Aluminum 1 - Steel Zero

[kcmillin]

Upon tear-down and inspection of the TI4, I noticed something strange.

Once I took off the oil pan I felt the slop on the connecting rods. With the pistons at TDC and BDC there was some slop, but in the oposing direction there was none. This would indicate that the connecting rod journals have worn into an oval shape. I had brass bearings .003" thick in the rod ends, which were completely disintegrated and had turned into powder in the oil. This left the 60-61 aluminum connecting rod in direct contact with the the CRS crankshaft.

Tearing apart the engine further I removed the connecting rods and pistons, I then decided to measure the connecting rod journal bore in two places. All four connecting rod journals were no more that .00125 out of round, and still within the original spec I made them. This seemed strange since the slop in the connecting rod was at least .010" when the engine was together. So I then decided to measure the crank journals. They all indicated an out of round reading average of .006". WHAT, I thought, this cant be, the aluminum is stronger than the steel!! scratch.gif

Can anybody explain to me what is happening here? This just seems weird to me.

I have now reamed out the connecting rod journals, and using emery cloth have made the crank journals round again. There is now more space for a larger bronze bushing which is working great for the mains. I hope this solves the problem.

Kel

 

[MachineTom]

A couple of things going on, the steel was being worn for quite some time while the brass was wearing into dust. That would be most of the wear on the crank, I assume that you did not run it very long after the last pieces of brass flaked away. AL is a better bearing surface than steel as it is much more ductile, and moves around as particials embed in it surface, steel will notdo that and just wold score from dirt and the like.

Lots of small engines have AL connecting rods with no inserts at all, I doubt they are 6061 though.

 

[doc1955]

Lots of small engines have AL connecting rods with no inserts at all, I doubt they are 6061 though.

I believe they would be made of 7075 t651 aluminum.
That is what I used in my compressor when it threw a rod and it has been running fine since knok on wood. I posted what the original rod looked like it was a forging and it was actually a very soft aluminum. So I know the rod replacement I made was made of a much more durable material.
My 6 cycle engine which has quite some run time on it shows no wear the rod was made of 7075 t651 and the crank journal was made of 1018 crs.

 

[Jasonb]

It will be the brass that has done the damage not the Aluminum, next time use a leaded bearing bronze.

Jason

 

[peatoluser]

I suppose it's rather like the process of lapping? Particles embed into the softer lap and wear away on the surface of whatever we are working on.

 

[tel]

I suppose it's rather like the process of lapping? Particles embed into the softer lap and wear away on the surface of whatever we are working on.

It is indeed! The softer material with usually outlast the harder for just that reason - I've seen steel line shafting worn down to almost 1/2 its original diameter - running in wooden pillow blocks.

 

[kcmillin]

Thanks Guys, it really makes sense now. It seems that my preventative measure is what caused the problem. I have bearing bronze bushings now, which I have had good luck with on the mains and cam.

Perhaps I should get me some of the 7075. It sounds like the cats pajamas.


Kel

 

[Tin Falcon]

A couple yeas ago I did some work on a vacuum cleaner for my church. the wheels were on steel shafts in plastic bushings. the steel shafts were quite worn I replaces the shafts and make bronze bushings.
Tin

 

[mu38&Bg#]

Many rods are 2024 in RC type engines.

 

[Swede]

The Zimmerman plans for the 1/6 scale deHavilland Cirrus call for sheet silver bushings for the connecting rods. Apparently, this was done quite a bit in the WW2 era, and the silver is an excellent bearing material.

When I did my Cirrus, I substituted sheet bronze, which was formed in a little jig. These separate pieces allow for a bushing when your con rod has a split big end, and the crankshaft middle journals are inaccessible. Works nice. Might give some guys ideas...

 

[Maryak]

Guys,

A word of caution regarding silver bearings. "Normal" engine oil contains Zinc Dithiophosphate as the EP additive, this will eat out the silver. If you use this type of bearing then you need an engine oil with Barium as the EP additive such as an oil suitable for Alco loco engines which have silver gudgeon pins.

Hope this helps.

Best Regards
Bob

 

[steamer]

Ron Collonna uses silver liners in his Offy engine....which he borrowed from the Cirrus.

Didn't know about the oil issue though Bob....I'll keep that in mind!

Dave

 

[Rustkolector]

Kel,
While material selection could be an issue here, it sounds more likely to be a lack of lubrication.

Jeff