Service Bulletin: Oil Pressure Relief Valve

Service Bulletin: Oil Pressure Relief Valve

Triumph Oil Pressure Service Bulletin

Triumph service bulletin dated 6-6-1973

Today's blog post is on a Triumph service bulletin dated June 6, 1973. This brief service bulletin describes the fine screen and coarse screen oil pressure relief valves used on twins and triples. Now your probably thinking to yourself why is this important? Aren't all oil pressure relief valves the same? According to Triumph some are not the same. We will also cover more information that this service bulletin does not include.

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What is an oil pressure relief valve?

Oil pressure relief valves are fitted to most vintage British (if not all) motorcycles to discharge excess oil pressure when the engine is cold. When an engine is cold oil is typically thicker causing pressure to build up in the crankcase. The oil pressure relief valve relives additional pressure and re-routes the oil to a separate passage until the oil warms and thins thus lowering pressure.

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Are all oil pressure relief valves the same?

 

Yes to a certain extent. I use the word interchangeable cautiously because most oil pressure relief valves look the same but internally some are different to accommodate certain models and engine sizes. It is important to use the correct oil pressure relief valve for your specific application.

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Fine Screen

Triumph Twins Oil Pressure Relief Valve 71-3447

 

The fine screen oil pressure relief valves where typically fitted to Triumph and BSA twins. Almost all reproduction oil pressure relief valves on the market today have the fine screen for both twins and triples. The best way to ensure a part is correct for your application is to check the screen but to also check the internal spring by removing the top cap. Most workshop manuals will list the proper spring length for each application.

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Coarse Screen

Triumph BSA Triples Oil Pressure Relief Valve 70-6595
The coarse screen oil pressure relief valves where fitted to all Triumph and BSA triples from 1968-1976. As you can tell both the fine screen and coarse screen valves look very similar. Triumph (and BSA) offered the coarse screen for triples only so one can tell the difference between the two. You should not use an oil pressure relief valve designed for a twin on a triple and vice versa. Reason being is that the triples have a stronger spring due to more pressure while the twins have a weaker spring due to less oil pressure. 
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There you have it - Oil pressure relief valves are a unique part and require some additional researching and checking to make sure that it is correct for your application. Failing to do so or installing a wrong oil pressure relief valve on an incorrect application may cause serious damage. As always, thank you for reading today's post. Feel free to comment in the section below. Read our other blogs.

5 comments

  • Matthew Epperson

    I have a 1970 BSA A-50 . What is the proper length the valve spring needs to be to allow proper oil pressure after the engine is running and hot , my oil pressure light always comes on after riding a bit at a idle and sometimes won’t go off until after I hit 1,500-2,000 rpm .
    Thank you

  • Shelby Flores

    1973 t140 at what pressure does the oil relief valve bipass?

  • Steve D

    Wasn’t aware of screen differences re my ‘73 T140V. I removed the OP relief valve and took it apart as I was blowing the points cavity and crank seals. Bike was relatively new to me. Low OP light began flickering despite oil flow in the OIF return. I measured the spring and it was too long. Rec’d advise it was appropriate to the 750 Triple and not the T140V. Measurement of spring is key (see manual GD section) as it determines when the OP relief comes into play. Screen itself doesn’t matter. It is only a visual designator when looking at a complete OP relief valve unit. If in doubt, remove spring and measure length. With proper spring installed, my ‘weekly’ replacement of seals is at an end. A real PITA as it required points timing and etc. Thanks for this blog.

  • Classic British Spares

    @eony – Thanks for reading and commenting. Many folks are not aware of this! I was not aware until I got a hold of the bulletin. Cheers!

  • eony

    well I was not aware of the difference . Now I will check as I seem to remember my Trident has a twin type filter, but maybe my mind is playing tricks. Still good info to know.
    Any more little known facts

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