The MGA With An Attitude
Speedometer & Tachometer LUBRICATION - ST-109

On 11/18/2010, John Greenlee in Texas wrote:
"What sort of oil do you recommend for the speedometer"?

Try a drop or two of Graphite oil.

When new and clean the speedometer (and tachometer) likely had no lubricant applied anywhere. The input shaft is polished steel running in pot metal with very light load. I have never known this journal to wear out, but it could get noisy if dry and a little loose, especially in very low temperature (cold start conditions). The shaft for indicator needle runs in bronze bushings with even less load (nearly nil). I have never known these to have any perceptible wear, but the shaft might get a little sticky after many years of accumulation of dirt, or when left in storage (non use) for some years.

The odometer drive mechanism is more of a problem, especially if not used for years, or if used very little on a regular basis. The two crankshafts and the ratchet pawl parts are usually okay even with neglect. The odometer number wheels and interconnecting parts are the more common issue. Application of mineral oil in the odo wheels can cause more harm than good, as oil attracts dirt, then eventually dries out to leave the gunky goop and dirt inside of working parts where it should not be. The goop and dirt causes friction drag in the odo wheels. Sometimes when the odometer tries to turn over the last one or two wheels at left end (and all prior wheels in concert), the wheels that have not moved for 1000 or 10,000 miles (possibly many years) will have enough friction and drag to cause shear failure of some teeth on a phenolic drive gears on one of the small crankshafts.

Best treatment for the odo wheel parts is a good cleaning with solvent, but be careful that solvent does not wash the painted numbers away like paint remover. Better to use no lubricant on the odo wheels than to dirty those parts up with oil. The interconnecting parts between the wheels are thin brass shims with positive engagement fingers (and positive stop lever catch). Those parts will operate reliably when dry and clean as long as they do not get gunked up with dirt.

If you feel inclined to use any lubricant on the odo wheels, I would recommend using the graphite oil. This works as a penetrant to dissolve dried oil and loosen stiff parts, then leaves powdered graphite behind for lubricant when it dries. With some luck the thin mineral oil part may dry out early before attracting much dirt. The same graphite oil is good for a touch on all other moving parts in the speedometer and tachometer. Use sparingly, just a small drop on contact points of the indicator shaft bushings and ratchet lever parts, and a drop or two in the drive shaft journals.

If you were patient enough to disassemble and reassemble the odo drive wheels for cleaning, then they may be left dry for clean running. Or you might add a light touch of dry film lubricant like Moly Lube (molybdenum-disulfide powder). This stuff should be good to use on any moving parts of instruments, as it is always dry and will not attract dirt. Apply to exposed surfaces with a small cotton swab (Q-tip). This is NOT a penetrant.

Above all else, keep engine oil out of the instruments. The speedo drive spindle on the gearbox has a rubber oil seal. Ditto for the tach drive spindle at back of engine. These seals should keep oil out of the flexible drive cables. You can disconnect the output end of the drive cable, and pull out the flex wire core. Clean the outer jacket bore if possible (but usually only when both ends of the jacket are disconnected). Clean the core wire with solvent, dry it off, and apply a few drops of light machine oil or graphite oil (wipe on), then reassemble.

If the flex cable assembly gets enough oil inside it will be driven up the cable into the instrument where it WILL cause problems. In this case the instrument needs internal cleaning, the cable will need thorough cleaning or replacement, and the seal on the spindle at input end (gearbox or engine) will also need replacement. While the tach drive is essentially on top of the engine block, it can still weep oil into the cable if the seal is bad. The speedo drive seal on the gearbox is below normal oil level and can leak oil into the cable when parked. This seal is often overlooked when rebuilding a gearbox.

Thank you for your comments -- Send e-mail to <Barney Gaylord>
© 2011 Barney Gaylord -- Copyright and reprint information