I have a 70's Takamine 12 string in for a reset. The neck was very difficult to get off txs to the unforgiving epoxy used and the fact once off I discovered it didn't have any tenon or dovetail joint but just glued onto the face of the body and attached with wood dowels (I had been pushing the neck up as you do with a normal neck joint not pulling it out)
Anyway I've cleaned up the repair area but found without a dovetail to pull the neck into the body I'm not confident the neck will stay at the right angle while I clamp and glue it. So I've decided to convert it to a bolt on which will ensure I can check the neck angle once it's glued.
I've never converted to bolt on before so wanted to get some advice on which bolts to use, it seems there's 2 options: 1) Dual thread lag screw that will screw into the neck heel with a machine thread end where the nut goes on inside the body. or: 2) Put a screw insert in the neck heel to screw the bolt into. Researching it a bit I found some guitar builders had a problem with the screw insert pulling out of the neck heel.
I'm also in a quandary about what size bolts to use 6mm or 8mm. Does anyone know a source for bolts specific for guitar building? (I'm in Australia but don't mind sourcing from overseas)
Having never come across this kind of construction before would be interested to hear any comments from those of you who have. Txs for your help with this!
It is definitely a good idea to convert from the dowels to a bolt on. I did one exactly like your job a while ago, and I do bolt-on necks for new builds. Two 6mm bolts is very adequate. Since you already have holes for the dowels the easiest thing is to just put threaded inserts into those. But, as you have read elsewhere, they often fail because the end grain of the heel is often too soft to hold them well and they can strip out. It can work, but but some inserts are more failure prone than others. If you look at the two types in my photo, the top one which has lugs or spikes in a spiral pattern is completely hopeless, while the stainless steel one with a proper thread is much better. It is also a good idea to use CA glue or epoxy as extra insurance, but obviously you need to keep it off the internal threads. You would need to assess the quality of the wood and the size of the existing holes to see if this is a good approach for your job.
A much better system is to drill a hole down the middle of the heel and insert barrel nuts internally, like the ones seen in the photo. More work because you need to remove the heel cap and then drill your hole, insert the two nuts with a bit of dowel between them as a spacer, get them aligned to the existing holes, and reattach the heel cap. But a better result in the end. Gerard Gilet and Trevor Gore, in their build book, go so far as to recommend a square bar in a square mortice the length of the heel - they say a round barrel nut can burrow its way through soft end grain. But I am not tooled up with a square morticing jig so I make do with a simple round hole.
As to the bolt, I use the common M6 furniture connectors shown. You can buy them in Bunnings. They have a nice big flat head to sit against the neck block and they don’t look too ugly so I leave them shamelessly exposed. But any M6 bolt, cut to length, will do the job and you can countersink and cover it if you want the hardware to be invisible inside the guitar.
Txs for the detailed reply and all the info, that really helps knowing the best size of bolt, I was considering the 8mm but it did seem a bit bulky so will go with the 6mm as you suggest.
I'd also discovered Bunnings do a variety of lag screws and hanger bolts, I'd like to physically check them out, but might have to go with buying online if they don't lift some travel restrictions soon. (I'm about an hour away from the nearest Bunnings in rural NSW)
I had also thought about the barrel nut but dismissed it having not thought of putting them in vertically, great idea, don't know why I didn't think of that! The heel cap came off when I removed the neck so no problem there!
Just a question with that though, do you think you can get enough torque with that kind of nut and small Alan key or would it be better to go with a standard bolt that I can tighten with a socket or ring spanner.
They're cheap enough so I'll probably get an assortment of different lag screw options so I can make the final decision which to use with the bolts and guitar in front of me
Txs again for the info
I find I can easily get sufficient torque with an Allen key - and you can't effectively use a socket wrench or spanner inside the guitar, through the soundhole.
Drilling a vertical hole for the barrel nut can be a bit hairy in a slender heel. The nuts that I use have an external diameter of 10mm so that feels like a bloody big hole to be drilling. Fortunately that Taka you are working on has a pretty beefy heel so it could probably manage it. You would drill from where the heelcap was directly up towards the fingerboard,going past the level of the upper bolt. I then stick a piece of dowel (temporarily) up the full length of that hole. While it is in there stick a pencil or nail down the bolt holes to mark the dowel at the appropriate points where the bolts will come through. Then remove the dowel and make your measurements so that you can put dowel - nut - more dowel - second nut - dowel in sequence to refill the hole and end up with the nuts lined up with the bolt holes. Having the bolt holes a bit over size gives you some wriggle room. Test fit everything, and once it is all good you can trim the dowel flush and cover it up with the heelcap again.
Cool, yes I'll probably go with the Allen key bolt then. There is a good amount of wood in the heel except the top where the cap goes is not so big. I did a mock up on the heel end making a 10mm circle and it's doable but might be a bit hairy when it comes to actually drill the hole as there's not an awful lot of room.
But like I said once I can get down to Bunnings I'll see what they have and pick up a few options so I can choose the best one once back in the workshop.
Txs again for the tips, all the best and have a great week!