Franklin 165 Heavy Case
vs. Light Case - Stinson Aircraft
When shopping for Stinson
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July, 1951, most cases have been replaced by now.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!"