Grain Mill Gap Settings

The numbers

Gap by process

These are starting points, not prescriptions. Your mill’s roller diameter, your malt’s moisture content and your water-to-grain ratio all shift the optimum a thousandth or two. Use the table to get close, then adjust by measured efficiency.

The principle

Why the sparge method decides the gap

It comes down to one question: does your process rely on the grain husks as a filter bed?

Fly sparging does. You trickle sparge water onto the top of an undisturbed bed while draining from the bottom, and the whole thing only works if the bed stays permeable throughout. Shredded husk compacts, channels and sticks. So fly spargers crush wider, accepting slightly lower conversion for a bed that drains.

Brew-in-a-bag does not. The bag is the filter, the bed gets lifted and squeezed, and husk integrity is irrelevant. So BIAB brewers crush as tight as their mill will manage, maximizing the surface area the enzymes can reach -- which is why BIAB efficiency can match sparged systems despite the simpler process.

Batch sparging sits between. You stir the bed when you add sparge water, so it is not relying on an undisturbed filter bed, but you are still draining through grain rather than lifting it out. A moderate gap suits it.

Terminology, if any of that was unfamiliar: mash tun vs lauter tun.

Setting it

Feeler gauge, both ends

1. Get an automotive feeler gauge set -- leaves in thousandths of an inch, for very little money.
2. Slide the leaf you want between the rollers and adjust until it is a firm but sliding fit.
3. Repeat at the other end. A mill adjustable at only one end pivots, producing a wedge-shaped gap where one side over-crushes and the other under-crushes.
4. Note the setting. Mill markings drift and you want to be able to return to a known number.

Adjusting

Change one thing, then measure efficiency

Gap setting is an optimization, and optimization requires measurement. The method:

1. Record your mash efficiency on your current gap -- your pre-boil gravity and volume against your grain bill. Most brewing calculators will do the arithmetic.
2. Change the gap by 0.002 to 0.003 inches, no more. Bigger jumps give you two variables at once.
3. Brew the same recipe and measure again. Same grain bill, same water volumes, same process.
4. Keep the direction that helped. If efficiency rose and the runnings still flowed, go tighter again. If the sparge stuck or slowed noticeably, you have gone too far.

A few points of mash efficiency is a realistic gain and it is worth more than any difference between brewing systems. It is also free once you own the mill.

Troubleshooting

What the symptoms mean

Note the second and third rows. A stuck sparge or a floury crush is more often a speed problem than a gap problem -- a drill at full speed shatters grain regardless of how carefully you set the rollers. Slow the mill before you widen the gap, or you will give up efficiency you did not need to.

Technique in full: how to mill grain.