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Martin OM plans/templates differences
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Author:  Colin North [ Fri Jul 10, 2020 2:32 pm ]
Post subject:  Martin OM plans/templates differences
I have ended up with 3 Martin OM "patterns", for a variety of reasons.

1) John Hall '31 OM template
2) Kenneth Michael OM template
3) StewMac OM plans

Quite interesting to see some of the differences -

SM plan is smallest body, length and width, rear shifted bracing
Kenneth Michael is longest body, "middle" sized waist and slightly larger upper bout, rear shifted bracing.
John Hall '31 OM template similar size across lower bout, but larger upper bout and waist - forward shifted X bracing (by nearly 1")

Some of the measurements differ by as much as 1/4 in upper bout", even 3/8" in the waist.

I suppose I could ask which is the "best", but I know the '31 OM was traced from a '31 top by John Hall, so that's the one I would build from, firstly anyay
But are these considered substantial differences? Or just manufacturing tolerances?
Comments/Observations?
I've read quite a bit about Dread forward shifted bracing, not so much about OM's - I presume it's for similar reasons to the shifted Dread bracing.
Author:  johnparchem [ Fri Jul 10, 2020 2:50 pm ]
Post subject:  Re: Martin OM plans/templates differences
I am not a Martin expert, but how stable was Martin's specification over the years especially with regard to the bracing.
Author:  joshnothing [ Sat Jul 11, 2020 6:32 am ]
Post subject:  Re: Martin OM plans/templates differences
Do all three fit in a standard OM case from your preferred case manufacturer? If one doesn’t I’d be inclined to disqualify it right there :P
Author:  doncaparker [ Sat Jul 11, 2020 8:14 am ]
Post subject:  Re: Martin OM plans/templates differences
joshnothing wrote:
Do all three fit in a standard OM case from your preferred case manufacturer? If one doesn’t I’d be inclined to disqualify it right there :P


Truer words were never spoken. Custom cases are very expensive. I try hard to make sure what I build fits in a decent quality, but low cost, standard hardshell case.
Author:  Colin North [ Sat Jul 11, 2020 8:35 am ]
Post subject:  Re: Martin OM plans/templates differences
Case size isn't a problem.....
Author:  klooker [ Sat Jul 11, 2020 9:27 am ]
Post subject:  Re: Martin OM plans/templates differences
I've built 4 using the Grellier plans & they fit the Martin case perfectly, FWIW.

http://www.grellier.fr/en/downloads
Author:  Clay S. [ Sat Jul 11, 2020 9:29 am ]
Post subject:  Re: Martin OM plans/templates differences
There is a note at the bottom of the page in the Longworth book that gives the dimensions of the models that there is some variation because they are hand built instruments. I've read elsewhere +/ - a quarter inch at the extremes, but generally closer to the nominal dimension.
"Forward shifted" bracing has always seemed like a misnomer to me, because that was the original placement of the bracing, and it was "rearward shifted" later to increase the strength of the construction to handle heavier gauge steel strings that became popular with some players.
Author:  Freeman [ Sat Jul 11, 2020 10:55 am ]
Post subject:  Re: Martin OM plans/templates differences
I have built two from the Antes plans which he says are "similar to a Martin(r) Orchestra Model". Both guitars fit nicely in a Martin thermoplastic case (as well as a standard TKL case), both play and sound good. Both guitars had braces located per the plans but shaped to try to get the best out of the tops. I know there are different ways to measure brace "shifting" - the plans call for the X 4 inches from the center of the sound hole and the angle measures 94.5 degrees between the legs. One guitar is Lutz over mahogany, one is cedar of coco. The latter is my personal favorite guitar.
Author:  Colin North [ Sat Jul 11, 2020 2:05 pm ]
Post subject:  Re: Martin OM plans/templates differences
Found a tracing/drawing on http://www.vintagemartin.com/ showing changes in upper bout shape around 1928/33.
Apparently Martins dimensions have varied less than Gibsons over the years, but still changed from time to time.
I still remember hen the Martin OM-28 EC came out thinking how it looked smaller than the OM shape I remembered.
Author:  bluescreek [ Sat Jul 11, 2020 6:27 pm ]
Post subject:  Re: Martin OM plans/templates differences
since I make the molds for Martin I can tell you that mine are what Martin has but this is a fact
Martin changed the shapes at least 4 times. KMG patterns were not from Martin. Grieller is also close but the one thing you can say
how close does it matter. I can say that no matter what yo use it will still sound like a guitar.
I use the pre war bracing locations on my templates. LMI ( antes plan ) isn't close to martin .
Martin in fact did not have a blueprint until the 70's and the first blue print drawn was of a neck block.

the tracing you show is not even close to a 31. The upper bout has more curvature. You are correct in that the shape is different. Also many expect a tighter tolerance of Martin that what actually was from the period . Remember the molds were hand made ,sides hand bent , and if it looked like a guitar it went out the door. I have seen as much as a 1/4 in difference in guitars from the same year. So don't get so hung up on exact specs

the bracing was placed with a jig and they were pretty tight in location. I can give you a fast history
34 to 39 forward era
In 39 the braces were moved backward
the neck block was changed to be smaller the transverse brace was change after a lot of top failures the Popsicle brace
Not sure when the bridge plates changed from the smaller one to the wider then again the life raft rosewood plate
scalloping didn't come back till the HD 28 the ends changed through time from scallop to taper
brace angle changed a few time from the 99 the eventually becoming 95
placement of the tone bars changed a few times as did the end of the scallop to the rim.

In all the years you can see the different hands on the scalloping. That is an interesting study also.
Author:  Colin North [ Sat Jul 11, 2020 6:49 pm ]
Post subject:  Re: Martin OM plans/templates differences
Good info John. Thanks for that.
Reminded me too that I have a Grellier OK plan,. I'll have a look at that as well, just to confuse the issue even more! laughing6-hehe
Author:  John Arnold [ Mon Jul 13, 2020 8:41 am ]
Post subject:  Re: Martin OM plans/templates differences
Since original OM's were not made after 1933, and only reintroduced into regular production in the 1970's, it would surprise me if any of them were ever rear braced. There are slight variations in body shape, but I have not seen anything close to the 1/4" variation cited. Most are within 1/16" of each other. Shapes did change somewhat with the introduction of CNC molds in the early-1990's, and IMHO the change was not for the better.
The OM drawing I made was from a 1932, and its shape is slightly different from the earlier ones. The early ones have a narrower upper bout, with a bit more slope of the shoulders. This is a subtle difference.

Sent from my SM-G950U1 using Tapatalk
Author:  bluescreek [ Mon Jul 13, 2020 4:40 pm ]
Post subject:  Re: Martin OM plans/templates differences
I got my tracings I took off a 1931 top that was in my shop. I documented that on the kit guitar forum. here is a link to that information .

http://www.kitguitarsforum.com/board/vi ... +OM#p47924
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