Guide
Do You Dimension a Bushing Bore Before or After Press-Fit?
A bushing bore is the inside diameter of a plain bearing sleeve, the surface the shaft runs in. Pressing the bushing into a housing squeezes it, so the bore closes by some part of the interference, and the drawing must say whether the bore size applies before or after pressing.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers who design a plain bushing, press it into a housing and then see the shaft bind. It explains why the bore closes, compares the three ways to put that on a drawing, and shows with ISO 286 band widths when finishing the bore after pressing is the only sound choice. Numbers come from the linked maker documents and from ISO 286-1 widths, and are examples of practice, not the capability of any shop.
On this page
Why does a bushing bore shrink when it is pressed in?
The housing compresses the bushing wall, so part of the radial interference reappears as a smaller bore. One carbon-graphite bushing maker states that the bore shrinks nearly the full amount of the outside-diameter interference and that the bushing and housing may need 24 hours to reach final size (Graphalloy installation sheet DS 1131, 2015).
How much of the interference shows up in the bore depends on the wall thickness, the housing stiffness and the materials. Bunting, a bronze bearing maker, gives its recommended press fits for housings rigid enough to resist distortion and for walls of about one eighth of the outside diameter, and it publishes an installed bearing bore for a given housing bore and shaft size (Bunting product catalog, 2020). Its nylon bearings are described as measured after installation in a nominal housing.
The housing edge matters too. Bunting notes that a smooth chamfer on the housing bore reduces installation time and avoids press-fit loss from metal being sheared off the bushing outside diameter. A burr on the housing changes the fit and the bore.
Should the drawing dimension the bore before or after pressing?
Dimension the bore after pressing when the shaft fit is what matters, and name the housing bore that the dimension applies to. Dimension only the free-state bore when the supplier never sees the housing, and accept that the closure risk then sits with the designer.
Catalog bearings already work this way: the table lists a recommended housing bore, a shaft size and the installed bore that results, with a note that the installed bore applies in a rigid housing. A custom drawing should carry the same three facts. Table 1 compares the three practical ways to write it.
Whichever option you pick, write it in words as well as numbers. A bore dimension with no note is read by most shops as a free-state dimension, and the closure then lands on the wrong side of the shaft fit.
| Option | What the supplier makes | Who carries the closure risk | Best when |
|---|---|---|---|
| A. Installed bore on the drawing, housing bore given | A bushing sized to meet the bore band once pressed into that housing | The supplier, if the supplier can test in the housing | The supplier also makes or gauges the housing, and the shaft fit is close |
| B. Free-state bore with a predicted closure | A bushing to the free-state size on the drawing | The designer | The supplier ships bushings only, and the shaft fit is loose |
| C. Press first, then finish the bore | A bushing with stock left in the bore; final bore cut after pressing | The process, since the final cut absorbs the closure | The installed band is narrow and the housing is precise |
How much of the interference reaches the bore, and who should estimate it?
The ratio cannot be taken from a standard, because it depends on wall, housing and material, so use the maker’s installed-bore data or measure a pressed sample. As a worst case, assume the bore closes by nearly the full interference, as the graphite bushing maker above reports.
The reason to bother is stacking. The interference itself varies by the sum of the bushing outside-diameter band and the housing bore band. Table 2 shows that for an IT6 outside diameter in an H7 housing, the spread is wider than the H7 bore band you might want after pressing, per the IT widths of ISO 286-1:2010; the spread column is simple addition of the two bands.
If the bore follows the interference nearly one for one, no amount of care in turning the free-state bore will give an H7 bore after pressing. The options are to finish the bore after pressing, tighten the outside diameter and housing bands, sort bushings and housings into matched groups, or accept a looser installed band.
| Size range (mm) | Housing H7 band (µm) | Bushing OD IT6 band (µm) | Interference spread (µm) | H7 bore band wanted (µm) |
|---|---|---|---|---|
| over 10 up to 18 | 18 | 11 | 29 | 18 |
| over 18 up to 30 | 21 | 13 | 34 | 21 |
| over 30 up to 50 | 25 | 16 | 41 | 25 |
When should the bore be finished after pressing?
When the installed bore band is narrower than the interference spread, or the shaft fit is a close sliding fit, finish-ream or bore the bushing after it is pressed in. The supplier then needs the housing, or a gauge housing, at the time of machining.
The sequence is: press the bushing with an arbor and an uninterrupted stroke, let it settle, then cut the bore to size in the housing. Graphalloy warns that stopping partway through the press stroke creates high stress at the housing edge and can crack its bushing, so the press method belongs in the RFQ as well.
Finishing in place also helps when two bushings must align in a pair of housings, because both bores can be cut in one setup. For a single bushing, the quality of the result still depends on the outside and bore being coaxial before pressing; turning the bore and the outside diameter in one chucking is the usual way to keep wall thickness even, which in turn keeps the closure even around the circumference.
Ask who presses and who finishes. If the supplier only ships bushings, the assembler must ream, and the drawing should say so instead of showing a bore that no one can hold.
How do you write the drawing note and the RFQ line?
Put the condition in the note: say that the bore applies after assembly in a named housing, or that it is free-state and for reference only. Then give the RFQ the housing facts the supplier needs to predict or test the closure.
- Installed requirement:
BORE Ø__ H7 AFTER ASSEMBLY IN HOUSING BORE Ø__ H7 (SEE DWG __); MEASURE AFTER SETTLING - Free-state reference:
FREE-STATE BORE Ø__ FOR REFERENCE ONLY; NOT AN INSTALLED REQUIREMENT - Finish after pressing:
BORE FINISHED AFTER PRESSING; HOUSING SUPPLIED
Include in the RFQ: the housing bore size and tolerance, housing material and wall, bushing material and wall, the target interference, the press method, who presses and who finishes, and the chamfers on both parts. For acceptance, name the bore gauge (plug, air or bore gauge), the measuring plane away from the ends, the settling time before measurement, and a first-article check in the real housing.
Saola turns bushings from bar at its lead CNC plant (CNC turning). Send the housing drawing with the bushing drawing through the quote form so the quote can state how the bore will be held and gauged.
Does the bushing material change the answer?
Yes. A carbon-graphite alloy closes almost one for one and takes large interference, bronze bushings follow catalog installed-bore data, and polymer bushings are often specified as installed.
Graphalloy says interference up to 0.5% of the outside diameter is not unusual for its bushings and that they have a lower thermal expansion than steel, so the fit must hold at operating temperature (Graphalloy DS 1131, 2015). Bunting’s sintered bronze guidance recommends 0.001 to 0.003 inch of press fit on outside diameters up to 0.760 inch, rising to 0.002 to 0.007 inch above 3.010 inch (Bunting, 2020). These are different materials with different rules.
Take the numbers as examples of how makers publish them rather than as values for your part. Your bushing maker’s data for your alloy and wall is the right input, and a pressed first article settles any doubt.
Frequently asked
Do I have to ream after pressing if the supplier sizes the bushing?
Only if the installed bore band is narrower than the interference spread, as in Table 2, or if the shaft fit is close. For a loose fit, a correctly sized free-state bore with a stated closure is often enough.
How long should I wait before measuring the bore?
The graphite bushing maker says the parts may need 24 hours to reach final size (Graphalloy, 2015). For other materials ask the bushing maker, and measure at a stable temperature after the part has settled.
Why does a catalog list an installed bore?
Because the free-state bore is not the bore the shaft sees. Bunting lists housing bore, shaft diameter and installed bearing bore together, with a note that the installed bore applies in a rigid housing (Bunting catalog, 2020).
What if my housing is thin or soft?
The catalog press-fit values assume a rigid housing, so a thin or soft housing gives a different closure. Test the real housing, and say on the drawing which housing material the bore dimension applies to.
Where can I read about the fit itself rather than the bore?
The guides on press-fit tolerance and on clearance, transition and interference fits cover the fit classes; this guide covers what the pressing does to the bore.
Sources
- Bunting Bearings, product catalog (press fits, housing chamfers, installed bearing bore tables), catalog dated 2020
- Graphalloy (Graphite Metallizing Corporation), installation instructions DS 1131, technical sheet, 2015
- ISO 286-1:2010, ISO code system for tolerances on linear sizes, Part 1 (IT tolerance widths), ISO, 2010