Soybean futures prices, propelled by a late wave of speculative buying, advanced about 7 cents a bushel today on the Chicago Board of Trade despite an Agriculture Department projection for a record harvest.

Corn futures values also rose shortly before the final bell, closing just a shade lower even though the government's estimate of a record 6.229 billion bushel yield had been considered quite bearish.

Wheat, about 1 cent beneath previous levels for most of the session, advanced nearly 5 cents a bushel on commercial buying and short covering Oats values lost a cent.

On the Chicago Mercantile Exchange, hog futures and pork bellies contracts scored gains, but live cattle prices closed fractionally mixed.

The upturn in soybean and grain delivers began with some short covering by day traders. But most of the late advance was triggered by large purchases of November soybeans by prominent local professional.

Soybean oil worked back from firmly lower levels to a narrowly mixed finish, while meal gained about $3 a bushel in nearby contracts.

Heavy rains in the Midwest minimized the bearish impact of the Agriculture department's crop report by raising the possibility of harvest delays and damage to corn to the Soviet Union encouraged some buying. Persistent exporter pricing helped wipe out most losses in the corn pit.

At the close of grain trade, soybeans were 5 1/2 to 10 1/2 cents a bushel higher. September 5.29 1/2, wheat was three to 5 1/4 higher, September 2.39 1/2; corn was 1/2 to 1 1/2 lower. September 1.95 1/4 and oats were 1/2 to 1 1/4 lower, September 1.14 1/4.

Pork bellies also rebounded from earlier losses to finish 27 to 57 points higher.