So a customer brought me his 1970 D28 and his fresh in-the-box K&K pickup for an installation.  It should be relatively easy, but noooooo....

The problem is that the tail block is too deep for a standard-issue endpin jack.   Hmmm.

I called K&K for any options they might have and was told "we run-across this every now & again with older Martins, and the only fix is to counterbore the tail block an amount that allows the jack to fit, lengthwise". 

Easier said than done, my friends.

I've attempted to squeeze my hand, along with a short 3/4" forstner bit and a flexible shaft inside the box, and the results were predictable... we all won't fit!  

Even if everything fit (which it doesn't) there's the problem if "hitting the hole square" to make the counterbore centered on the existing 1/2" hole.  Yikes. 

Years ago... somewhere on, I saw that Frank had a "reverse bore" drill he made from scratch. Of course (a). I can't find it now and (b). it would probably take some skills and equipment that I'm not in possession-of.

It occurred to me that maybe someone manufactures something like that...  ideally it'd be a "kit" with interchangeable bits and perhaps a couple of different-diameter shafts.  Any thoughts from the crew?

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Since I made that tool, I've learned that machinists use what's known as a "reverse counterbore" or r"everse spotfacer."  Not exactly cheap, but available. 

Or, skip the problem and use the Anderson end pin jack, that threads directly in to the hole.  You drill out, tap the hole for 9/16" standard coarse threads, and screw the jack in from the outside.  Here's one on Amazon:

Maybe something like this? Remove the bit and replace it with a straight shaft that goes through the hole from the inside and attaches to the drill.

Great minds think alike, Norm.  I'd considered something like that, but wouldn't know how to knock-out the slug cleanly, particularly since the needed depth is only about 1/2".  

Frank has, yet again, nailed-it.  Other than reverse counterboring (and you're right about the cost of those danged tools) the screw-in jack seems to be the remaining viable option. 

I had it in my noggin that -somehow- they were just a jack and didn't have the necessary strap button that the customer dearly needs... but I see that they certainly do have it.  

Case closed. That shall be the solution and I thank you for it!  That being said, my eyes will remain open for "bargain reverse counterbore" options, as that thorny little issue does raise it's head from time-to-time.

By the way, Frank, where on was your old homemade counterbore? Drives me nuts that I couldn't find it...  but that's a real short trip :)

Perhaps a bit crude...but... I have done this several times-with the right dremel router bit, you can reach inside from the heel of the guitar and working slowly, hand rout out that recess you are looking for. I've been suprised just how round and flat it can turn out if done carefully. The bit I was using seemed about right as the shaft acted like a depth gauge as I worked around the hole. The key was using a wrench to tighten the router bit as it was almost maxed out in the dremel chuck to get it deep enough into the guitar.


I'm not actually suggesting it, particularly since Frank pointed out that a screw in jack would probably be a more elegant and simple answer, but you might be able to sharpen the back edges/shoulders of a paddle bit then pull the shaft through the hole and run it in your drill. I'm not crazy about paddle bits and think you would need to be pretty easy going to not tear up the block. It's possible that jamming it could break the block, now that I think about it so maybe it's not such a good idea after all but it's the only cheap, fast way I can think of to actually widen the hole on the inside. 

Definitely filing all of these ideas away for future use, but will be going with the Anderson screw-in endpin jack this time around. 

The Dremel idea has lots of possibility, Rory.  One of those "why didn't I think of that" ideas, but that's a great example of what makes this entire forum so very valuable.  Thanks to all. 

Correct me if I am wrong, but wouldn't you need a stereo jack with three prongs rather than the standard two to shut off the 9V battery when it is unplugged. The Anderson jack shown in the link looks like a two prong jack. Just wondering?

The jack I'm looking-at (StewMac #4654) has 3 points and is suitable for stereo or mono, however the pickup in question for this particular customer is the K&K "Pure Mini" which is mono and passive, needing no voltage. 

By the way, Mike, fish the leads out the endpin hole, solder to the jack and screw the jack in before you glue the spots to the bridge plate. Avoids ending up with a snarled mess of twisted wire inside the guitar.

Thanks, Greg... sounds like the voice of experience there!  Good tip. 

Adding onto Greg's warnings:

1) Test everything before screwing the end pin in. It might seem obvious, but...

I have placed a male and female RCA plug combination inside the guitar when installing a magnetic pick-up. I leave enough slack at the end pin end to allow the connection to be made, but not enough to allow the female RCA to knock against the guitar if the pick -up is taken out for repair, replacement or aesthetic reasons (not that anything Mike installs will ever need repair). The RCA plug will fit easily through the resized end pin hole. This also makes repair or replacement of the end pin jack easier.

Sunrise pick ups has a set up like this because they recommend removing the pick up when flying with the guitar. Apparently the pick up is pretty heavy.

Probably a good idea to leave a picture of the set up in the case for the next guy.


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