Resizing Connecting Rod Big Ends

acotrel said:

When being resized, the faces of the cap are probably ground, then the cap and rod are machined as an assembly.

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In order to retain the original big end bore, both faces (of the cap and the rod part) need to be machined prior to the drilling operation. This will render a slightly shorter rod. In addition, careful measurement of the second rod is required, and resurfacing cap and rod may deviate from the first rod.

The small end bore center is the datum for for this operation (assuming straight non-twisted rods). Now, if the big end has become oval, I expect the small end to be oval as well, and this will require the small end to be bushed and drilled off-center in order to restore the correct rod length. The (oversize) bush can be fitted by either enlarging the bore, or by use of an expanding technique using the old bore.

- Knut

Interesting article. Note the comment that eye to eye length is much more important in diesel engines. I would think that the change in length from resizing a big end would be irrelevant in our engines.

I always have the rods resized on all my rebuilds, never had a problem.

Stephen Hill said:

Interesting article. Note the comment that eye to eye length is much more important in diesel engines. I would think that the change in length from resizing a big end would be irrelevant in our engines.

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A reduction in conrod length of 0.25mm (0.01") is a reasonable figure for an aluminum rod, I think. Steel rods will deform far less (typically 0.003").
This figure will reduce CR in a 750 engine from 9:1 to 8.83:1 . In this example, the power and torque lost will be noticeable on a dynamometer only. Power drop aside, fuel efficiency (MPG) will also drop, and I'd say this is a highly relevant consequence these days.

- Knut