Turbo Overhaul By Mathew Thomas Levett (MTL)

http://www.300zx.co.uk

Welcome to my little FAQ on how to overhaul your T2/T25 turbo. This is not

recommended for beginners as you need to be very careful as a turbo is a precision

engineered device. At full boost the turbine and compressor can spin at speeds of

over 130,000 RPM.

Ok, first you need to remove the turbo from the car. Once you have done this you

then need to prepare a clean work area. Once you have your work area you can start

to strip the turbo down.

Fig 1

Figure 1 shows the complete turbo ready to strip down. Start by removing the oil and

water pipes from the turbo.

Next you need to remove the wastegate actuator from the turbo. (Fig 2)

Fig 2

You should have a turbo that looks like fig 3

fig 3

Now you need to spray the bolts with penetrating oil and leave it to soak for a while.

Then undo the 4 bolts that hold the exhaust turbine housing on. Start by loosening the

bolts as shown in fig 4.

fig 4

You will notice that you can not remove the bolt all the way as it will hit the CHRA.

(Centre Housing, Rotating Assembly) as shown in fig 5

fig 5

When you get this far you will need to tap the housing off the CHRA, but be very

careful as you need to tap it off evenly so it does not bind on the turbine.

Once it had been removed it will look like fig 6

fig 6

Next you need to remove the compressor housing from the CHRA. To do this you

need to remove the big internal circlip shown in fig 7 and then tap the housing off in

the same way as the turbine housing.

fig 7

After the housing is removed it will look the fig 8 (compressor) and 9 (turbine).

fig 8

fig 9

Mark the nut, shaft and compressor so that you can line it all back up. Now clamp the

triangle ‘nut’ on the turbine in a vice or use a 14mm socket to hold it still while you

undo the compressor nut with a 8mm socket. Be careful not to bend the shaft. Once

the nut is removed you can lift the compressor wheel up off the shaft. Your turbo

should now look like fig 10.

fig 10

Now pull the turbine out the other end with the heat shield as in fig 11.

fig 11

Now its time to disassemble the CHRA.

First you need to remove the circlip holding the seal plate in place shown in fig 12.

fig 12

Now remove the seal plate and you can now see the ‘Thrust bearing’ and ‘Thrust

collar’. (fig 13)

fig 13

Now remove the ‘thrust plate’, ‘o’ ring and ‘thrust collar’ and you should then see the

first brass bushing (bearing) as shown in fig 14 (bearing lifted out of its seat)

fig 14

Under the bushing there is two circlips, the first stops the top bush from falling out the

centre and the second clip stops the bottom bush from falling out. You need to

remove them both and then the bushes will fall out.

Fig 15 shows all the turbo parts in order.

fig 15

Fig 16 shows the piston ring seal on the thrust collar

fig 16

Fig 17 shows the rear oil seal that commonly goes when you decat.

fig 17

Here in fig 18 you can see the stock piston ring on the left and the race spec step gap

piston ring that I have fitted to mine on the right. (The reason I fitted this one is that

when a Z32 is decatted the loss of back pressure causes this seal to leak). As you can

see the step-gap one makes a much better seal.

fig 18.

Here is the parts kit that I got to rebuild the turbos (fig 19)

fig 19

As you can see this kit has almost all the internal parts that we removed from the

turbo. Next reassemble the turbo in exactly the same order as we stripped it, but

remember to dip all the new parts in oil before fitting them. Reassemble all the parts

in the reverse order to the removal and ensure that the marks on the nut, compressor

and shaft all line up. Then get the CHRA VSR balanced and refit the housings.

VSR Balancing

The high speed balancing process

Turbochargers are assembled from component parts which are separately balanced using conventional low-speed, hard bearing balancing machines, and both the turbine wheel and compressor wheel are normally balanced in two planes. At the turbine end, the balance is not affected by assembly into the cartridge, but at the compressor end, small errors in the wheel, the shaft, the thrust collar and the nose nut can cause an accumulation of balance error. At high speed, this can result in a noisy turbocharger, and in a severe case can cause premature bearing failure.

This imbalance can be corrected by running the assembled cartridge at high speed on a flexible suspension, measuring the vibration response, and either changing the assembly position or removing metal from the nose to achieve an acceptable balance. This operation usually requires the unit to be run at speeds close to the normal service operating speed, typically 100,000 to 200,000 rev/min, depending on wheel size.

In operation, the CHRA is mounted in a slave turbine housing adapter, using quick-release clamps to hold it. The turbine housing is in turn attached to a flexibly-mounted air nozzle assembly which directs air into the housing, rotating the turbine shaft. An accelerometer attached to the flange measures the vibration of the complete assembly.

The compressor wheel is covered by a shroud for safety, and to reduce air ‘windage’. The nose nut or shaft end is magnetised, and a coil in the centre of the shroud converts the rotation of the magnetic field into a voltage signal, which is processed as a speed signal.

The accelerometer and speed signals are then processed electronically to remove unwanted frequencies, giving a display of vibration level (g-level ) against speed as the CHRA is accelerated up to the maximum speed. At the same time, the angular difference between the speed signal and the accelerometer signal is displayed to indicate the imbalance position.

Here you can see the turbo being balanced.

That’s it! Job done :D

By Mathew Thomas Levett (MTL)

http://www.300zx.co.uk