Bearing Housings: H7 Fits, Press-Fit Reality, and Cylindricity That Matters

The bore is the part

A bearing housing is three holes and a footprint, and the only feature that decides whether it works is the bore that receives the bearing. Everything else — mounting face, bolt pattern, oil ports — is supporting cast. I have quoted hundreds of housings and the pattern is always the same: the bore tolerance drives the price, and the bore tolerance is where drawings get either too vague or impossible.

The standard callout for a bearing seat is H7. On a 40 mm bore, H7 means a tolerance band of +0.025 / 0 mm. That number is achievable on a CNC mill or lathe — the question is what happens around it: cylindricity, surface finish, and how the bearing actually gets installed.

H7 explained without the textbook

H7 is a hole tolerance: the bore may be exactly nominal size up to nominal plus the H7 band, never smaller. That is deliberate — a bearing seat must never be undersized. The fit with the bearing outer ring is then decided by the bearing's own tolerance, which for a normal P0/P6 class bearing is around 0 to -0.01 mm on the OD. The result is a light interference or transition fit, which is exactly what most ball bearing applications want.

• H7 on 10 mm bore: +0.018 / 0 mm
• H7 on 40 mm bore: +0.025 / 0 mm
• H7 on 80 mm bore: +0.030 / 0 mm
• H7 on 120 mm bore: +0.035 / 0 mm

If you need the bearing to slide in and out for service, call H7 with a note, or move to a looser fit like H8. If you need it locked solid, that is where press fits come in — but a press fit on a bearing OD is a special case, because pressing on the outer ring loads the rolling elements. Most designers use a light press only, or a clamping flange, instead of a heavy interference fit on the housing.

Cylindricity beats roundness

Here is the trap: a bore can be perfectly round at the mouth and bell-mouthed 20 mm deep. That is why the drawing should call cylindricity, not just roundness. Cylindricity controls the whole surface — roundness at every depth plus straightness along the length. Typical production cylindricity on a fine-bored housing: 0.005-0.010 mm. The tool that gets you there is a single-point boring bar with a carbide insert, taking a finishing pass of 0.1-0.2 mm at low feed — not an end mill interpolated in a spiral, which leaves a lobed surface.

I mention this because end mill interpolation is how most shops cut a bearing bore when the part is already on a mill, and it can hold H7 diameter fine while leaving poor cylindricity. If your housing spins at high RPM, say so in the RFQ — the shop should fine-bore or ream it instead.

Press fits and installation reality

The practical advice on press fits: tell the shop how the bearing is installed. If the housing gets heated and the bearing dropped in, the shop can hold a tighter band. If a hammer and a block of wood are involved — and I have seen drawings that assume this — the tolerance needs to be friendlier, because that installation method will not repeat.

Materials: aluminum, steel, cast iron

• Aluminum 6061-T6: machines fast, but the bore wall expands more under press-fit load and under heat. Keep the wall thick enough or the housing will relax the fit over time
• Cast iron (GG25/HT250): the classic housing material — damps vibration, holds tolerance, and the graphite content makes it self-lubricating at the fit. More expensive to machine than aluminum
• Steel 1045 or 4140: for heavy loads and shock. Fine-bores beautifully. Requires more rigid fixturing and better tooling

For aluminum housings, one more consideration: if the housing is anodized after machining, the bore grows by roughly half the coating thickness — often 0.01-0.025 mm. Precision bores should be machined after anodizing, or the fit class re-calculated for the coated size.

What to check before you send the RFQ

• Is the fit class realistic for the installation method? A press-fit housing installed with a hammer is a contradiction
• Does the drawing call cylindricity, or only roundness? Add it if the bearing is long relative to its diameter
• Is the bore surface finish called out? Ra 1.6 or better on bearing seats, or the measured fit changes with roughness
• Are the mounting face and bore perpendicular? A perpendicularity callout between the bore axis and the mounting face is what keeps the bearing from binding under load

Frequently Asked Questions

• What does H7 mean on a bearing housing bore?

  H7 is a hole tolerance where the bore can be exactly nominal size up to nominal plus the H7 band, never smaller. For a 40 mm bore that is +0.025 / 0 mm. It gives a light interference or transition fit with a standard bearing.

• Can an end mill hold a bearing bore?

  It can hold H7 diameter, but interpolated end mill bores often have poor cylindricity — a lobed surface. For high-speed or long bearings, specify fine boring or reaming.

• Should I use a press fit for my bearing?

  Only a light press (H7/p6 with +0.01 to +0.03 mm) and only if installation is controlled — heated housing or hydraulic press. Heavy interference on a bearing OD loads the rolling elements and shortens bearing life.

• Does anodizing affect the bearing bore?

  Yes. The coating adds about half its thickness to the bore — typically 0.01-0.025 mm. Machine precision bores after anodizing or re-calculate the fit class.