Stewart Mossman and company built some great Guitars in the 70’s and 80’s. But not 100 percent great ones.
I have owned three of these guitars and currently play a Tennessee. By no means does this make me an expert on Mossman guitars. But it does provide a reasonable perspective on a problem apparently shared by all Mossman acoustic guitars. This opinion article describes the problem.
Generally, every guitar has to resist the tension created by six strings. This tension wants to collapse the neck and body structure by pulling the gear head towards the bridge:
The entire guitar body structure must be designed to resist these forces. The neck must be structurally connected to the upper bout sides via the neck block and to the upper bout sides via the top wood. This allows the upper bout to work with the rest of the guitar body as a torsion box. Unfortunately, the top wood is not very strong above the sound hole because of the sound hole.
Due to poor design Mossman guitars made before 1977 may crack and fail along the fret board extension. After 1976 at some point Mossman changed the neck block and upper bout design. These guitars most likely won't need the repair described below.
In my opinion, Mossman’s arched or cantilevered neck block design is defective. The Mossman design has no structure preventing the fret board extension from rotating down into the guitar body.>
Mossman’s arched neck block fails because the only thing connecting the neck block to the rest of the top is the 3/32 inch thickness of the top on either side of the neck block fingers. This design allows the neck block and finger board extension to roll towards the sound hole. The top attached to the neck block fingers shears away from the top on either side. Please see the diagram below.
As you can see from the diagram, there are four sets of braces on either side of the neck block. However, these serve no useful purpose at all. The builders could just as well left them out. These braces are not connected to the neck block fingers. Therefore, the only connection between neck block fingers and the rest of the top and sides is the thickness of the top. And in that area, the connection is with the grain. The top splits on both sides of the finger board extension allowing the block to roll towards the sound hole.
Mossman’s dilemma at design time was to install one or more contiguous cross braces just north of the sound hole (Martin style) or depend on the neck block fingers. He chose to depend on the fingers. That was a bad choice.
In short, the Martin full length cross brace structure is a much better design. In my opinion, all Mossman guitars with the arched neck block structure are doomed to failure as described above. The only way to fix the problem is the "Agony of D Feet" repair. This requires neck removal, probably a reset, cutting away the ends of the neck block fingers, and installing a cross brace between the existing short side braces. If your guitar has a shaved bridge and/or a very low saddle, this repair is definitely worthwhile. Your high action Mossman can be made playable again. Mossman guitars are definitely worth the work.
My understanding is that after the fire, some Mossman guitars had an improved upper bout structure. If your never repaired Mossman has a full width T-1 cross brace in the upper bout, you have a guitar with the improved design or a repaired guitar. If not, consider having your otherwise excellent Mossman repaired. Contact Steve Mason at firstname.lastname@example.org. or contact us. We are now performing this repair.
By the way, this repair is normally done without removing the back.
The image below shows the top crack caused by the failing neck block structure. White lines show where the neck block fingers are cut. Back is not normally removed for this procedure. The guitar shown needed a new back.
©2018 D.R. Hanna