#!! Hey what the heck--Im just gunna run it as is. I just wanted to see if anyone else noticed this. --Paul

Ron B.
F4Fguy

Don't worry about it unless you've got a really heavy press fit in the piston.The wrist pin bosses in the piston run about 275-300 deg F at running conditions.if you have a tight push fit at ambient,you'll have a very good fitting pin at normal run temp since the pin runs about 100 deg cooler.The fox pins are on the small side any way,so too loose a fit in the piston will result in wearing the pin bosses in the piston.This is far worse than wearing the rod,as the rod is easily replaced,whereas wearing the piston means a new sleeve as well.

Hi Ron,--Good point, and a big thanks to all you guys and your help!! I new I came to the right place!! -Paul

An interesting question and series of responses so far. I took apart my new Fox .35 40th anniversary engine after a lean run to devarnish it. The wrist pin fit seemed fine to me, but I was surprised that the pin was solid. I assumed that this was because it was cheaper, not because the core material was adding much stiffness or strength. So I'm looking for a replacement that is hollow. That would help reduce the balance problem and increase the available power.

I don't know the exact composition of the piston or liner in a Fox, but if you assume the piston is a cast steel/iron (mehanite, perhaps?) and the wrist pin is a carbon steel, there is probably about 2 parts per million per degree F difference between their coefficients of thermal expansion (CTE). If the piston runs around 275 to 300 as Ron stated and maybe the pin is 100 degrees cooler (a complete guess on my part based on the assumption that the fuel charge assists in cooling) that would mean a line to line fit would open up about 25 millionths of an inch or .000025" on the diameter. The resulting tiny gaps would only allow sufficient penetration of the fuel/oil mix to establish boundary lubrication of the bearing surfaces. So the friction from the wrist pin to piston joint, doubled because it happens on both sides of the piston would mean that the rod/pin bearing would want to rotate first. Not to mention the drag on the pin from the retainers.

Seems to me that if the stresses were high enough, you might get fretting of the piston or wrist pin eventually, but the engine would be fine until you tried to take it apart..... Also, the life and reliability might be improved if the pin floated so the wearing surface had no preferred orientation.

Finally, some of the Fox engine tuners are adding oil feed holes in both rod ends to promote hydrodynamic separation of the bearing surfaces. While my engine is apart, I'm going to try this mod.

Can anyone fill in some of the blanks here?

LAST EDITED ON Mar-18-04 AT 01:34 PM (CDT)
 
>An interesting question and series of responses so far. I
>took apart my new Fox .35 40th anniversary engine after a
>lean run to devarnish it. The wrist pin fit seemed fine to
>me, but I was surprised that the pin was solid. I assumed
>that this was because it was cheaper, not because the core
>material was adding much stiffness or strength. So I'm
>looking for a replacement that is hollow. That would help
>reduce the balance problem and increase the available power.
>
>I don't know the exact composition of the piston or liner in
>a Fox, but if you assume the piston is a cast steel/iron
>(mehanite, perhaps?) and the wrist pin is a carbon steel,
>there is probably about 2 parts per million per degree F
>difference between their coefficients of thermal expansion
>(CTE). If the piston runs around 275 to 300 as Ron stated
>and maybe the pin is 100 degrees cooler (a complete guess on
>my part based on the assumption that the fuel charge assists
>in cooling) that would mean a line to line fit would open up
>about 25 millionths of an inch or .000025" on the diameter.
>The resulting tiny gaps would only allow sufficient
>penetration of the fuel/oil mix to establish boundary
>lubrication of the bearing surfaces. So the friction from
>the wrist pin to piston joint, doubled because it happens on
>both sides of the piston would mean that the rod/pin bearing
>would want to rotate first. Not to mention the drag on the
>pin from the retainers.
>
>Seems to me that if the stresses were high enough, you might
>get fretting of the piston or wrist pin eventually, but the
>engine would be fine until you tried to take it apart.....
>Also, the life and reliability might be improved if the pin
>floated so the wearing surface had no preferred orientation.
>
>Finally, some of the Fox engine tuners are adding oil feed
>holes in both rod ends to promote hydrodynamic separation of
>the bearing surfaces. While my engine is apart, I'm going to
>try this mod.
>
>Can anyone fill in some of the blanks here?
>
>Dave
>LA_Flyer

Ron B.
F4Fguy

Dave:

A few points:

The temps I'm talking about are the pin bosses,the piston crown is much hotter,~500-550 deg F.

The difference in coefficients is not the major factor,the delta T is.If you assume 50-150 deg F(all the data I've seen indicate the higher)and .000006"/"/deg F* for both parts you get ~.00005"-.0001".For hydrodynamic lube this is probably too small,but the small end always operates in boundary conditions,since it only occillates,it doesn't revolve.A thinner gap encourages capillary flow into the bearing,a large gap allows loading to squeeze out the oil.

* Cast Iron=.00000655"/"/deg F
Steel=.00000633"/"/deg F

In this application,the sacrificial part should be the rod bearing.The rod and/or pin replacement is simpler and less costly than the cylinder/piston.

I would recommend the oil hole in the big end,if its location is carefully chosen to be in the unloaded portion of the bearing.If not,the hole will allow the hydro-wedge to collapse,making matters worse rather than better.On the small end,I would recommend against drilling,as all you'll accomplish is weakening of the small end.

On the hollow pin,if you do this,do not bore straight through at the final i.d. Bore through at a small diameter~1/4 of the o.d.,then taper bore to the final i.d.(~1/2 the o.d.at the ends)leaving about 1/4 of the length in the center at the smaller bore.This procedure not only removes unwanted weight,but utilizes the material properties in a much more efficient way.

I think I have asked this before, but why do we really care if the piston pin can rotate? Afterall, isn't the connecting rod to piston pin the place where the rotational movement mainly occurs? Couldn't the pin be solidly fixed to the piston (except for assembly purposes of course!).

I suppose it might be desirable for the piston pin to slowly rotate to perhaps prevent it from taking a permanent set due to the combustion forces. Is this the reason?

Thanks, I am just curious.

Ron B.
F4Fguy

Alan:

It's a question of wear.Ideally the pin would float as Dave indicated above.The pin would oscillate randomly to a new position each stroke and wear would be distributed between all three bearing surfaces.In practice this almost never obtains.My comments were to the point that if anything is tight,it should be the piston,since the rod is replaceable without destroying the P/C fit,and is therefore less expensive.

In full size auto practice,the pin is often captured in the rod by either a press fit,or(if you go back a few years)a clamp bolt.This has more to do with saving the cost of a small end bearing than with good design.Those same engines,set up for racing,use full floating pins.Wrist pins(gudgeon pins for our overseas friends)are the hardest working,worst lubricated parts in an engine.Probably the best thing we could do for them,is make them as large as will fit through the small end and piston bosses and still leave sufficient metal in those parts to take the applied loads.A pressure oil feed through the rod would help also but,in a fuel mist lubed engine,it's hardly practical.