I've always thought that egging-out of the bushing was caused by a lack of sufficient/proper lubrication. Turning the engine over while in or near hydraulic lock would most likely cause a bent connecting rod or crankpin, I would think.


Ed Cregger

The root cause of the ovality is most likely over-reving if the largest diameter of the hole is between 4 and 10 o'clock. If the largest diameter goes straight up and down along the center of the rod (between 6 and 12 o'clock), the reason is possibly a tight ABC setup.

If lack of lubrication is the reason, there should be signs of overheating in the bearing bush. (carbon deposits or blue-shine in metal)

Are we talking about wearing an egg shape in the bushing area of the connecting rod's wrist pin? Or are we talking about a mechanical distortion where the bushing material is still present, but deformed?


Ed Cregger

Ed,


'Egging out can be due to wear patterns... But it could also be a result of actual elongation of the 'yoke', whether it is at the top-end, or at the bottom-end of the rod, or both.

Excessive loading of the con-rod, from the piston repeatedly being pulled down, out of a seize at TDC (a very cold and rich break-in in a tight ABC type engine), will do that. Over-revving a four-stroke engine can also eventually cause that (unless floating valve-piston contact occurs before...), but this will take much, much longer.

Egging out due to wear is more likely to be due to excessive compression loading of the con-rod, like when running the engine for long periods on the verge of detonation; especially with a grossly exaggerated prop size.

...Or, as was suggested, spinning the engine with an electric starter, when it is 'nearly' hydro-locked...
That will wear down the near-sides of both bushings.

--------------


Why would a four-stroke rod deform from over revving when a two-stroke ran at the same rpm would not?

I can see why there might be valve problems because the springs are too weak to track the cam profile, eventually causing interference with the valve and piston.

Are four-stroke connecting rods built to a much lower standard because of anticipated lower rpm?


Ed Cregger

The 4 stroker has the E. valve open every other stroke and so there's no compression there to slow or cushion the piston and it wants to go flyin' right out thru the head [X(]

Ed,


In a two-stroke engine, firing normally and constantly on every revolution, the connecting rod is always under compression load.
This load does alternate.
It is stronger as the piston is closer to TDC (by compression and combustion forces from above the piston and from inertia of the crankshaft and prop-load from below) and weaker as the piston is near BDC (prop-load and crankshaft inertia 'try' to slow the piston as it descends and then crankshaft inertia pushes it back up).
At mid-stroke; both on the way up and on the way down, the forces on the con-rod are virtually zero.

But they are never at a real negative value.

I.e. a two-stroke engine running normally, is never 'trying' to pull its con-rod apart...


In a four-stroke engine it is different. At nearly all phases of operation, it is very much like a two-cycle engine and the rod is under compression, but at the end of the exhaust phase and the beginning of the intake phase, there is virtually no force applied to the piston from above.
...And this coincides with the highest piston acceleration (deceleration, actually) rate... The piston 'wants' to continue going up, but is pulled down by the con-rod. The forces applied to the rod are of tension and as RPM is increased, these forces are too, at an exponential rate.

So, a four-stroke engine 'naturally' applies tension forces to its con-rod, whereas a two-stroke never does, unless it is very tight (to the point the piston catches at TDC) and is broken-in at a cold four-cycle...


...And, not all four-stroke engines are designed for lower RPM...

One reason that is also likely, is running the engine too lean and/or with fuel containing too high much nitro. This can cause pre-detonation and excessive maximum fireing pressure. Then the con cod will face very high compression forces. This, together with a heavy prop and low RPM will give poor running condition for the crankpin and bushing.

When I started this thread my guesses were:

"Is this type of distortion caused by high compression with too much nitro, i.e., pre-detonation? Could using an electric starter on a flooded engine cause this?"

It looks like my initial reasoning was about right.

I ordered a rod and ring; I'll probably replace the bearings, too.

I have the perfect project lined up for this engine:

http://www.rcuniverse.com/forum/m_6146821/tm.htm

I even had a few rib sets laser-cut to make the build go faster.

David

I've had one engine egg out the small end of the rod. I noticed my Rossi 45 was getting some play at the prop tips after maybe 20 hours so I stripped it down. The small end (unbushed...in a Rossi??) was egged by .012" only on the bottom side of the bush. Obviously this also changed the timing and lowered the compression. Not being one to buy something if I think I can fix it I made a bush centred on the original rod length. I have no idea why it egged because the fuel was 80/20 all castor, never been lean it's life, always hand started and never thrashed with high revs.

Castor is a fine oil -- but not necessarily any better than the newer alternatives, particularly when it comes to issues of film-strength in high-pressure areas that operate at lower than cylinder-head temperatures (ie: the big-end/crankpin interface).

You'd be rather shocked at the low levels of lube I've been playing with (on disposable engines). The better synths are bloody impressive and I'll gladly run them at ratios much lower than would be even remotely sensible with castor.

Feel the force :-)

-----------------


Has the bushing material been lost due to wear? Or is this a deformation of the connecting rod end? I'm still not clear on this. TIA


Ed Cregger

"What causes conrod bushing damage?"

1-Using a starter to force an engine to start will sooner or later cause conrod damages. On a flooded engine it is even worse. Thats probably the main reason for a conrod bushing damages /failure. It is better to find out why my engine needs a starter in the first place.
2- The lack of lubrication "lean runs" is another reason for conrod damage and I also think castor oil is needed for superior lubrication in that area.
3-Wrong tolerances "usually too tight" between crankpin and big conrod end is another reason for bushings damages

I'm aware of those things, Flyer 95. What I'm not clear about is precisely what kind of damage was done and is being questioned. Wearing of the bearing material can be attributed to poor lubrication from lean runs or bad fuel (lack of lubrication).

Starters bend connecting rods and crankpins, they do not normally cause a deformation of the wrist pin end of the connecting rod. This is what I'm trying to get out - without success.

Dar Zeelon pointed out one way that deformation can occur in a four-stroke connecting rod's bushed areas. I accept that, but we're all just guessing without more information.

A few photos photographed close-up to the damaged connecting rod in question would be a tremendous help.


Ed Cregger

Here you go, Ed:

OK, here's another opinion. Lets first examine the lower bearing on a connecting rod. The crank pin rotates, thus providing a good film of oil to ride on. This film of oil will tend to cause the crank pin to ride somewhat near the center, if lubed properly.

The upper end, on the other hand, just pivots back and forth. If all castor is used, and the engine has been run very lean a lot, there may be a film of varnish covering the upper end. If the oil hole is clogged by varnish, it may be blocking adequate lube. To extreme, this can freeze the upper end of the conrod to the wrist pin and transfer wear to the piston to wrist pin bearing surface. BTW, this same effect can be caused by an engine just sitting for years until the upper end lube hardens or corrosion freezes the conrod to the wrist pin. For this reason, if you get an old engine it is a good idea to check that the upper conrod pivots freely on the wrist pin.

Just offering another possibility. If this was covered and I missed it, apologies.

George

David,


This definitely appears to be compressive overload damage...

This con-rod seems not to be bushed at either end.

Or limited lubrication... Non bushed conrods need better lubrication. The conrod has definitely been overheated. You can see the black varnish from carbonized oil. This overheating can be a result of compressive overload together with bad lubrication. I have never seen carbonized oil on conrods at normal operating temps before.

Thanks, Dave.

No bushing? That does look hammered.


Ed Cregger

Neither end is bushed, correct.

Looks a lot like the K&B .40 rods I mentioned...whupped!
Over-heated... maybe by too much prop, and too lean?

The bottom end looks like either it has remnants of a bushing, but it might also be (no bushing ) just discoloration from varnish and or a little rust residue in the chamfer?

You can also see where the top end of the rod was rubbing the pin bore's boss in the piston...must have been fore/aft thrust loads...it may be a slightly bent rod too, or just rocking caused by the wallowed out hole?

That is exactly what my MVVS .15 diesel looks like. i ran it quite a bit when I started the hobby. I never thought I ran it over compressed, but it did run it with a tuned pipe on it. Being so long ago I'm not sure. I had black carbon near the small end of the rod. Aluminum gets soft when it gets hot, so I think that is the main cause.

I have heard alot about too much load and was too lean and too much nitro and yadda ,yadda.
How about the engine just has many-many hours on it cause it is at least 30 yrs old. Once the wear point reaches too loose it doesn't take long to beat the egg shape. Regular wear will be at top and bottom of con rod because this is where weight of piston takes a violent change in direction and more so at bottom because of added compression and ignition. I just got an old Webra .91 with the dynamix accelerator pump carb. Of course seller said it was good. Upon examination compression was down , con rod egg shaped, Bearing rough with back and forth play. Ring was stuck ,but 3-N-1 oil freed it up to increase the compression. Compression was still down. My conclusion was this engine was not abused but totaly well used. If you take a car engine with 200,000 miles on it the bearings (bushings)are going to be egg shaped and loose. Now if this was done in first season (being a brand new engine) I'd go with the YADDA -YADDA...........We don't even know if the bottom end has been rebushed in it's lifetime ! Or that there wasn't a flaw or air bubble in the metal on the top end.

speedster 1919, that's a good point. For all I know this engine may have given several hundred hours of reliable service to a previous owner. It may have run so well that it was simply used until it couldn't give anymore. It is a Webra, after all.

David

Sorry, but true: Nothing one engine are without abration.. Abration are a processive lost of material when the parts are running together.