Cirrus Construction Log: Conrods


Created: March 2005

I was unable to obtain 0.010" silver for the bearing shells as called out by the plans, but did have some 0.008" bronze which seems to have worked out well enough. Notice the post in the jig plate (photo 10). The height of this post was set to be flush with the face of a rod when clamped in the pillow blocks and faced to size. This allowed both faces to formed by simply flipping the rod on the jig with the post set to run true and facing back to the top of the post (not shown in the photographs). The post and block also provided correct alignment for drilling and reaming the hole for the little end bush (photos 11 and 12). After pressing in solid bushes (photo 9), the rod went back in the jig to drill and ream for the wrist pin.

The bearing shells formed easily enough, but required "running-in" against a drill rod blank with some fine valve grinding paste to hone away the slight bulge at the point where the tabs bend out. I chose to ream the big end hole 5/16 (0.313"). Take away 0.016" for the shells, plus another 0.001 for the running fit and we end up with a shaft journal size of 0.296". The plans had 0.312" for these, but I figured reaming the big ends would give more consistant sizes, and be much easier. If you look closely, you'll see the 0.016" aluminium shim used to separate rod from cap for the big-enf drilling and reaming operation. Photo #2 shows one of these being trimmed to size.

Photo #8 shows the block used to align the blanks in the mill vise for facing, drilling, and reaming the big end hole. The milled cut forms a "V-block" like pair of edges that aligns the blanks longitudinally. The orientation placing the cap bolts horizontal was simply eye-balled and the vise tightened up while pressing the blank down onto the jig. The hole in the jig block provides clearance for the 19/64" drill and 5/16" reamer. Crude, but sufficiently effective. Photos #1 and 6 show the Radford-style "up and over" ball turning tool in operation for the end caps and little ends respectively.

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