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Oil Thermostats

OIL THERMOSTATS - APPENDIX 1, ADDENDUM

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EVALUATION OF FRANKS TC OIL THERMOSTAT - ADDENDUM

1. BACKGROUND

At the end of Appendix 1 which attempted to diagnose the fault(s) in Franks TC oil thermostat the conclusion was inconclusive in that no definite cause of malfunction had been identified.

I was not happy with this conclusion so further thought and examination of Franks oil thermostat was undertaken.

The results of this process are detailed in this addendum.

2. FURTHER THOUGHTS

In mulling over possible causes of a malfunction in Franks TC oil thermostat the thought occurred to me that if the shuttle valve (see Diagram B) was in some way tilted over with respect to the wax capsule rather than being always at right angles to it then because the shuttle valve internal diameter is a close fit on the outside diameter of the wax capsule then it might wedge against the outside of the wax capsule and not move.

The tilting of the shuttle valve might arise from one of the springs involved (the main return spring and the shuttle valve retaining spring in Diagram B) exerting a force on the shuttle valve that was not totally in line with the axis of the valve.

3. FURTHER EXAMINATION OF FRANKS TC OIL THERMOSTAT

With the above thoughts in mind a further detailed examination of Franks TC oil thermostat was made.

Disassembling the oil thermostat a careful examination was made of the shuttle valve.

The shuttle valve is really nothing more than a pressed out washer.

However looking at it carefully it was clear that one edge of the internal diameter was radiussed quite generously whereas the other edge of the internal diameter had a very sharp edge. These edge appearances are typical of a pressed out washer.

Further thinking would indicate the possibility that if the shuttle valve was tilted by the springs then the sharp edged internal diameter could well “dig in” to the outside diameter of the wax capsule.

With this in mind the outside diameter of the wax capsule was examined using a magnifying glass. There was a definite ridge on the outside diameter extending about a third of the way around the circumference where it was clear that the shuttle valve sharp edge had dug in.

Offering up the sharp edge internal diameter of the shuttle valve to the outside diameter of the wax capsule with no springs in position showed a very close match between the sharp internal diameter of the shuttle valve and the indentation on the outside diameter of the wax capsule.

4. DISCUSSION

From the further examination there is no doubt in my mind that Franks TC oil thermostat malfunction resulted from one of the springs exerting a non axial force on the shuttle valve, thus tilting it slightly and causing the sharp edge of the internal diameter of the shuttle valve to “catch” and dig into the outside diameter of the wax capsule.

For this to happen probably needed a combination of:

  • A particularly sharp edge on the internal diameter of the shuttle valve
  • The assembly of the springs giving rise to a small non axial force
  • Perhaps a slight undulation on the outside of the wax capsule giving a start point for the “catching”.

It would seem that this combination of factors is unusual as the vast majority of these thermostats work well over the course of many thousands of miles. However the fact that one malfunctioned in the way described means that others could do likewise.

It is perhaps significant that Franks thermostat malfunctioned after only a 1000 or so miles—relatively early on in its life. I suspect that the sharp edge on the internal diameter of the shuttle valve in Franks oil thermostat was worse that the norm and that in the vast majority of cases the two surfaces—the sharp edge of the internal diameter of the shuttle valve and the outside diameter of the wax capsule “wear” together and thus produce no problems.

It is perhaps a good idea to keep a close eye on oil temperatures /oil pressure for the early usage of the bike following the installation of one of these thermostats.

5. CONCLUSIONS

I conclude that the malfunction of Franks TC oil thermostat was due to a combination of factors that would not be repeated except in a very small number of cases.

The use of the Trucool oil thermostat by triple owners is therefore still a valid recommendation.

It is however well worth noting that the failure mode of Franks Trucool oil thermostat cannot occur in the DM (Dave Madigan) oil thermostat due to differences in the detailed designs of the two thermostats.

6. FRANKS TC OIL THERMOSTAT

Just to round off the story I filed a generous radius on both inside edges of the internal diameter of the shuttle valve of Franks TC oil thermostat.

Using a Swiss file I also filed smooth the outside diameter of the wax capsule where the shuttle valve had dug in.

I then polished both surfaces and reassembled the unit.

The unit was then returned to Frank in Germany.

RAB
02/18/2019
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