banjoT1 - Posted - 08/13/2025:  20:59:21

Regarding radiused fretboards of the RB7, RB12, and RB 18, does anyone have the multiple radii numbers from nut to bridge?
Easily enough I could likely get close enough with radiused templating but I'm primarily looking for the theoretical or engineering specs (then double checked with templates).
Or.....directing me to a link or 'reissue' production drawing would be ducky.
I am setting up a 7 that's beckoning me to try various bridge weights and minor deviations but the Snuffy that was as received is too heavy and I don't want to thin it down - plus, I'd like to proceed knowing the specs first.

In advance I appreciate your replies - muchas gracious !

Maurice McMurry - Posted - 08/14/2025:  17:52:45

Gold tone has reproduced something similar with the Bela Fleck signature neck. They will likely be a challenge to get any specs from. I have had one in my shop and could venture an educated guess... I did not measure.

Maurice McMurry - Posted - 08/14/2025:  19:16:55

12" at the nut, 14" at fret 11, and 16" at fret 22 is my best guess.

 

banjoT1 - Posted - 08/14/2025:  19:36:56

...thanks Maurice.



I've read many archived threads about radiused fretboards and many radii numbers, guesses, and assumptions quoted - including what radiused bridge dimension or 'range' works best - but I've yet to read from any banjo builder or well known neck meister, any actual engineering specifications related to the 'reissues' mentioned in the OP. This is not a problem mind you - I'm simply surprised that no one with detailed knowledge of the Gibson banjo world has replied with a definitive answer.



Oh well, it's summer......a lot of folks are on vacation, baling hay, or busy jamming.



Using fretboard templates in this case essentially reverse engineers for SWAGS as answers - but not as trustworthy as production specs.



...case closed.

Edited by - banjoT1 on 08/14/2025 19:38:08

steve davis - Posted - 08/15/2025:  07:50:44

I've read that pre-war top-tensions have a 10" radius at the nut.

Maurice McMurry - Posted - 08/16/2025:  04:35:30

I used the same concave sanding blocks that I made for guitars on a few radiused banjo necks. My technique is not very scientific other than being a copy of guitars I like. Data for Guitars is readily available on the web. This from StewMac might help.



stewmac.com/video-and-ideas/on...PONtwcO2Z

Edited by - Maurice McMurry on 08/16/2025 04:48:10

 

banjoT1 - Posted - 08/16/2025:  07:05:10

thanks again Maurice.

I too also found several site links to conical math and calculation equations....but, being too tired tonight to do calcs I worked with a radiused fretboard template and then confirmed accuracy of the template radii by striking an arc with a beam compass > never know how a document (i.e., template) may shrink or expand when copied.

So, I found that regarding the specific radiused fretboard I'm working with that at 10th fret the rad is 8" - @ 22nd fret rad is 14", and at bridge distance - using the outside dimen of the conical extended neck binding as reference, the bridge rad is close enough to 20".

One must not assume though that the rad numbers I found can be applied to all similar models whether 'pre-war' or 'reissues' (referring to Gibson), nor for radiused fretboards of other makers - rather, the frequency of anomalies of all types and kinds don't support generalizations.

Alex Z - Posted - 08/16/2025:  07:43:54

What does the radius at the nut come out to be?

banjoT1 - Posted - 08/16/2025:  10:04:20

AlexZ.........I need to take time for a coffee - this thing has been a challenge to find optimal combination of tweaks and twerks to release its best attributes. Its birthday was 2004.02.28 and I have reason to believe its never been set-up properly during its 26 years of life or been fitted with the right bridge.

I'll answer your Q when I return back here.

steve davis - Posted - 08/16/2025:  12:09:08

I like having a full set of StewMac radius gauges.
They tell me that my one year old Gold Tone radiused neck (150/not Fleck neck) has a 14" radius from frets 1 through 22.

Alex Z - Posted - 08/16/2025:  14:05:53

quote:

---

Originally posted by banjoT1

AlexZ.........I need to take time for a coffee - this thing has been a challenge to find optimal combination of tweaks and twerks to release its best attributes. Its birthday was 2004.02.28 and I have reason to believe its never been set-up properly during its 26 years of life or been fitted with the right bridge.



I'll answer your Q when I return back here.

---

OK. Thanks. No hurry. 

 

The first thing that hit me about the 8" radius at the 10th fret was "that's Fender Telecaster territory".  : )

The second thing was, if it's linear back to the nut, we're heading to zero!

The third thing was, as Mr banjoT1 is saying, all these banjos can be different regardless of specs.  No telling what was going through the mind of the person with sandpaper or scraper in hand while whacking away shaping the fretboard.  : ) 

Edited by - Alex Z on 08/16/2025 14:08:22

banjoT1 - Posted - 08/16/2025:  17:49:28

AlexZ......you're right......the FB could be going to zero but the other possibility is that the FB is not truly compound radiused all the way to the nut - meaning that from the 10th fret (to nut) it violates the theory of a cone and might actually and nearly flatten out. This is just supposin' but a template check will tell us for sure - however the nut is radiused to what rad?...again I didn't check that yet. BUT, you did raise an interesting point. I was compelled to solve it's buzzing riddles up to early this morning so my brain was buzzed too.

But, speaking of buzzing, I received it with a very, surprising bad, Snuffy bridge installed by the man back in 2011. Sorry to the faithful, but the string slots and cap angle (rear edge of course) were apparently cut by a beginner apprentice, and weight was too heavy. So, we detailed an alternative bridge and bingo bango, that eliminated buzzing at that point.

The Gibson tailpeice also was a source of buzzing due to a surprisingly aggressive upward 'curl' of the front string paddles - me never seen a Gibson Presto, or any other Presto type, with such a degree of upward lift - a head scratcher for sure but a short strip of felt between the strings and front paddle edges solved that buzz #2.

Then, the TT-type armrest, not properly jigged up when brazed to the post, also was a buzz source (#3). Then....I examined the nut but that too was either shaped and filed by the same beginner as I mention above or clobbered up by a T-Rex. Plus, the nut was shimmed up - so, after another coffee I'll replace the nut as called for - that could be a buzz source #4.

As I said in last post, this unit (RB-7 TT) has been a challenge. But then there's more......as i've found with many banjos and from all makers, I frequently find the upper co-rod holes are drilled way too F'n close to the upper rim edge. Since I pretty much disassemble every banjo I work on I find most upper co-rod rim washers overlap onto the toe edge of the tone ring - effectively hindering tone ring in both vibrational characteristics and dimensional roundness.

Geeesh, but February of 2004 must have been a bad day at Gibson. But now, a larger point I need to make is that frequently I see BHO posters refer to 'dud' banjos however I conclude that there are many 'duds' created during machining and assembly that inherently prevent a banjo's physics from working properly. And oh yes...forgot to include ...the tone ring was fit way too tight and could only be heated off.

But, the good news is that the machining and alignment of the neck, neck heel, and rim quality and roundness were as perfect as any other banjo I've laid my paws on. Call that the "A Axis", that is a very big deal that presents a very good foundation for energy transmission - then, most other set-up problems can be systematically remedied to 'un-dudify' a disappointing banjo. (.....details, details...)

Then Steve Davis.....obviously your Gold Tone has a single rad fretboard (not compound) but then you know that already. And, whether or not the Stew-Mac radius gauges are accurate, all I can say is I'd have to personally confirm their die-struck radii by 'proofing' them against a standard or quickie drafting check. I say that because once upon a time I simply took for granted that feeler gauges were the thickness as marked. But, when 'proofing' thickness accuracy with a micrometer I was taken aback by how many thousandths of thickness (over/under) some of the feelers were.

I've taken space and time here way beyond today's allotment for now but will get back to you Alex about what the 1st fret radius is.

(...time for another coffee.)

Alex Z - Posted - 08/16/2025:  18:13:30

Speculating that something may be different in the nut-to-5th fret area, other than an extension of the cone that is used from the 5th fret up.  Wouldn't be surprised if the radius is constant in that area. 

 

Transition around the 4th to 6th fret is a real art, to get a good feeling neck and a good playing radiused fingerboard. 

banjoT1 - Posted - 08/16/2025:  18:56:39

Alex....thanks for hanging with me on this OP - you've brought up good points that I'm factoring in.

banjoT1 - Posted - 08/17/2025:  00:01:15

AlexZ....



From nut to bridge ( @ ~ 26 3/8" scale length) using confirmed template radii, here are the compound fretboard numbers for unit re: Gibson RB-7, 7-040201 [....but not assuming all models (incl 'reissues') with radiused fretboards are machined identically - accounting for human factors, specs of production drawings, and QC during production.]



4" rad - nut to 3rd fret.

4" to 8" rad - 3rd to 10th fret

8" to 14" rad - 10th to 22 fret

14" to 20" rad - 22 fret to 3rd bridge slot



With these numbers - compared to the numbers derived from the conical equation provided by Stew-Mac - a feller might likely entertain himself if not for any reason other than curiosity. The fretboard is what it is. There's no real point of the original post except to narrow down the specs of the relevant bridge to match the math. As said above, but without going into the details, the bridge installed when I acquired this unit was quite sorrowful.

BTW....the transition at the 5th fret was artfully done.



out........

Edited by - banjoT1 on 08/17/2025 00:03:52

Maurice McMurry - Posted - 08/17/2025:  04:38:26

That is certainly rounder than I expected, and rounder than I have ever seen. The smallest radius caul that came with my fret press outfit is a 6" and I have never had occasion to use it. Building a radiusing sander was on my to do list during the busy years at the factories. I still have not gotten that done. This one from Grizzly  does not look capable of going down to 4" without being modified.

Edited by - Maurice McMurry on 08/17/2025 04:56:08

 

Maurice McMurry - Posted - 08/17/2025:  05:07:34

A closer view, It is or guitars. The Bella Fleck Gold Tone neck appeared to be all done with CNC.

 

Alex Z - Posted - 08/17/2025:  07:48:00

4" radius at the nut seems off.   Double that would still be a fairly tight radius. 

 

Is there any evidence that the neck has been sanded down after it was made, such as inlays on the first and third frets being very thin at the edges?

banjoT1 - Posted - 08/17/2025:  10:05:59

Alex.....no, there's no evidence of any post-production fix-its or tampering. Not even slightest chip out clues as if it had any fret work - partial or otherwise.

Inlays very healthy. Overall finish condition is quite excellent - headstock all around, pristine - with resonator nearly so.

Although you're surprised at the 4" rad from 1 to 3........all I can say is that I was very thorough and careful to take accurate template readings. At first I eyeballed 1 to 3 and sez, hey, that's gotta be 6" but the template was way off so to satisfy my curiosity I cut a 4" rad template and found it was a near perfect fit.

As an aside, for those folks who like to chase down buzzes, dead frets, paranormal anomalies and general lack of snap crackle and banjo pop, I just didn't like the nut workmanship or fitting or quality of nut slots. Long story short....when removing the original nut, upon 10X magnification it appeared that the bone was cracked or had a natural (oblique) 'faultline' defect that easily separated into two pieces during removal. So, after making a new nut with subsequent tuning-up the sound improvement was remarkable by several measures.

Alex Z - Posted - 08/17/2025:  10:20:06

Glad you're satisfied.  As long as the outcome works for the player, that's what counts. 

steve davis - Posted - 08/19/2025:  05:24:55

The smallest radius in my StewMac gauges is 7.5 inches and I've had no need to use that yet.

In my automotive experience since 1970 I have found feeler gauges to be very reliable and accurate and my cheap "Omega" 1" micrometer to be as accurate as more expensive ones.

Measuring equipment has been suitably accurate for a very long time and I fully trust a StewMac radius gauge to be right on the money.

The radius gauge is one of the simplest pieces of measuring tools.I would trust theirs over any made by hand.

I believe Gibson's pre war 10" TT radius to be the tightest.Don't know if they used that in modern times.I use a flashlight to back-light my radius measurements.

Edited by - steve davis on 08/19/2025 05:41:20

Maurice McMurry - Posted - 08/19/2025:  05:54:17

This thread started me thinking about the radius of a Fiddle, Cello Or Bass compared to a banjo, guitar or mandolin. We has a few fiddle players ask for a tight radius on mandolins. Those were 7.5 or so, still a big departure from 1.6" standard for a fiddle. That's a big leap for fingers to get used to.  Bowing of course requires the tight radius. Fiddle seems impossible to me.



thestrad.com/lutherie/analysin...t%20hand.

Edited by - Maurice McMurry on 08/19/2025 06:06:38

banjoT1 - Posted - 08/19/2025:  16:01:10

I have a working practice of double checking and when doubt still exists I'll triple check my calcs. In this case I'm hearing a cacophony of doubt from you folks so I'll do a quadruple check about the 4" radius.

Just to clarify......we're talking here about an 8" diameter obviously - with 4" rad.

....be back later.

steve davis - Posted - 08/19/2025:  16:51:39

I'm talking about 30s Gibsons using a 10" radius.

10" diam. circle arc.

Edited by - steve davis on 08/19/2025 16:54:49

Maurice McMurry - Posted - 08/19/2025:  16:56:30

quote:

---

Originally posted by banjoT1

I have a working practice of double checking and when doubt still exists I'll triple check my calcs. In this case I'm hearing a cacophony of doubt from you folks so I'll do a quadruple check about the 4" radius.



Just to clarify......we're talking here about an 8" diameter obviously - with 4" rad.



....be back later

I sure don't doubt it.  I am here to learn.

---

Alex Z - Posted - 08/19/2025:  17:44:34

Gold Tone's "Bluegrass Heart" model designed to match Bela Fleck's banjo has a compound radius of 5" to 10".

Mr. Bela's fingerboard always looked very rounded to me.  Now we know why.

Fathand - Posted - 08/20/2025:  13:01:00

quote:

---

Originally posted by banjoT1

I have a working practice of double checking and when doubt still exists I'll triple check my calcs. In this case I'm hearing a cacophony of doubt from you folks so I'll do a quadruple check about the 4" radius.



Just to clarify......we're talking here about an 8" diameter obviously - with 4" rad.



....be back later.

---

As information, a 4 " radius is apparently common on a Portuguese Guitar, sometimes known as a Fado Guitar, so not unheard of.

banjoT1 - Posted - 08/20/2025:  17:37:04

OK then......here are two fretboard metrics at the nut:

1st photo = 4" rad
2nd photo = 6" rad
(order of photos may reversed - see rad written on template.)

....who wants to argue and split hairs?.....whether 4" (my preference), or 6", IMO really doesn't matter except as an academic discussion.

(...say you?...)

2025.AUG20 - 6'' FB RAD @ NUT - for 7-040201

2025.AUG20 - 4'' FB RAD @ NUT - for 7-040201

banjoT1 - Posted - 08/21/2025:  06:15:55

My original question was assuming that the reissue 7's, 12's, and 18's were radiused the same or that production specs for all three models were identical. But, although I've templated one specific banjo (including the template overlay photos in last post), I'm less certain after all that the radiused fretboards are consistently accurate from nut to end of fretboard, or may show some slight inconsistencies/differences when comparing the reissue fretboards generally.



Does anyone have a list of the reissues that were actually made? I ask the question(s) because I'm thinking that the radiusing operation was more likely "handcrafted" (rather than jigged up and machined) because there were so few produced as to not warrant batch runs. But then, I have no clue how many reissues were produced so therefore I basically have no evidence of anything - nonetheless, for folks with sufficient "production" experience it's often a personal curiosity, or discussion topic, to query "how did they do that?" But only guessing mind you, that the fretboards of the pre-war 7-12-18 series were shaped the same as were the reissues.



The same question(s) could be asked regarding the specs and methods used by other manufacturers as someone mentioned above about Gold Tone's "Heart" model.



Anyway, from time-to-time a thread emerges where folks report their impressions of, or "feel" and playability of whatever radiused fretboard they've had experience with. But, during my recent exercise I'm thinking that a bridge without the proper math likely doesn't "match" a specific fretboard. 

.....not sure though how big or small of a deal this is. 

Edited by - banjoT1 on 08/21/2025 06:18:04

DRL777 - Posted - 08/21/2025:  06:48:01

Most bridge should be able to match to the radius required. ...and should be able to give you a reliable way to measure said radius to determine it for the bridge.

Maurice McMurry - Posted - 08/21/2025:  07:28:14

A call to a shop that has had rb 7's etc pass through over the years might be a way to find out. Some where like Carter vintage or Gruhn Guitars. Good pictures on the web are few and far between. I don't see a whole lot of web content for the originals or the re-issues.

A copy from a BHO contributor,
banjo.com/product/gibson-rb-7-replica/

banjobarn.com/banjo/74546

fretboardjournal.com/columns/c...son-rb-7/

reverb.com/item/198604-gibson-banjo

guitars.com/inventory/ja6436-1...p-tension

Just pictures, No data. They all look fairly similar to the Bella Fleck signature neck from Gold Tone.

Alex Z - Posted - 08/21/2025:  08:01:46

"But, during my recent exercise I'm thinking that a bridge without the proper math likely doesn't "match" a specific fretboard.       .....not sure though how big or small of a deal this is."



I do play a banjo with a radiused fingerboard.  The radius of the bridge doesn't have to be exact.  In fact, I've tried and used straight bridges.



    -- And the reason is that my desired string height is a lot more exact than the precision of any radiused bridge.  So I end up cutting the notches to the desired depth anyway.  For example, I like the 12th fret action to be between .125 and less than .130 -- so maybe .127 +- .002.  The slop in the bridge notches therefore has to be +- .004.  Not going to get there just by specifying a radius for the bridge -- have to fine-tune the notches.  KInd of reverse engineer the adjustment.  



    -- With a straight bridge of a height that works with the 3rd string, you can cut the other notches to the height you want, and then for cosmetic effect put a little curve in the top of the bridge.  I don't do the cosmetic effect, because if the bridge sounds good, I'm not going to remove any wood from it.



In short, have to fine tune the bridge notches anyway to get the exact action height with a radiused fingerboard, and thus the nominal radius of the bridge doesn't matter much, except for cosmetics.



Hope this helps.

Edited by - Alex Z on 08/21/2025 08:03:28

banjoT1 - Posted - 08/21/2025:  09:39:27

....in reply sort of, to the last 3 posters (to AlexZ) here's a few thoughts - not all have conclusions.

First, there are obviously sources of radiused bridges on the market - some of which are available from individual "bridge makers" but I'm generally unaware what data points and numbers are requested that enables the maker to provide the "right match" .....I mean, this is the customer's expectation right? [...fwiw, I make my own bridges so in doing, this transaction is omitted] But, I earlier mentioned the totally inadequate bridge and fitting in 2011 when the 7 was first set-up by a well-known guru.

And AlexZ.....you make some good practical points - your experience cover a longer period than mine. In my BHO archival search there's bits and pieces about radiused fingerboards sprinkled here and there since ~2008 (as I recall), but you're correct.....not much info re for banjos - but I have found more tech articles pertaining to radiused fretboard guitars.

Also AlexZ, after setting up with a straight bridge - that seemed to work fine - mind you, the 20" rad that I needed is nearly flat anyway.

Zooming way out though, as someone posted, there's not much history in print specifically about the 7's - 12's - 18's nor specifically the reissue series. OTOH, the copies are interesting.

Finally, FWIW, I watched the Pikelny video (link above by Maurice) - I can't provide much of a comment due to recording/playback limitations but will step out on a limb by saying it ("it" - the banjo) sounded not much better that a poorly set-up standard flathead. OTOH, the 7 I'm currently setting-up has quite a different tone than the standards and is very powerful (not just referring to loudness), is very precise in note separation, and clarity - actually quite different in set-up characteristics. (I'd welcome your impressions...anyone?)

(....this has about run its course but I also welcome other POV's. My time is up.)

Maurice McMurry - Posted - 08/21/2025:  11:19:24

Research ideas, Our library will bring these in as an ILL.