SMOH vs TBO Explained: Engine Time Terms, Calendar Limits and Reserve Math

This page is the reference for the engine-time abbreviations you see in aircraft listings and logbooks, and for the arithmetic that turns engine time into dollars. It is written for buyers reading listings and for owners setting up an overhaul reserve.

What do SMOH, SFRM, SNEW and TBO mean?

Listings compress an engine's history into a few letters. The table below defines each one and says what it does and does not tell you. The legal definitions behind "overhauled" and "rebuilt" are in 14 CFR 43.2. The rule that lets a factory-rebuilt engine start a new logbook is 14 CFR 91.421.

Overhauled vs rebuilt: why the logbook matters

Under 14 CFR 43.2, an overhauled engine is disassembled, cleaned, inspected, repaired as necessary, reassembled and tested to approved standards. It can reuse parts that are within service limits. A rebuilt engine must meet the same tolerances and limits as a new one. Only an engine rebuilt by the manufacturer, or by an agency the manufacturer approves, can go back into service with a new logbook and no prior operating history (91.421). That is why "zero-time" is a factory designation. A field overhaul, however good, is logged as SMOH against the engine's existing total time. For how that choice affects cost, warranty and appraised value, see field overhaul vs factory rebuilt vs new.

Do Part 91 owners have to overhaul at TBO?

Generally, no. In a July 1, 2026 article, AOPA's legal services team wrote that "Part 91 operators generally are not required to overhaul an engine at the manufacturer's recommended TBO" (AOPA Pilot Protection Services). The legal test is airworthiness: the engine must conform to its type design and be in condition for safe operation, as confirmed at each annual or 100-hour inspection. Commercial operators are different. Part 135 operations, for example, often have overhaul intervals built into their approved maintenance programs.

"Not required" doesn't make TBO irrelevant. Three groups still act on it:

• Buyers and appraisers price a past-TBO engine as run-out, so the overhaul cost comes off the value whether or not the engine is still healthy.
• Insurers may ask about engine time and condition at underwriting. Terms vary by company and policy, so ask your broker before you fly well past TBO.
• Lenders lend against appraised value, and a run-out engine lowers it.

Owners who run "on condition" past TBO normally rely on trend monitoring: oil analysis, filter inspection, borescope images, compression trends and engine-monitor data. That is a maintenance decision to make with your mechanic, not a budgeting shortcut.

What is calendar TBO, and why does it matter for low-hour engines?

Lycoming's current TBO schedule, Service Instruction 1009BF (issued July 20, 2026), states that "all engine models are to be overhauled within twelve (12) calendar years of the date they first entered service or of last overhaul". It explains that the calendar limit exists to mitigate deterioration that occurs with age. Continental's TBO letter, SIL98-9, also lists a 12-year figure next to the hour limit for most models. Check the current revision for your engine.

Calendar time matters because a lightly flown engine is not a fresh engine. Moisture and acidic combustion byproducts can corrode cam lobes, lifters and cylinder walls in an engine that sits. A listing that shows "600 SMOH" on an overhaul done 15 years ago describes an engine past its calendar TBO, whatever the hour meter says. Lycoming's position in the same SI is that, for Part 91 operations, only an appropriately rated and qualified maintenance person can allow the 12-year period to be exceeded, with concurrence from the civil aviation authority. Treat it as a condition question, not a formality.

What TBO applies to common piston engines?

TBO is set per model, and sometimes per serial number or configuration. The ranges below come from the manufacturers' documents. Always confirm the figure for your exact engine model and dash number.

One trap: the Cessna 182 did not always use the same engine. Normally aspirated Skylanes built through the 182R (1986) use a Continental O-470. The 1997-and-later 182S and 182T use a Lycoming IO-540. They have different TBOs and different overhaul prices. Lycoming's SI 1009BF also describes a 200-hour extension for factory-overhauled engines. The same extension is available for field overhauls that use only genuine Lycoming parts and Lycoming-approved procedures. Your logbook needs to show that the work qualifies.

How do you calculate an engine reserve per hour?

Reserve per hour = expected overhaul cost ÷ TBO hours. Use a current written quote that includes removal and reinstallation, accessories and shipping, not a price-list headline. The figures below are illustrative budgets, not quotes.

If you buy a mid-life engine

Say you buy an airplane whose 2,000-hour engine shows 1,200 SMOH, and you start your reserve at zero. You now have to save $40,000 in the 800 remaining hours: $50.00 per hour, not $20.00. The difference is the $24,000 of engine life the previous owner used (1,200 hours × $20.00). That amount belongs in the purchase price negotiation. The buyer's guide to engine time and aircraft value walks through that negotiation.

If you fly fewer than about 170 hours a year, check the calendar

A 2,000-hour TBO spread over 12 years is about 167 hours a year. Below that, the calendar limit arrives first. At 50 hours a year, a freshly overhauled engine reaches 12 years at only 600 hours. The same $40,000 overhaul then costs $3,333 a year, which is $66.67 per hour flown. You don't have to overhaul at 12 years under Part 91, but a buyer or appraiser will look at calendar age. Budget as if the calendar will matter.

How does engine time affect price and financing?

Buyers and appraisers turn engine time into dollars using the same reserve rate. An engine with more remaining life is worth more, and a run-out engine is priced as if the overhaul is due now. Lenders then lend a percentage of that appraised value. With 15% down as a typical minimum for post-1960 singles (AOPA), a lower appraisal on a run-out engine can mean a larger down payment for the same purchase price. Financing an overhaul after purchase usually means refinancing the aircraft loan. The mechanics are covered in these guides:

• How engine time moves purchase price, and how to negotiate a high-time or past-TBO engine: engine time and aircraft value.
• Field overhaul vs factory overhaul vs factory rebuilt vs new, and what each does to appraised value: field overhaul vs factory rebuilt.
• Ways to pay for an overhaul, with payment examples: engine overhaul financing.

How to verify engine time in the logbooks

• Find the overhaul entry itself. It should name the facility, the date, the standard used (for example, new limits or service limits) and the major parts replaced. A factory-rebuilt engine should have the manufacturer's signed statement of the rebuild date required by 91.421.
• Divide hours by years since overhaul. 300 hours over 10 years is 30 hours a year, a corrosion question worth a borescope.
• Separate top overhauls from major overhauls. STOH is not SMOH.
• Check the exact model and serial number against the TBO document. Serial-number ranges and configurations can change TBO.
• Look for trend data. Oil analysis history, filter findings and compression history make a mid-time engine easier to price and easier to insure.

For a full logbook walkthrough, see how to read aircraft logbooks. For the inspection a buyer should order, see the pre-buy inspection checklist.

How this guide differs from related guides

This page defines the terms and the reserve math. For what a buyer should pay for a given engine time, see engine time and aircraft value. For choosing between overhaul types, see field overhaul vs factory rebuilt vs new. For loan options and payments, see engine overhaul financing. For building a broader repair fund, see planning for unexpected aircraft repairs.

Frequently asked questions