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A customer is asking about solutions for a pickup jack in his Harmony Sovereign. I seldom work with pickups.
Harmony Sovereign tail blocks are 1 1/8" thick. Martin tail blocks are typically around 3/4"
Will most pickup jacks work with a tail block that is 1 1/8" thick?
Or, do you need to counter bore the hole inside, so that the threads will reach.
Does anyone make an extender that will fit most of pickups that won't work with a thick tail block?

I just counterbore the hole inside.
I've made a counterbore by filing/grinding a spade bit in the past, you don't need to buy one if it's just a one-off.
Drill a hole to suit the spade bit shaft first, counterbore to suit, then ream out the hole for the jack with a stepped reamer.
Metric usually suits at 12mm but check yours first.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Here’s a thread with some details about the spade bit counterbore that Colin described.

Countersink From Inside Closed Body?
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We use a counter boar that we made. Guilds from the past can have very thick tail blocks too.

I've done the same thing as Colin, but I made my counter bore from a Forstner bit. It makes a smoother hole. You have to run it backwards, of course...

Will any pickup jacks work with a tail block that is 1 1/8" thick?
What is the max thickness that most pickups will work in?

I have a home made reverse counter bore, that I made from a spade bit, but the customer is in Canada. I am in AL

Does anyone make an extender that will fit most of pickups that won't work with a thick tail block?

I think you will have to counterbore we did and do. I'm unaware of any extenders and even if they exist we didn't go that route which tells me it's not necessary or advisable.

Our's is made from a Forstner bit like Alan's also run backwards.

There is a bit of an investment in the tooling to do one of these or Guilds if you only ever will do one of them but it gets the job done nicely. We use our counterbore several times a year so the costs and time are sunk for us.

Here’s mine made from a spade bit in 3 minutes with the belt sander.

Fits a 9/32 through hole and also runs backwards.

I’ve always used a reverse spotfacer. A centering bushing if needed can help keep chatter down and get a cleaner recess.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I used one of the long threaded jacks stewmac sold.
I ended up having to buy the weird sized tap to create the threads. Drill, tap, and screw the jack in.
Easy but expensive.

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Wah

Depends on which way you grind it ...

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.