Rod Bearing Renewal

Special thanks to William Bailey for preparing this information.
All photos on this page taken by and used by permission of Larry Rouse.

Connecting rod bearings on these Winton engines are not removable precision shells as on modern engines, but instead are bronze bearing boxes lined with babbitt metal, hand-scraped to fit.

To renew a worn bearing, it is sent out to a specialty firm where they carefully prepare and tin the bearing box halves, then centrifugally cast new babbitt metal (an alloy of tin, lead and antimony) using a special machine. It is then precision-bored to the requested size, based upon careful measurement of the crankshaft journal.

But that is only the beginning --- the bearing must be hand-scraped to fit the journal properly.  While the brass shims between the bearing halves set the running clearance on the top and bottom parts that bear the major loads, due to wear on the journal you must still scrape the high spots to match the bearing to the journal with the proper clearance. Also, the side clearance is not affected by the shims, so the bearing must be scraped from the joint down at least 45 degrees.

To check the clearance, a blue paste is applied to the bearing half,

 

then the bearing half is temporarily fitted and rubbed on the crankshaft.

 

The high spots on the babbitt will have the bluing rubbed off. Here, my brother William is scraping the high spots with a bearing scraper, which is a curved tool with a knife edge. By carefully drawing it on the babbitt, the high spots can be removed. It sometimes takes many tries repeating the bluing and fitting, before we are satisfied.

 

Here is the top half, which has an oil connection that feeds a copper tube (with check valve) on the connecting rod. The check valve keeps oil from draining when stopped, and provides a pumping action due to the reciprocating motion of the rod.

 

Once both halves of the bearing fit properly, the bluing is cleaned off and you are ready to check the running clearance. Here is a scraped bearing half ready for installation. Note that the sides require more babbitt removal since the shims added between the halves do not affect the side clearance much.

 

The two halves are then lightly drawn up on the crankshaft without shims, and the distance between them are measured, then stacks of shims are calculated to give that clearance plus the running clearance of .010" - .012". The bearing box halves are assembled on the crankshaft with the shims, using Plastigage® to double-check the bearing clearance (we used to use the traditional lead wire, but Plastigage® is better and can be read more easily). Each time the shim stacks are handled, they are measured with a micrometer to ensure that individual shims are not dropped or lost from the stack.

Once the correct clearance has been confirmed, the bearing halves are cleaned, reassembled with the correct shims and then bolted to the foot of the connecting rod (with another shim set under the rod foot, which sets the piston clearance).

Here is the assembled bearing on the crankshaft, bolted to the foot of the connecting rod, as viewed through the crankcase access door. Note the setscrews that are used to hold the bolts in place and keep the halves from separating when the nuts are removed to allow the connecting rod foot to lower onto the bearing box. The crankshaft journals are nine inches in diameter, and each bearing half weighs almost 100 pounds. Four large bolts with castellated nuts and cotter pins secure the assembly to the rod.

 

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Copyright © 2001 Brian F. Bailey, W4OLF
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