As we have had so much rain over the last year, perhaps now is the time to look at converting the TR7 tachometer
to run with the V8 engine. The following was first published by the previous TR8 Registrar, Paul Towle - I have amended and updated a few details. My thanks to Paul for a very comprehensive and well
thought out article.
There are two reasons to change the rev counter (tacho). Firstly the new tachometer has to take account of
the extra four cylinders that are being installed under the bonnet and secondly the red line occurs from 5500 to 7000 rpm on the TR8 version whereas the TR7 red line from 6500 to 7000 rpm.
The original TR8 tacho (BL Part No. TKC 2587) is no longer available. However, it is possible to convert the
standard TR7 tacho for use in the TR8, the cost of the conversion being less than £5 in parts. This conversion should be well within the scope of most people, provided that it is done with care and in a good
The following items will be required for the conversion:-
- A small tin of 'HUMBROL ENAMEL SATIN RED' paint No. 132 (This is available from most modelling shops).
- 'HUMBROL' paint thinners.
- A roll of 1" wide sellotape.
- A small modellers paint brush.
- A scalpel or razor blade.
- A 15 - 25 Watt soldering iron with a 2mm bit. (approx.)
- A small length of 'MULTICORE' solder, (suitable for electrical/electronic work)
- A 47k (47 kilo ohm), 0.25 watt, 1% tolerance, metal film resistor. These are available from any
obliging TV/Radio repair shop for about 50p
- A pair of fine wire cutters.
- A jam jar lid (or similar) with a diameter of approx. 63 mm.
I must emphasize that it is essential to use a soldering iron with a fine bit as suggested above. If you
do not have one, try to borrow one. This job can not be done with a blowlamp!!
Assuming that you have already removed the tacho from the instrument pod,
- a) Examine the rear of the tacho. It should be possible to see four components which look
like 'liquorice all sorts' standing on legs!!!. (See Fig. 1.). These components are 'capacitors'. The capacitor on the right should have five coloured bands, brown, black, yellow, black and red,
reading them from top to bottom. Locate the 'leg' of this capacitor which is closest to you when viewed as shown in Fig 1.
- b) To the right of the capacitor located in a), there is a red wire which is soldered to
the thick film substrate, (i.e. the ceramic base to which the components are soldered). Locate this connection.
- c) Using the tweezers, cut and bend the leads of the 47 kilo ohm resistor as shown in Fig. 2.
- d) Very carefully solder either lead of the 47 kilo ohm resistor to the leg of the
capacitor located in a), and solder the other lead of this resistor to the connection located in b). See Fig. 3.
- That completes this section. All that remains is to mask off the red line area of the scale and paint it.
Fig. 1. below.
The tacho circuitry prior to modification. Note that the 'substrate' or ceramic base
is very brittle and can be damaged very easily if handled without care.