Heisler Engine Construction Part V
Connecting Rod, Strap, Bearing & Crankshaft Counterweight
Nelson Riedel, Nelson@NelsonsLocomotive.com
9/14/2009, last updated 04/27/2017

Connecting Rod-HM-123 and Rod Pin HM124: These are described in Engine Design V.  Both rods are machined from a single piece of  1" square steel bar as shown on the right.  The first step was to drill holes for the connecting rod pin and for the bottom of the yoke.   Next, excess material was sawed off.  
After the material was sawed off, the flat surfaces were finished to dimension on the mill.  Photo show one side completed
At his point most the excess material has been sawed off the four sides and it is beginning to resemble a pair of rods.
An end mill was plunged  vertically in the area shown to make the rounded interface between the bottom of the yoke and the large round area where the pin fits.
This shows the nearly complete pair of rods.  The yoke remains to be finished.
The bar was sawed in half and the remaining material removed from the inside of the yoke using the mill as shown on the right.
The rounded end of the yoke was finished on the mill by rotating the rod against the side of an end mill as shown in the photo.    This is a simple way to make a really nice rounded surface in the home shop.

The milling marks were removed with a file and a 1/2" sanding drum in the rotary tool.

The last step was to drill the holes for the bearing straps.   The holes were drilled in the strap first and the strap then used as a template to drill the holes in the rod.

The photo shows the finished rod and the rod pin. (The fabrication of the rod pin is straightforward.)

The rod, strap and bearing are meant to be a unit.  The three dots visible on the rod are also punched in the bearing and the strap so that the thee pieces can be kept together.


Rod Bearing Strap HM125:   The basic strap was formed by silver soldering 1/8" X 1/8" strips to a 1/2" thick 1" wide steel flat.  The FH screw and tension pin keep things straight during the soldering operation.

This shows the strap after the lower end had been sawed and milled and the holes drilled.   In hindsight, it might have been easier to saw the basic strap out of 1/2" thick plate. 
Rod Bearing HM126:   The rod bearings are made of bronze bearing stock.  The round stock seems to be the least expensive so the first step is to machine a pair of rectangles for each bearing.   The red dye was used to identify a pair (the other pair had blue dye).

The bearing design information on the Engine Design V page suggests the bearing be made extra long and the sides then trimmed to exact size required after the connecting rods have been installed. 

The cylindrical surfaces were turned on the lathe using the 4 jaw chuck as shown on the right.

The nest step was to mill out the 1/2" wide slots on the sides.

This shows a finished bearing.  The 1/4" hole at the joint is under the grease fitting in the strap.  

After the engine was assembled and operated a while on air I found that there was too much side slop in the rod bearings.   Some thin bronze thrust washers were made to exactly match the gaps at the side of this bearing (the other bearing fit snug against the crank web).  The bearing halves were clamped together and the washers attached to the bearings with #2 FH screws and then soft soldered in place.   The washed were then cut at the bearing seam.    These thrust washers are visible in this photo.      

Crankshaft Counterweight HM101 & Counterweight Strap HM102:  These parts are described in Engine Design I.   The circular part of the pair of counterweights were made from from a cast 1" thick ductile cast iron disk obtained form McMaster-Carr.  The sides and outer edges were  finished on the lathe, the result shown in the photo.

The disk was then sawed in half  and the surface that mates with the crankshaft was sawed to the correct shape and finished on the mill. 

This photo shows the counterweight attached to the  crankshaft.  The holes for the screws between the counterweight and crankshaft were drilled in the counterweight first and then into the crankshaft web from each side using the counterweight as a guide.   The first holes were drilled 1/8" (previously erroneously shown as 1/4")and then reamed one 1/32" at a time up to 3/16".    The final step was to enlarge the holes slightly with a #12 drill for a tight fit for the 10-32 screw.

The HM102 strap was cut slightly too long and the ends treaded.  The middle was then heated red hot and bent over the web to the correct shape.  The excess length was cut off after it was installed. 

The photo shows the installed crankshaft with rods, bearings etc.  

The engine had been run on compressed air for over an hour at this point and the bearings have become smooth.  Note the shim thrust bearings on the main bearing to eliminate crankshaft end float - also note that the crankshaft is slightly off center to give the best match with the rods.  

One of the rod bearings has also been shimmed to eliminate rod bearing end float.  

That is black grease round the bearings 

Another view of the lower engine with the crankshaft at a different angle.
Still another angle of the crankshaft
Crosshead Modification: When the crosshead and connecting rod were test fit inference between the bottom of the crosshead and the bottom of the rod yoke was noted.    This was expected.   The bottom of the crosshead was thinned slightly on the mill and the outer edges were thinned more plus a small recess was cut on each side to give the desired movement of the rod.  I forgot to take a photo at the time.   The photo on the right shows these modifications on an installed crosshead with the rod removed.

Assembling the piston, rod, crank:  The pistons have been installed once and not removed.   Everything in that area fit perfectly.  A  screw type adjustable hose clamp was used as a ring compressor on the piston.    The upper end of the cylinder bushing was beveled slightly using a 1/2" sanding drum in the rotary tool to make it easier for the rings to slide into the bushing.   The cylinder was coated with way oil, the hose clap tightened around the piston and rings and the piston then carefully tapped into the cylinder.  No problem.

A couple Viton O-Rings and the packing gland had already been fitted into the lower head.    The piston rod and crosshead were then pushed up into the crosshead guide with the rod going through the packing gland and eventually into the piston.    The piston wrench (Engine Construction I)  was then used to screw the piston onto the end of the rod.  Once the piston was tight on the rod, the wrench was used to hold the piston while at lock washer was put on the rod  and a nut screwed on the end of the rod and tightened against the piston.    This part went without a hitch.

The crankshaft and connecting rod bearings required a bit more effort to fit.   When initially put together the crosshead is a tight fit and the rod has very little side movement. The sides of the main bearings and rod bearings should be trimmed/shimmed as necessary so that the rods remain in that position with no more than a couple thousands end float in the crankshaft and slide slop in the rod bearings.  

Finishing up the valves in next in Engine Construction VI         


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