I was going to put on a wing that had a little more area,

and an unbroken spar ,

to stop flex

and my connecting rod needs some work , and the piston had a build up of like a tar ,

soot about 1/2 mm thick

Well mate, remember the journey of a thousand miles starts with a single step.

What's your first task to be?

If it were me I'd do the rod. Is that misdrilled? or worn? If worn the carboned up piston could be a contributor.

Isn't this a fun hobby? Always challenges.

Greg

It use to be in good shape , but it went egg shaped , I thought the big end was the one that worked hard ,

that is still on size ,

But I guess the little end is a lot hotter , up near the piston

I used the free machining 2011 grade of aluminium and its done about 20 flights and 2 hard crashes

Im tempted to make one out of a concrete nail I saw down at Magnet Mart , looks pretty tough

Did you put in something for lubrication holes or grooves or something into the rod ends? usually they have a small hole on both ends on the crankshaft side to help feed oil into the rod. Yes the little end of the rod can get quite a bit of abuse when the engine is running, as it is near the piston and all the heat and it doesn't rotate all that much either. So it needs a little extra help. I like to look at what some of the successful engine designs do for the connecting rod and use those ideas or techniques myself when I make a rod too.

Will just try good old Aluminium again with maybe bigger lube holes or maybe slots and a good slow run in period

[QUOTE=earlwb;11760260]Did you put in something for lubrication holes or grooves or something into the rod ends? usually they have a small hole on both ends on the crankshaft side to help feed oil into the rod. /QUOTE]

Um, why let the oil out prematurely by drilling a hole or cutting a slot somewhere along the along the big end rod eye?

In a single overhung crank engine oil feed is directional, going from the high pressure end of the oil slinger crankweb to the much lower pressure end of the back plate.

Sure, the little end may benefit from an exit hole or slot because its oil feed is equal from both ends and similarly with any double overhung crank but on a single overhung crank I think that its a mistake to follow sound thinking from full sized engines.

I have seen many engine manuals extolling the virtues of oil exit holes but none have shown examples of single hung cranks. - has anyone seen different?

Cheers.

Why not just use 2024 or 7075 Aluminium Alloy like everyone else does then?

See: http://controlline.org.uk/phpBB2/viewtopic.php?t=11068

I was thinking of just mild steel , as I thought the Cox had steel , is that possible

You may find that Cox engines used hardened steel and consider what you are running the conrod up against - the big end undoubtedly will be steel and the crank pin will be similar.

Can you guarantee enough of a hardness difference to overcome galling?

And what about the extra weight of steel in an engine bigger than an 049?

I wonder how much hardness difference there is between a Cox conrod and crankshaft

Donno,

but what I do know is that a whole lot of manufacturers seem to have settled on either 2024 or 7075 Al alloys for rods with hardened steel gudgeon pins and the same materials for crankpins. Some use Bronze bushes on the rods as well. If you PM me your mailing address I can send you a little bit of 2024 suitable for a rod.

Cat, about 12 years ago I bought two Webra SilverLines, a .40 and a .61. Wile breaking in the .61, (very carefully) it developed a clicking internally after about an hour of run time. It turned out that the upper rod bushing was just a hair tight and the wrist pin was rotating in the piston's wrist pin bores and wore them oblong. It's amazing what can happen. Paul at Paul's Model Supply in Fargo, ND fixed me up with the necessary parts and all was fine.

Recycled,

The oil holes in the rod ends, as I understand it, are not to let the oil out. However close the rod fits are at the crankpin and piston pin, there is a change of clearance due to operating loads. At the very least, at direction reversals - around TDC and BDC - oil can be drawn into the bearing surfaces. As the pins migrate from loaded against one 'side' of the bearing to the other, they can pump so as to draw ambient "stuff" into the bearing spaces.

These bearings are "dry sump" lubricated in a dense pressurised mist of fuel and air during the downward 'base compression' phase. There isn't likely to be a significant 'oil-slinger' effect from the enclosed face of the crankweb; the rod swings across that volume, too, remember.

Oil delivery is important. I can't place the reference just now, but I recall a discussion of why rear rotary valve engines almost always have the air/fuel inlet path off-center from the shaft center. It may have been that the rod, sweeping across the chamber, obstructs the incoming charge less when the path is offset while the valve is open. In that case, if it were sweeping 'open' past the valve 'window' when open, it may even enhance delivery of fresh charge for the next ignition up top...

So, most rear rotary valve 2-cycle, single-cylinder engines use a disk, driven by an extended pin on the crankshaft, with its port window to one side from the shaft center. Centered drum valve engines have been developed, but apparently, offset inlet flow path has proven better.

(edited to add) The accelerations on the rod and piston are very heavy. Occasionally, I understand, a thin hole has been drilled through the length of the rod, from one oil hole to the other. As the accelerations reverse direction, oil drawn into the bearings also enters this lengthwise drilled volume. The pumping is easier to understand, here, as oil in this 'channel' tends to continue in its - say - rising direction as the piston reverses its direction past TDC, and inject pretty high pressure oil to the pin bearing. ...Same, vice versa round BDC...

(For an insight to the accelerations on the piston, and much more, find Gordon Jennings' Two Stroke Tuners' Manual, available on-line, which is about 40 years old, but still pretty much the ultimate word for what goes on in our engines...)

Well in model engines, I disagree. The oil holes simply allow passageway and in most engines it is artificially pumped into them but here we have to rely upon naturally existing pressures and the greatest pressure or wicking will be from the crank web end.

With a single overhung crankshaft as compared to a double one the difference is that there is only one crank web and therefore only one oil slinger and pressurised crank web joint, so the oil should be flowing from a high pressure end to a low pressure one - or from the centrifugal slinger to the static backplate.
Providing an oil hole along a pressurized oil gallery will allow passage, let that pressure drop and bleed away.

Plus the fact that most oil holes in model two strokes that are drilled into the lower part of the big end eye are simply eclipsed more than half of there rotation by the very close fitting crankcase anyway.

Rear rotary valved engines have the inlet path off centre as to better aid the oil supply to the crank pin (the most critical part of the engine then gets the coolest, freshest oil supply possible) and (if there are crank web cut outs present) to directly oil the main bearing.

To sum up, oil relief holes or slots have great purpose when the pressure is presented from both ends of a doubly supported crank pin but only provide an early exit when one end is not supported.

Cheers.

"The oil holes in the rod ends, as I understand it, are not to let the oil out."

This may not be the intention, but it is the fact, and it is detrimental to our little gems.

I had a talk to one of Sydney's noted engine tuners about this and his findings were that where an engine had oil hole/s in the big end of the conrod, there was significantly MORE wear on the back plate side of the crankpin.

His answer for this phenomena was pretty simple, simply do away with oil holes and add a crossed slot on the face of the conrod where it contacts the crank web. This acts as a pump to force oil along the entire length of the crankpin.

Greg

Andy Kerr? That was the guy who got me believing all of this.

The one & only. Andy's probably forgotten more about engines than most will ever know.

Greg

Agreed.

Recycled , how much compression does it take to run these engines at high power ,

more than in a car engine ?,

or less

A model diesel with a typical kerosene/ ether based fuel?

Can be up to 18:1 secondary trapped compression ratio and 1.6:1 primary, so yes more than a petrol car engine.