The government conveyor belt carrying the Labor Department's grain elevator safety standard to the Federal Register has stalled again, this time at the Office of Management and Budget.

Christopher DeMuth, head of the OMB's Office of Information and Regulatory Affairs, told a congressional subcommittee last week that his office has "some questions, difficulties with the proposed rule."

While meetings between the two agencies to iron out the problems began shortly after the Thursday morning committee hearing, it remains unclear how long this new hitch will delay publication of the standard, which Thorne G. Auchter, head of the Occupational Safety and Health Administration, declared a "high priority" last year.

The crucial question appears to be OSHA's determination to make operators reduce loose dust in their elevators--as measured on floors and other surfaces--to one-eighth of an inch.

If other conditions for an explosion are right, relatively low humidity and the presence of a spark from machinery or welding, grain dust can act like gunpowder. Since 1977, 143 elevator explosions have killed 108 people and injured 321 more.

OSHA first announced its intention to propose a grain elevator safety standard in February, 1980. Hearings were held later that year. In September, 1982, a draft rule was circulated among industry and labor representatives. Between March and May of this year, the key draft of the standard was mislaid at the Labor Department. It was sent to the OMB May 11.

Last month, the OMB announced that it wanted more time than its allotted 60 days to review the rule. Among the OMB's questions, according to Robert Bedell, DeMuth's deputy:

* Why was the maximum level of grain dust--achieved by either ventilation systems, which can be costly, or basic sweeping and housekeeping--set at one-eighth of an inch? "There's no justification of that level," said Bedell.

* Are the rule's estimated benefits, especially those based on number of lives saved, skewed? Bedell said that the proposal's benefit calculations are based in part on figures from 1976 and 1977. Since then, he said, fatalities have been reduced 80 percent. He asked whether the calculation of benefits shouldn't be based on the lower fatality rate.

* Would other methods, including OSHA's proposed controls on ignition sources and the voluntary standards of the National Grain and Feed Association, be sufficient to reduce the danger of explosions?

Those questions trouble the chief spokesman for the AFL-CIO's Food and Beverage Trade Division, which has lobbied hard for key elements of the OSHA proposal.

"OMB has bought lock, stock and barrel the argument by industry, which says, 'We are taking care of the problem; we don't need a standard,' " said the union's Debbie Berkowitz.

" . . . There will always be an ignition source you don't catch," she said. "Unless you clean the dust up, the elevators are going to blow up."

OMB's Bedell said Friday that he is intent on keeping the negotiations between the two agencies amicable, unlike prior OMB-OSHA confrontations. OSHA spokesman Doug Clark said, "Sometimes, if Thorne Auchter has made his mind up, he can make a forceful case for a standard ." Has he made his mind up? "He signed the standard."