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Franklin 165 Heavy Case vs. Light Case - Stinson Aircraft

Added 7/16/10



When shopping for Stinson airplanes with Franklin 165 HP engines, one will often see reference to a "Heavy Case." What is this all about?  And how can one tell if they have a heavy case?  The details are spelled out in Airworthiness Directive (AD) 51-15-02.  Essentialy, the AD called for either replacing the original crankcase with a reinforced "heavy" case, or called for repeated inspections of the webbing near the main journal area for cracks.  Since the AD required compliance by July, 1951, most cases have been replaced by now.  And those that have not are probably not likely to crack at this point.  Many "light case" 165's are doing just fine, though the top cover must be removed for regular inspections. The numbers on the crankcase castings will tell you what you have.  Below is the text from the AD

51-15-02 FRANKLIN: Applies to All Franklin 6A4-165-B3 Engines Serially Numbered 33046 and Below Incorporating Original Crankcase (Left Half No. 18305; Right Half No. 18306). These Two Parts Form Crankcase Assembly, P/N 18553. The Number of Each Crankcase Half is Located on each Casting Below the Number 1 and 6 Cylinder Location.

To be accomplished by July 15, 1951. Effective on and after this date, all applicable crankcases with 500 hours of operation since new or 250 hours since last overhaul should be inspected as follows: Remove crankcase cover and visually inspect the webbing near the main journal area for cracks. (1) Crankcases found to be free of cracks should be inspected at 250-hour intervals thereafter. In the event that the conditions described in (2) and (3) are detected, the provisions of (2) and (3) will apply. (2) Crankcases found with (a) surface indications, hairline cracks, or small wall cracks and (b) cracks starting at main bearing stud hole on the opposite side from main bearing support, may be operated further at the option of the owner. Such crankcases should be inspected at 50-hour intervals thereafter to determine progress of cracks. (3) Crankcases found fractured or with cracks that have progressed to the extent that they enter the main bearing supports (usually from back  near (a) main bearing stud hole and (b) drilled oil hole) indicate that a complete break soon will occur. Such crankcases should be replaced with the reinforced crankcase assembly, P/N 18925, at which time no further inspection is required.  Crankcase assembly P/N 18925 may be identified by casting No. 18905 appearing below No. 1 cylinder location and casting No. 18906 appearing below No. 6 cylinder location.

(Franklin Service News No. 10 also covers this subject.)

Note that this AD only applies to the 6A4-165-B3 engine (165 HP), not the earlier 6A4-150-B3 (150 HP).  The bottom line:  If you see casting number 18905 below the #1 cylinder, and 18906 below the #6 cylinder, you have the heavy case.  Or as one Stinson owner has noted,  "If it's got nines, you're fine!"


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