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51 Factory - Merlin Engines

The 51 Factory is dedicated to overhauling and restoration of Rolls Royce and Packard, V-12 Merlin aircraft engines. We have a tremendous inventory of new original parts, in addition to literally hundreds of tons of low time parts, sub-assemblies, and complete engine cores. In the past 14 years, we have acquired numerous inventories filling over twenty 48’ semi-tractor trailers, totaling well over 600,000 lbs of Merlin engine cores and parts.

We have every component necessary to restore an engine in any condition.  

The bulk of our inventory is traced back to Bristol Aero Engines Ltd, Montreal, the last commercial Merlin overhaul facility, operating in the 1950’s up to the early 1960’s. Bristol Aero was supported directly by Rolls Royce, receiving shipments of new overhaul components/parts, tooling/fixtures, and extensive documentation on processes and approved repair procedures, stamped by Rolls Royce engineers.

The entire inventory of parts, tooling, and documentation was acquired by Cavalier Aircraft Co. in the early/mid 1960’s.

From Cavalier, the remaining inventory transferred to Paul Szendroi in the 1970’s. He was employed at Cavalier rebuilding Merlin engines for their P51 conversions and other customers. Paul set up his own shop called ‘Universal Airmotive’ in the Chicago area, and continued on with rebuild/overhaul of these engines for over 30 years. He is a very accomplished machinist, and had purchased and prepared a number of machines specifically for rebuild of Merlin engines. Some of the machines he set up/modified include:

  • precision line bore machine for re-boring the main case bearing journals

  • crankshaft grinding machine to restore crankshaft journals to new condition

  • dedicated cam shaft grinding machine with precision cut masters to exactly duplicate the original inverse radius cam lobe used on the Merlin cams

  • surface grinding machine and fixture to re-grind pads on the rocker arms to as new condition for perfect mating surface to new ground cam lobes

  • cylinder liner lapping machine and fixture

  • cylinder liner flange grinding machine

  • cylinder head surfacing machine

  • dynamic balancing machine for the supercharger rotating assy, water pump impeller assy and supercharger planetary gear assys

  • valve and valve seat grinding equipment and fixtures

In 2003/2004, the entire inventory of parts, engine cores, complete machine shop, Rolls Royce tooling/fixtures and all the documentation was acquired by the ‘51 Factory’ and moved to our facility in San Martin, California.

The chief mechanic/machinist at Universal Airmotive also moved to California to continue on with a career of restoring Merlin aircraft engines.

The 51 Factory has further supplemented the operation established by Universal Airmotive, with CNC milling machines, lathes, and other specialty machinery and equipment, taking advantage of modern machining capabilities, ensuring first class engine restorations.

Virtually all processes are performed and controlled ‘in-house’ including cylinder head crack/damage welding, machining and installation of new valve seats and guides, and all other machining and grinding operations necessary to properly restore a Merlin aircraft engine with fits and clearances to ‘factory new’ specifications. 

Our inventory contains hundreds of new in the box main bearings and fork rod/blade rod bearings, including over/under sizes to accommodate oversize bore on the crank case journals and undersize crankshaft journals ground below standard.

We include a full set of NEW (not used, re-plated) main bearings and connecting rod bearings on every engine we overhaul.   

Our investment in the extensive inventory of parts, machines and tooling is substantial -  with the result that we will be able to re-build an engine in virtually any condition.

Most importantly, if your engine has parts that are out of spec/below minimums, or marginal in any way, we will have either a new in the box part, low time original, or in some cases, a new reproduction item to replace it.  

Our chief mechanic has been overhauling Merlin engines since 1994. He has built many Merlins of various types at Universal Airmotive, and is continuing high quality engine restorations now at our company in San Martin.

With our 'one of a kind' inventory, extensive machine shop, specialty tools, tooling and many years of experience, we are providing the highest quality restored engine possible for your prized Merlin powered warbird. 

If you are in need of a Merlin engine overhaul/restoration, we encourage a visit to our facility, where we can explain and demonstrate various processes, and provide examples of ‘before’ and ‘after’ overhaul for each significant part/sub-assembly of the engine.

The real quality of an engine overhaul/restoration is internal, and we welcome visits from our customers at each phase of the rebuild process to personally experience the workmanship in each component of your engine.   

51 Factory
13545 Sycamore Ave
San Martin, Ca 95046
tel 408-779-4163, 408-683-2531
fax 408-683-2533



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