Hi Team,
a "Search" found this discussion and the recommendation that 6061 aluminum alloy was suitable for replacement pony main bearings.
I believe it was Mike Meyer that found out that the pony mains were very similar to Toyota piston alloy and that 6061 was the nearest available alloy and deemed as suitable.
Your experience may vary--I stand to be corrected.

www.acmoc.org/bb/search?query=pony%20mai...ldforums=1&catids=35

Cheers,
Eddie B.

Eddie, Thanks for posting the search links particularly the part about the composition of 6061 Aluminum. 6061 would work for a bearing material (for a while) but it surely is not the optimum material. So called "Aluminum" bearings in IC engines usually use an Al alloy that is 3% to 4% Silicon whereas the nominal range of Silicon in 6061 is only from 0.4% to 0.8% Silicon. The silicon helps polish the journal hence adding to longer life. In addition it is common to fuse steel backing to the "insert" for durability rather than just a piece of machined Aluminum. Sometimes it is impossible to get the part one needs and it requires some improvisation to make it work but when available I firmly believe in installing quality replacement parts, particularly if the replacement requires a lot of labor.

Very late John Deere model A,G and number series Waterloo tractors used AL bushings for the main bearings on their tractors. Someone tried to make some main bearings out of bronze and found that it did not work overly well for these tractors. This company is reproducing the JD main bearing in AL.
www.motormachining.com/html/john_deere_main_bearings.html

Neil wrote:

That's interesting Jim, is there a standard source for the 4% alloy in generic pre-made bearings (beyond the OEM part of course)? And does that alloy have a trade name?

The various alloys of Si and Al with high Si content come under the general heading of silumin. (I think I spelled that right but hope it is close)