Thermo 49i Pump Rebuild: Step-by-Step Diaphragm and Valve Replacement

A Thermo 49i pump rebuild means replacing the pump’s diaphragm and valve plate. It restores the analyzer’s vacuum and flow without buying a whole new pump. For most iSeries analyzers, the job takes one kit, a few hand tools, and careful attention to the order of the parts. This guide explains how to confirm your pump type, pick the right kit, and carry out the work.

What the pump does in a 49i analyzer

The internal pump does three jobs at once. It pulls the sample through the analyzer, draws ambient air through the ozone generator, and keeps the vacuum the measurement needs. The internal pump serves three essential duties: it draws the sample through the analyzer, draws ambient air through the ozone generator, and maintains the vacuum for the reaction. 

Because every one of those functions depends on one small diaphragm pump, a worn diaphragm or tired valve plate can affect the whole measurement, not just the airflow.

When a rebuild makes sense

A rebuild makes sense when the pump motor still runs smoothly, but the pumping action has weakened. A diaphragm and its valves are wear parts, and they are what the rebuild kit replaces. Typical signs are low flow readings, pressure values drifting out of their normal range, or a pump that runs but struggles to pull the expected vacuum. Check these in the analyzer’s diagnostics before opening anything, because capillaries, filters, and leaks can produce similar symptoms.

Thermo also treats the pump rebuild as routine maintenance. Its guidance for shutting down a 49i for the off-season lists changing the filter, rebuilding the pump, cleaning the cells, checking the lamp, and inspecting the scrubber. If you want to plan the other wear parts around the same service visit, our Thermo iSeries spare parts replacement guide covers when to replace the pump, diaphragm, pressure sensor, and filter tape.

If the motor is noisy, the bearings feel rough, or the pump housing is damaged, a rebuild kit will not fix the problem. In that case, replacing the whole pump is the better choice.

Rebuild kit or complete pump?

A rebuild kit is the cheaper route when only the diaphragm and valves have aged. A complete pump assembly is the better route when the motor or bearings are the problem, or when you want to swap the pump quickly and rebuild the old one later on the bench. For a swap, the 111553-01 Thermo 49i pump assembly is the complete replacement pump. For a rebuild, you need the Thermo 108002-00 pump rebuild kit.

Identify your pump before ordering

Before ordering, read the part number on your pump and match it to the correct kit. Thermo’s iSeries pump bulletin states that rebuild kit 108002-00 is shared by pump part numbers 111553-01, 111553-00 and 101426-00, and that it includes replacement shims. The same bulletin lists the 49i and 49i-PS, along with the 43i, 48i, and 146i, as models that use these pumps.

The shims are where mistakes happen. In the same bulletin, the 111553 pumps are low-vibration, shim-less designs, so the shims in the kit should be discarded. The 101426-00 pump comes in two forms: the “old style” is shim-less and uses a different kit, 8606, while the “new technology” version uses shims. The old shims you remove must go back in the same quantity, so keep them in order.

Note that this bulletin is dated December 2011, so confirm current part numbers with your supplier before ordering. The 42i family uses a different pump and kit (107751-00), so do not assume the same kit covers every analyzer in the series.

Tools and preparation

You need very little: a #1 Phillips screwdriver, a 2.5 mm Allen key, a marker or pencil, and the rebuild kit. Thermo’s procedure also calls for Teflon tape when refitting fittings.

Before you start:

• Disconnect the pump from power. Do the work on a clean bench.
• Sketch or photograph every tube and fitting position.
• Clear a tray so small parts cannot roll away.
• If you are rebuilding several pumps, keep each pump’s parts separate.

How to rebuild the pump

The steps below follow Thermo’s published procedure for the 111553 and 101426 pumps. Always read the instruction sheet supplied with your kit, since it governs your specific pump.

Remove the head and old diaphragm

1. Mark the parts. Draw a line across the head plate, intermediate plate, and compressor housing so they go back in the same orientation.
2. Open the compressor housing. Remove the screws on the aluminum cover and keep the gasket for reuse. Clear any debris from the bottom of the housing.
3. Take off the head plate. Remove the four head screws, lift the head plate, and note how the valve plate sits relative to the valve ports.
4. Remove the valve plate and intermediate plate. Clean the parts without scratching them.
5. Remove the diaphragm. Rotate the fan so the diaphragm sits at top dead center, which helps unseat its edge. Unscrew it counterclockwise using both hands, gripping at about the 10 and 4 o’clock positions. Thermo says not to use tools on the diaphragm, and notes that high initial force may be needed to break it loose.
6. On shimmed pumps, secure the shims. Hold the support cup and shim rings on the diaphragm stud as you lift it out, and record how many there were.

Fit the new diaphragm and valve plate

1. Install the new diaphragm. Screw it into the connecting rod until snug, then tighten firmly by hand. On a shimmed pump, put the support cup and shims onto the new stud first. Holding the rod at a slight angle helps the thread start.
2. Seat the edge. Turn the fan until the diaphragm lies flat at mid-stroke, then press its edge firmly into the groove of the compressor housing.
3. Stack the head. Place the intermediate plate using your reference marks, then the new valve plate, then the head plate.
4. Tighten the head screws. Tighten them snugly in a diagonal pattern. The bulletin specifies a maximum torque of 6 to 7 inch-pounds. That is very light, so do not use a long driver or full force.
5. Check free rotation. Turn the fan by hand. It should move without binding.

Reassemble and refit the fittings

Refit the compressor housing cover and gasket, but do not over-tighten the cover screws. Remove all old Teflon tape from the fittings and apply fresh tape. Thermo advises two layers, kept on the threads only, because stray tape can tear off and lodge in the valves. Reconnect the tubing from your sketch, and avoid over-tightening metal fittings.

Testing the analyzer after the rebuild

After reinstalling the pump, power the analyzer and check flow and pressure readings in the diagnostics menu. Compare them with the values you recorded before the rebuild, or with your analyzer manual’s normal ranges. Listen for unusual noise, and check the fittings for leaks. If flow is still low, look at the capillaries and the sample filter next, since the pump is no longer the only suspect.

Run your normal calibration or verification routine before returning the instrument to service. A pump change can affect the sample path, so treat it as a maintenance event that needs documenting.

FAQs

Does kit 108002-00 fit the Thermo 49i?

Yes, for the common iSeries pumps. Thermo’s bulletin lists it for pumps 111553-01, 111553-00 and 101426-00 used in the 49i, 49i-PS, 43i, 48i and 146i. Check your pump label first, because the “old style” 101426-00 uses kit 8606.

Do I need shims for a Thermo 49i pump rebuild?

It depends on the pump. The 111553 pumps do not use shims, while the “new technology” 101426-00 does. Reuse shims only where the pump originally had them.

What tools does the rebuild need?

A #1 Phillips screwdriver, a 2.5 mm Allen key, a marker, and the kit. Teflon tape is needed for the fittings.

How long does the rebuild take?

Thermo’s bulletin does not give a time, and it varies with experience. Allow time for careful disassembly, drawing your tube sketch, and testing afterward.

Should I rebuild or replace the whole pump?

Rebuild when the motor and bearings are healthy and only the diaphragm and valves are worn. Replace the pump if the bearings or motor are damaged.