Selecting Pilots – Short and Sweet

We offer two different types of pilots: solid tapered pilots and expandable pilots. Solid pilots are recommended for a number of reasons detailed further below.

Selecting Solid Pilots

In most cases it's a good idea to have at least two solid pilots for a given nominal size. The exceptions would be 4mm, 4.5mm and 5mm. One pilot is all you will need for these sizes most of the time.

Ultimately it's the valve guide bore size we’re after, but if you don't know the bore size and don't have an accurate means of measuring it then we can use the valve stem diameter to figure out the guide size.

Using a micrometer or calipers, measure the valve stem and add .001" (0.025mm). That will tell you best size for the first pilot. The second pilot should be .001" bigger or smaller than the first pilot. When working with replacement valve guides or "like new" guides, choose the .001" smaller pilot. When working with existing guides with some wear, choose the .001" bigger pilot.

In most cases, following the steps described above should land you at a “standard” size for your first pilot, as in 11/32” standard, or 7mm, rather than 11/32”+5 or 7.08mm (examples of two unusual sizes). Odd sizes do exist but if you’re in doubt please give us a call. We know our stuff and we’re here to help.

Selecting Expandable Pilots

Choose the pilot that is the same nominal size as the valve guide. The pilots can expand about .015" to .017" (0.43mm) bigger than the stated size.

Make sure you select pilots that are compatible with the valve seat cutters you will be using. Small series pilots, .297" (7.54mm) top diameter, work with small series cutters (part number on cutter starts with "1"). Standard series pilots, 3/8" (9.52mm) top diameter, work with - you guessed it - standard series cutters (part number on cutter starts with a number other than "1").

Selecting Pilots - Longwinded and Detailed

The pilot is absolutely critical to getting a concentric valve seat. It “pilots” the cutter by indicating off the same bearing surfaces that the valve stem will be riding on, and carrying that centerline up to the cutter. If the pilot is not straight and concentric, the cutter cannot form a concentric seat. Neway pilots are centerless ground, a process that enables us to hold extremely tight tolerances.

Neway pilots are “dead” pilots. That is, they lock into the valve guide rather than a straight-shanked “live” pilot which turns in the valve guide as part of the cutting tool assembly. Live pilots are used in many seat and guide machines and rely on the machine operator to be especially attentive to the clearance between the turning pilot and the guide bore. Excessive clearance can result in a loss of seat-to-guide concentricity. Dead pilots do not carry this potential for operator error.

Neway offers three different series of pilots. Each pilot series has a distinct top diameter and works with the corresponding series of cutters. Standard series pilots have a top diameter of .375” (9.52mm), small series pilots have a top diameter of .297” (7.54mm) and heavy duty series pilots have a top diameter of .572” (14.53mm).

Within each series of pilots there are two types of pilots: solid and expandable. We recommend using solid pilots.

How Part Numbers Relate to Actual Size

A solid tapered pilot is, of course, fixed at a specific size. Our standard series solid pilots (.375” top diameter) will measure the stated size in the middle of the tapered portion and will measure about .001” (0.025mm) bigger at the widest part of the taper. With small series solid pilots this rule gets broken a fair bit, but it hasn’t put us out of business yet since the variances are very small. Both of these diameters are stated on the respective page for every solid pilot we make.

An expandable pilot is named for the nominal size it services. The collet can expand up to .017” (mm) bigger than the stated nominal size. Whatever the stated size, the collet will generally measure .003” less. In some cases, a metric size and a fractional size are nearly the same, and the same expandable pilot may be used for either size. The collet will be marked with both sizes and will measure .003” less than the smaller of the two sizes. For example:

  • 11mm = .433”
  • 7/16” = .437”
  • Mark on pilot: 11mm & 7/16
  • Collet size: .430”

Solid Pilot Details

Solid pilots have a very slight taper ground into them, so that they lock into the valve guide. The rate of taper is just .0007” per inch (7 microns/cm). This very slight taper means that the pilot will be hanging onto enough of the length of the guide bore to accurately capture the centerline. The flip side of having such a slight amount of taper is that a single pilot can only accommodate a small amount of variance between guide bore sizes, so you will usually require more than one solid pilot for any given nominal size.

If you are tooling up to cover a range of engines, and those engines happen to be older, American-made engines, you may well require a set of three or more solid pilots for each nominal size that you expect to service. This is because of the novel approach to valve guide sizing that American engine designers have taken over the decades. As an example, let’s consider the nominal size of 11/32”. As your pocket calculator will confirm, 11/32 in decimal terms is, was, and ever shall be, .3437. However, in the engine rebuilding trade when a guide is referred to as being an “11/32” guide this does not necessarily mean the valve guide ID will actually be .3437”. It may be .3437”, or it may just be in the neighborhood of .3437”. A given manufacturer may design one 11/32 guide with a stem diameter of .338” and a guide bore of .340” while for another model the 11/32 guide has a stem diameter of .344” and a guide bore size of .346”.

The same approach has been taken with the other fractional sizes used by American engine manufacturers, sizes like 3/8, 5/16, ¼, 7/16, 29/64, etc. This is why, when you look at the pilots in our Small Block Chevy/Ford Kit, the Harley-Davidson kit, or the fractional pilot kits, you will see so many under/over sizes for a given nominal size. They are there because you may well need them. If you don’t need them you can simply drop them from the kit and pick the sizes you need.

For metric nominal sizes a set of three solid pilots is all that you would ever require in most cases (e.g. 5.98mm, 6.0mm, 6.02mm, or .235”, .236” and .237”).

As stated above solid pilots will measure the stated size in the middle of the taper, so a 5/16” standard pilot will mic .3125” in the middle of the tapered length while a 5/16”+1 would mic .3135” at that same location. All under-sizes and over-sizes – whether fractional or metric - are ground in .001” (.025mm) increments because it happens to work quite well; if a pilot is slightly too big for the valve guide, the next smaller size - by .001” - will fit just right.

When it comes to part numbering, we do not mix systems of measure by calling a pilot 8mm minus .001”. We call it 7.98mm. Two thou undersized from 8mm would be marked 7.95mm. The following may be helpful:

Keeping in mind that .001” = .025mm ...

Stated Size

  • 3/8” STD = .375”
  • 3/8”+1 = .376” (.001” oversize)
  • 3/8”+2 = .377” (.002” oversize)
  • 7.98mm = .314” (.001” undersize)
  • 8.0mm = .315”
  • 8.02mm = .316” (.001” oversize)
  • 8.05mm = .317” (.002” oversize)
  • 8.08mm = .318” (.003” oversize)
  • 8.10mm = .319” (.004” oversize)

Expandable Pilot Details

The advantage here is that you can have one expandable pilot rather than multiple solid pilots for a given nominal size. Expandable pilots can cover a range of about .020” (0.5mm) in bore variance, starting from .003” smaller than the stated size to about .017” over.

We recommend solid pilots over expandable, but it hasn’t escaped our attention that we still sell thousands of expandable pilots every year. There are plenty of situations where expandable pilots make sense, and your situation might be one of them.

An expandable pilot is made up of four components; the stem, the collet, the expander, and the nut. To locate replacement parts go to Expandable Pilot Parts. Those pages include dimensions for the components that will make it easy for you to figure out what you need.

Solid Pilots vs. Expandable Pilots

Here are the arguments for solid pilots over expandable.

  1. Economics. For certain Neway customers the choice will be to purchase one expandable pilot or two solid pilots for a given size. Since the cost is nearly the same (a few dollars difference), and solid pilots are easier to work with, why not spend a few bucks more?

  2. On small valve guides, 6mm or less, solid pilots are less likely to flex. A solid pilot has more steel to hold the cutter on center. It is one solid steel shaft as opposed to the four piece assembly of the expandable pilot. There are ways to prevent, or defeat pilot flex – when using either type of pilot – but this is made easier when there is more physical integrity to the pilot. Of course the potential for flex is relative to the size. A 3/8” expandable pilot is not going to flex because the stem diameter, under the collet, is plenty big enough. At 5.5mm or 6mm however, an expandable will be more vulnerable to flexing than a solid pilot of the same size.

  3. Solid pilots are easier to work with. You simply insert the pilot to the guide, giving the pilot a slight twist as you insert it. With an expandable pilot, the operator must spend a few moments to position the collet correctly in the valve guide. If the collet is not completely within the guide when the operator expands it, the collet may be damaged. I know. I’ve done it.

Preventing/Defeating Pilot Flex

Pilot flex problems are rare, but it can happen. It’s most likely when you’re working on heads with very small valve guides. The trick is to know how to deal with it.

When a pilot flexes off center you will end up with a seat that is not as concentric to the centerline of the valve guide as it needs to be. The problem becomes apparent when you install the valve and notice that the valve is only contacting the seat on one side. Here’s a scenario to illustrate the problem and the fix.

Let’s say I have a motorcycle head with 4mm valve guides. I’ve installed a new valve guide so the centerline of the valve guide is slightly different from where it was before. The valve seats have been severely work hardened. When I place the cutter on to the valve seat the blades contact one side of the seat, but on the opposing side there is a gap between the blades and the seat. We call this a side-load situation since all the cutting action (the resistance) will initially be on one side only. In a scenario like this it’s a sure thing that, when I begin cutting the cutter will be pushed off center, flexing the pilot. The pilot is so small that there is not enough rigidity to hold the cutter on center with so much side load. The fix is simple: eliminate the side load before you do any cutting of the 45 degree seating surface.

We do this by cutting the narrowing angles first, typically 60° and 30°, until the 45 degree seating surface is very narrow, maybe half, or even less, of our target valve seat width. So if we wanted the 45 degree seating surface to be 1mm (about .040”) when it’s finished, we will cut the narrowing angles until the width of that surface is .020” or less. In an extreme case the 45 degree surface might disappear completely, leaving a ridge where the 60° and 30° angles meet. This way, when we go to cut the 45° surface there is very little chance of the cutter being pushed off center because our initial cut is taking place against a very thin surface with little or no side load.