Machining connecting rods and bearing inserts

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gbritnell

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This thread is to answer a question about split bearing inserts for connecting rods but will cover the making of the rods and inserts.
These rods were made for my flathead V-8 engine. They were made from 4140 steel. I could have made them from 1144 but would have had to cut round stock into a rectangular section.
I cut the stock to thickness, width and length. The length was left long to cut the cap material off and then mill to size. The stock was stamped with 1/16 number stamps to match the caps and rods. The stock was drilled and tapped and clearance drilled taking into account the metal to be removed by cutting the cap of. I also drilled for the pin hole which will located the bearing insert.
 

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The mating caps and rods were then assembled and the bolts lightly tightened. The assembly was then squeezed in a vise to align the faces true and the bolts tightened. The assemblies were then set up in the mill vise using a stop to index each assembly the same. The pin and throw holes were then center drilled, drilled, reamed (pin hole). The throw hole was enlarged with and end mill then bored to the finished dimension.
 

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The extra stock on the sides of the rods was then sawed off.
 

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An aluminum fixture plate was then made to mount the rods for profiling. Each of the holes were machined a little deeper so that the brass locate bushings would accurately locate the rod rather than have the screw do it, which isn't the most accurate way of doing it. The plate was then mounted on the rotary table and indicated and registered to the centerline of the rod.
 

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The last step for the rod beam was to machine a lightening slot into it.
 

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The first picture show a bushing that I use in my .50 R-8 holder. I use end mills with primarily .375 or .50 shanks so rather than continually switching holders I just use this knurled bushing.
To set up the rods to machine the radii on the big and small ends I first insert a brass rod with a 30 degree angle to roughly located center. I then indicate the brass plug true. The rod is then mounted with the other half of the plug and bolted in place.
 

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The radius was then milled on the big end. The procedure was reversed and the small end was finished in the same manner.
 

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Now for the bearing inserts. There are several ways of accomplishing this job. One is to turn the bushing to size, inside and out then cut it to length. It can then be mounted on a mandrel and split with a thin slitting saw. This will leave the insert with a slight gap which won't hurt a thing in use. For those who want a little more accuracy it will require a little more work and material. The bearings are made as before but when splitting them the saw is offset by the thickness and the insert cut. This will leave one half perfect and the other half shy so another bushing is cut the same way and now you have 2 perfect half inserts and 2 scrap inserts. A third way is to soft solder 2 rectangular pieces of bearing stock together leaving one a little long. This will facilitate setting up in the 4 jaw chuck for true center. The bushing is first drilled and bored, I don't like reaming because if the reamer isn't real sharp it will put pressure in the hole and sometimes break the solder joint. The O.D. is then turned and parted off. A little heat is applied and the two halves pop apart. The only lost material is the slight thickness of the solder joint.
For my bearings I turned the I,D.and O.D. and parted them off. I wanted somewhat perfect inserts so I went for the double cut procedure. A mandrel was made with provision for a small clamp. The sides of the mandrel were relieved for saw clearance. The inserts were mounted and cut apart.
 

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The last step was to put the hole in for the locating pin. I used an extra piece of bearing that was cut exactly at the center line and mounted it on the mandrel. It was then indicated perfectly flat. Each of the 'good' bearing insert pieces was set onto the mandrel against the shoulder and locator bushing and drilled for the located pin.
My locate pins are pressed into the cap with just enough material projecting to keep the bearing from spinning.
 

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A most excellent write-up! Thanks for the descriptive instruction here. I may never get to build a split bearing con rod, but some of these techniques lend themselves to many other projects I may tackle in my shop.

Thanks again for a great bit of instruction!

John W
 
To cut the radius on the big end I removed one of the cap screws and substituted it for a modified flat head screw. I cut the head down until there was just a small amount of the taper left. The combination of the brass clamping disc and the other cap screw held the bearing cap tightly enough so that it didn't move while cutting. I did one side then the other.
gbritnell
 

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