Diamonds and pyramids

A bit about my approach to crack repair!


A crack in a bass top is either a result of impact trauma, or due to shrinkage in dry climate. In either case, if the crack is re-glued promptly, the glued joint should be as strong as the original wood. However, in practice there is often a delay in re-gluing a crack, and this delay causes the wood to move further so that a perfect alignment is difficult. Or, dirt gets into the crack over time, compromising the glue surface. 

Adding cleats to the crack supports the repair and increases the glue surface by two-to three times.

I use diamond shaped cleats for most top and back crack repairs. I make them from spruce in various sizes, and I make them twice as long as wide. The wood grain runs parallel to one of the edges, so that the grain will sit diagonally across the crack when installed. 

I glue them in place with hot hide glue, about 4-6mm thick, then trim to a neat pyramid shape feathering out the edges to nothing. The finished diamonds are strong along the long axis and in the centre of the diamond. At the edges, the cleat is progressively more flexible.

Some luthiers use square cleats, or straps of thin wood running across the grain, covering several cracks at once. I think both these approaches are a bad idea for double bass repairs, because they actually CREATE cracks, either by constraining the wood’s natural movement, or by stiffening and creating stress lines that are more susceptible to damage than an unrestrained top.

By repairing each crack individually, using carefully placed diamonds, I am able to support weak areas while maintaining the plate’s ability to shrink, expand and flex. When placed close together to support heavily cracked area, the diamonds interlock to provide strength but allowing the plate to remain flexible. 

Square diamonds

For rib repairs, where the wood is thin and curved, I find that the longer diamonds I use for tops and backs are harder to fit and don’t provide enough lateral support.

So, I use “square diamonds” for rib repair, where the bent hardwood - sometimes less than 2mm - needs support both along the crack, and transversally to avoid buckling. As with top and back repair, the repair needs to be light, strong, removable and also needs to allow the structure to flex and shrink and expand with changes in climate. 

The square-diamond cleats are made of spruce and grain oriented at 45 degrees. Each one is shaped to fit - or to correct - the curve of the rib underneath. The strength lies in the finished pyramid shape, both along the rib grain, and across, but the edges of the cleats are feathered away to .5 mm to allow them to flex as well. The layout is always an interesting process, wherein I try to tie in the support needed along the crack or split, with any local stiffening across the rib, all without creating new stress-lines along or across the rib.

The pyramids usually end up 2-4mm tall at the apex. They weigh about a gram each. As to whether they affect the sound, well, they certainly improve the sound of a buzzing crack!

“Eyebrow cleats” are often installed above or below F holes to support cracks in those areas. I find a suitably placed large square diamond is a very good way of doing this. The diagonal grain provides support both along and across the weak area. The cleat starts out thick and is fitted to the curved area, and shaped afterwards so that the thin edges do not cause stress points.


In areas which in my opinion are not broken but vulnerable - for example overly thin - I sometimes apply strips or diamonds of high quality belgian artist linen, always cut on the bias and glued with hot hide glue, and often overlapping the linings at the edge of the ribs.

Linen is a useful material to use across thin ribs, as it allows for wood movement while providing significant support to the thin wood underneath. Linen is also useful to provide lightweight, flexible support to vulnerable areas.

Bolt on neck?

I’m wondering why some people hesitate at getting their bass converted to bolt-on neck? Particularly if the neck is for some reason or other, already out...


Bass is more easily and safely transportable in two separately packed parts …           

Neck to body joint is more secure than glued in neck … it cannot “pop out”           

In some versions, the neck angle can be changed slightly …           

Ability to switch necks for other instrument combinations eg: five string neck etc.



“Bolt fixture may be slightly visible.”   Well, it is SLIGHTLY visible. But most people won’t even notice. The bolt cap is smooth and varnished to match the ret of the heel.

“It might affect the sound of the bass.”  Actually, a bolt on neck sounds as good, if not better than a glued-in neck

“It might affect the value of the bass.” Well actually yes. And it might INCREASE its value! But the way I do it, if for some strange reason you don’t like the bolted-on neckl, you can just glue it in position the conventional way.

.            ..


But will it affect the sound?

Well, this is a question I hear often. And I suppose it’s reasonable, given a player knows and usually likes the sound of their instrument. And I’m about to do something - a repair, as setup adjustment, change something - that will change the way the instrument vibrates.

“Will it affect the sound?” Well, yes. Just about every change you make on an instrument will affect the sound in some way. Sometimes significantly, sometimes hardly at all, but everything has the potential to affect the sound. I can understand this is a scary prospect for as player.

However, I like to look at it this way; when a bass is in for repair or adjustment, this is usually to correct something already broken, faulty, worn, or a potential problem. Just this factor alone will also affect the sound, usually in a negative way. 

The work I will do, will result in an instrument that is intact, functioning, feels good and vibrates properly.  Yes, of course the work I do will affect the sound, but the effect of all the changes minor and major, will almost invariably be a positive result!

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