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I'm doing a repair on a 1967 Martin D-18 that is almost identical to the "1949 OOO-28 Top Failure" in the Guitar structural section of Frets.com.  My question is about the spruce reinforcement patch inside the guitar.  What would you think the thickness of the patch should be.  In my mind, I'm thinking somewhere in the range of .060" to .070"?  Over .070" seems too thick for me, but would like some opinions before I proceed.  

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whats up dude!  exactly i go for 60-70.  diamond shape cleat like the old center seam reinforcements.  thats what i do, I'm sure other people will have a different opinion.  i age the cleats  so they arent stark white compared to the yellowed top when you look in with a mirror.

Long time no see Alex.  This is more than just a cleat.  The bridge plate had split long ago and yada, yada, yada... there is wood missing under the bridge like in the article I mentioned above.  This is a reinforcement patch on the inside of the guitar where the new bridge plate will glue to.

Here is a link to a photo album of a 45' D-18 I did a while back. I can't remember exactly how thick the reinforcement was but must have been around .050"-.060" feathered to nothing at the back end. I heard from the owner and it is still flat as a pancake behind the bridge.

https://plus.google.com/u/0/photos/114790507557400134246/albums/597...

I wondered if you had questioned the wisdom of laminating a spruce patch under the top?  The '67 D-18 will have the large rosewood bridge plate used in that period so you would have to add a lot of spruce to cover the whole area and that would add a fair amount of mass to the top as well as glue, stiffness etc.  A 67 martin has enough mass under there already without adding more.  As an alternative you could replace the existing bridge plate with one where the grain runs parallel to the top, and add 3/16 of length to the front (or back if the top is broken there) which would no doubt result in less tone liability than a big spruce patch would.  For those who gaze at bridge plates it probably wouldn't appear to be quite as big a hit to originality either.  With a 67 the collectors value isn't as much of a factor as a 1940's model so a slightly oversized bridge (from to back) would help with the structural integrity of the top too.

The bridge plate was split right down the middle.  It had a 1 5/8" maple bridge plate (it was my understanding that '67 was the last year they used maple for the D-18' but I could be mistaken). When the bridge plate cracked, the bridge lifted up and took some of the spruce with it.  There is missing wood beneath where the bridge sits, not just a crack.  As I mentioned before, this repair is identical to the article I referred to above.  

my bad, i thought you were just asking about cleats.

That spruce patch in the repair above was .060" and 4 grams of spruce before tapering so maybe 3.5 after. It is by far the most stiffness bang for the weight buck. It would allow you to easily use a vintage size maple plate and end up with more stiffness and less mass than before the repair. It's not easy, but is a way more efficient fix in terms of sound than resorting to an oversized bridge or plate I think.

I feel the same.  I think .060 sounds like a winner.  Nice pictures, by the way.  I'm going to go with the original plate size as well.  Thanks.

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