For the last several months, Oyster Bah (1962 N. Halsted St.) has been offering a sweet Thursday-night deal in its tiny downstairs space: all-you-can-eat king crab and half-pound burgers, $59.95.

Only regular customers hooked up to Oyster Bah’s social-media channels were in on the deal. But, beginning Oct. 10, Crab Cellar (as the place is now called) will expand the deal to five nights a week (Wednesdays through Sundays) and welcome all comers.

The only catch, if you can call it that, is that Crab Cellar accommodates only 25 seats among seven tables, plus a 10-seat bar.

“It’s small in space and simple in idea,” said Bill Nevruz, executive partner of Oyster Bah and Shaw’s Crab House. “Typically, all-you-can-eat doesn’t translate into the highest quality. What we set out to do is serve the best Alaskan king crab and the prime burgers (using Slagel Farm beef) that we already serve upstairs. If you came in and ate only a half-pound of crab and half a burger, it’d be worth it.”

There are a few add-ons available, like mac and cheese, fries and jalapeno hush puppies. Unlimited lobster is a market-price add-on, and bottomless alcoholic slushies are on offer as well.

The response to the Thursday night “secret” deal has been so impressive that the decision to expand to five nights was easy, Nevruz said.

“I’ve been doing this (restaurant business) for 30 years, and I’ve never been part of something where I’ve had 100 percent satisfaction from every single table,” he said. “We’ve seen one table start talking to another table, buying each other drinks. It’s like a souped-up man cave, and you’re having a party.”

Those seated at the bar have the extra advantage of being able to order a la carte.

“Sometimes we find a party in which one person wants all-you-can-eat and the other just wants a burger or a piece of fish,” Nevruz said. “That’s perfectly fine, but only at the bar.”

Reservations are being accepted by phone (773-248-1737) or at crabcellar.com.