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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Thanks, Dennis.... yes, that's exactly what would've done the trick, the only drawback being they cost a fortune!  Of course, if I were installing 20 pickups a day into overly-thick tail blocks, I'd grab one in a heartbeat, but...  alas. 

Good thinking re: the connector, Joshua. Yes, reversibility is a wonderful thing. Got me to thinking that somewhere in my box of goodies is a pack of mini-connectors and a pair just may fit the bill here.  They're plastic, small & solid. They come from Micro-Mark tools and weren't expensive at all.  Thanks for the idea of leaving a note for the next guy, too! 

I've done this many times. You regrind a simple spade bit and feed it from inside the guitar into a power drill. Then you pull the bit so it cuts the recess, as in the sketch below the photo of the before-and-after spade bits.

Very elegant solution - Nice.


I think that's what we do IIRC... but Dave made the counter bore bit so I can't remember for sure.  Anyway we do what Paul is doing and it does work great.

Not to be a nag but remember too a 15/32" hole is a better fit for the jack than 1/2" and does not encourage the thing to come loose as the oversized hole does.

jack installation

I've used the knurled nut from a three way toggle switch, the kind that has a thinner shaft that goes into the hole with threads. It's a bit like a T nut with a knurled part where the T would be. If it's close that will work.

Well, hot doggies!.... I'm swimming in "reverse counterbore spot-facers"!  To back track: the job that brought me here (installing the jack in the too-thick tailblock) is out the door.  

I used the 'Anderson' screw-in jack.  The customer's happy, I got paid. It's done & gone.

In the meantime, once Frank supplied the proper nomenclature, I did some digging-around eBay and found a guy selling some used aircraft tooling... including 2 "counterbore spotfacers".  Oh happy day. They were cheap enough and arrived this morning. They will definitely do the job, at least when (or if) the job ever surfaces again.

One is 5/8" and the other is 1".  Both employ a unique attaching/detaching feature.... after the shaft gets inserted, a simple twist locks it in place loosely. When the bit is turned in the cutting direction, the bit locks down on the shaft and cuts. when the direction is reversed, the bit simply falls-off the shaft. Pretty ingenious. 

Any number of makeshift bearings can be used to increase the diameter of the shaft to fit an existing hole, making the centering of the counterbore easy. Or the hole can be temporarily re-dowelled and drilled to fit... either way works.

And then Paul H. tells us about grinding the 1" spade bit and...  all of a sudden... it's an embarrassment of riches!   I had two 1" spade bits, so one became a dedicated counterbore when I removed the original point to facilitate handling.

Can't wait for the next job to rear it's ugly, 'cause I'm ready!  

Looks to me like you need to go out and find another guitar with a thick block so you can play with  your new toys.

Holy crap! That's the same as the end mills that pen turners use!

Woodcraft pen end mill

Edit:  I see this has already been answered

Lets not forget the EMG ultrajack, good solution to this problem.

Stephen... couldn't find much written about the EMG Ultrajack that differentiates it from other "long" jacks.  

Does it have an expandable length to accommodate extra-thick tailblocks, or something else to solve the issue?  Hmmm, do the small circles midway along the shaft have anything to do with holding it in place? Are they some sort of detent-balls that lock into a raceway?

Maybe I'm missing the obvious, but always open to new discoveries!  

I really don't like those jacks. They fail inexplicably. The other one I don't like looks like this:

It seems like they should never fail, but they do. 


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