If you're talking about the bolts that retain the frog to the beam on the early tractor plow bottoms...
Frog mounting bolts could be square headed or carraige bolts or both. See if the frog has square holes in it. Carraige bolts were needed for some top bolts because the brace had a square or slotted hole in it. Bottom frog mounting bolts could be a #7 or #3 head plow bolt that went through the landside as well as the frog and beam, or a square headed bolt that only went through the frog and beam.
If you're talking about the plow bolts used to retain wear parts to the frog...
IH used a constantly changing variety-pack of plow bolts for mounting wear parts through the years. #3, #4, #6, #7, patent, and clipped head were the most common found on plows.
Some people cram whatever they can find in the hole with the thread size they need, regardless of the original head style. The #7 head plow bolts commonly used years ago by many manufacturers haven't been an easy find at dealers or hardware stores for 30+ years.
Some older bottoms used a #4 head. It uses a four-cornered neck that tapers and has an outward looking square head. #3 with a square neck won't interchange with the #4. #6 and #7 look somewhat similar, but the "keys" are different, and do not interchange, either.
Some aftermarket wear parts used a now common #3 head plow bolt with the square neck while IH was still using #7 head plow bolts with the "reverse key" protrusion for their version of the same part.
Originally steel wear parts were usually "finished" or "polished" with the bolts installed so the bolt head would be flush with the wear part. Chilled parts weren't finished, so the bolts usually didn't come with them and had to be ordered seperately. "Repair" head bolts were available to use in worn parts to somewhat compensate for wear, but still usually protruded some.
A #8 plow that early would be hard to find, as I think they started making them in 1927 or 28.
AG
This post was edited by AG in IN at 20:05:01 08/16/13.
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