Pleased to present for your consideration. . . . . . . . .

Also, I'm a hobbyist, not a professional repair person.   

I just acquired a 1977-80 Yamaha FG-375S, Taiwan,  with a broken truss rod.  The allen key nut looks like its been over tightened, and has the tip of the rod maxed out of threads.  Probably bottomed out in the nut.

Has any one ever replaced a rod in this era Yamaha?  Any clue if it's a single rod only, or a rod in a inverted U channel?

I'm not going to have time to pull the FB until after Christmas to see what's under there, too much family stuff to do.  Hoping someone with previous experience might be able to provide some hands on insight.

A previous owner was going to try to repair it, roughly removed the FB binding and balked at that point.  So it hung in his basement around all his woodworking project gathering dust and overspray from finishing processes.

Now it's found its way to me.  Nice solid top body with only one clean crack in it.  Worth it to me to invest my time to try a repair, so why not try myself.

Thanks in advance.


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Stewmac have this thread detective

Looks much like theHardware store thread sizer I was talking about, except the thread checkers on that one are mounted on a flat board.  You can't take it home but you can use it free at the store.  $50 is OK if you need this a lot but for an occasional need it's a lot of cash.


You can get one here for $29.99, no shipping or tax.

Everyone, thank you for your insights.

Part 1

I'm in neutral with my guitar.  I need to read up on installing a 2 way truss rod assembly properly.  Determine if there is one available to fit the guitar, build a functional stable jig for doing the routing process correctly by holding everything securely.  The jig I built years ago was not stable enough, and all I kept was the rail and sled assembly.  So I have some jig education in my near future.

Any pictures of what anyone uses would be very beneficial. 

Part 2 

Dave, re-reading your second response I see that you state the rod is broken on your guitar.  If I follow your description correctly, the rod must have snapped off at the back end of the barrel nut if you want to cut more threads onto the remaining rod.  Do you have the barrel nut?  If so, can they remove the broken off piece from the nut, and get thread specifications from it or the piece of rod?

Out for any insight from anyone, this is what I have done.

I've run into an unexpected result when going to the hardware store, they only carried course threads, industry standards say that would be a .8mm pitch for 5mm, and the rod and nut will not thread.  Fine threads should be .5mm pitch.  4mm bolts and nuts are too small.  6mm, too large.

Here is what I have measured from my rod and nut with my digital calipers.

Diameter rod :            5mm

Diameter of threads:  4.65mm

Pitch*:                       .8mm

*This I measured by marking the rod and barrel nut, measuring the depth of the rod inside the nut.  Making 10 full revolutions, re-measuring the depth of the rod to get a second reading.  Subtract first from the second to get the distance 8.1mm, that the nut traveled, divide by 10 =approx. .8mm 

I'm going by a shop tomorrow that will measure the pitch for me.   

Am I making a wrong assumption that a 5mm rod would equate to a 5mm thread?  

Here a some other measurements from the rod.

Total length, base plate and rod combined:  366mm

Length from base plate to end of rod:           363mm

Length of threads:                                           15mm

Base plate

Thickness:                                                        3mm

plate dimensions:                                             10mm x 10mm

Barrel nut

Length:                                                                 20mm

Diameter:                                                             9mm

Allen key:                                                             5mm

Dave maybe you would want the option of just removing the neck, leaving the fingerboard attached, this would give access to replacing or re-threading the rod.  Give it a neck reset at the same time and be ready for the next 30 years. Just my thoughts as mine becomes more complicated.

More practical, experienced minds needed, again......

I need some practical experience from someone who has done this conversion from a single rod to a 2 way truss rod.  Follow me through this preliminary scenario and let me know if you see any pitfalls on the horizon.  I'm breaking this into 3 phases.

Brian and Russell, thanks for your repair advice earlier in this post.  I'm proceeding with that as my foundation.

Phase1 :  Calculate what 2 way rod will fit the repaired neck. This I need before I build my routing jig or position my dowel holes.

Here's where I have no experience at all in correctly determining what length rod will fit and function properly. The old single rod came completely through to the inside of the dovetail on the heel.

I see 14 1/4 inch rods.  I see rods that are threaded at both end, some that are welded at the non-adjusting end.  Advantages, disadvantages each?

First, Do I need to leave a specific distance behind the end of the rod to the heel? 1/4 -1/2 inch? 

Second, Does the threaded rod assembly, behind the nut adjuster, have restrictions on how far forward it can come towards the nut?  Does it need to stay back say 1/4-1 inch to get a solid gluing surface between the fingerboard and the neck?

A truss rod channel of 3/8 inch deep will cut down into the repaired break in the neck/heel.  Is that a concern to remove a small foot print of the glued repaired surface?

Phase 2:  Repair the neck/heel break


I have removed the rod. 

I'm going to dry align the broken neck to the still mounted heel, and then drill from the top of the neck 2 alignment holes for dowels, through the neck into the heel.

Steam out the remaing portion of the heel.

Clean up, align and glue up the neck and heel with the dowels.

Phase 3:  With all the dimensions determined by Phase 1, build the neck jig for routing out the old hardwood cap and making the new channel.

I plan to make a jig that the whole neck will mount solidly below.  Hopefully not having to remove the neck until all routing is complete, removal, back filling, creating the new channel.  The old hardwood rod cap strip is less than 1/4 wide so if the new rod requires a 1/4 slot, I can remove and rechannel all with the same bit.

First, Route, with multiple passes, down to flatten out the old rod channel.  

Second, back fill the channel with a mahogany filler strip, glue in.

Third, Route for new rod depth and width, make clearance for adjuster nut.

Reattach the fingerboard, test the rod.

Perform a neck reset if needed.

Thanks again everyone.



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