Oil Pressure Switch Thread Form

650 Unit Triumph Motorcycles

Triumphrat.net, Posted By: Atlanta Bonnie
I want to test the oil pressure on a pre-69 that has a 3/8 CEI threaded opening in the timing cover? Do any vendors in the USA offer anything to address this CEI thread connection? Would like to test pressure with a gauge, than put plug back in

Unless someone else reading this thread knows of a source of CEI-threaded hose connections, imho my earlier suggestion to Tobin is probably the quickest way.

Triumphrat.net, Posted By: Atlanta Bonnie
It's my understanding that pre-69 timing covers used a 3/8 CEI blanking plug to seal this opening before OP switches were introduced in 1969.

Only on the 650's, pre-'69 500's had the timing cover done differently. The CEI-threaded hole was just a manufacturing drilling, and Triumph threaded it CEI 'cos that was the finest 3/8" thread they used - normally they used BSF for engine threads but 3/8"BSF would've been coarser and not such a good seal. Then, when they decided to fit everything with o.p. switches, that manufacturing drilling, with just a different thread, was an easy mod.

Triumphrat.net, Posted By: MikeinBiddeford
I put in a fitting with 1/8" IPS threads. It wasn't until some time later that I found I was the one that was wrong. Some folks put the IPS threads in so tight that they crack the case. IPS threads are tapered and SAE are running threads. So the IPS wedges it's self in, I used pipe dope on the threads and not teflon tape.

Strictly-speaking, the o.p. switch thread isn't IP but NP (National Pipe), and can be one of two - early '69, Triumph (and BSA) had 1/8"NPT- (Taper-) threaded switches/holes, then during the '69 season, swapped to NPS- (Straight/parallel) threaded switches/holes.

Compounding the problems today:

  • The trade uses 60-2133 for the taper-threaded switch, when Triumph originally used it for the first (of two) parallel-thread switch.
  • (One of?) the switch maker(s?) released a batch of taper- threaded switches that tapered up mad from the nominal o.d. (0.405") instead of down - screwing one of those into a timing cover or crankcase will ruin your day too. frown I came across the problem several years ago so I don't know whether it persisted on subsequent batches, or how long a bad batch takes to catch out all the poor customers.

3/8"CEI is 26tpi while 1/8"NP is 27tpi and a slightly larger o.d.; if your pipe thread fitting was tapered is more likely why it appeared to 'fit' - very few threads were actually engaged - and also why it appeared to 'wedge itself in'.

Either switch into a correspondingly-threaded hole shouldn't require "dope", tape, whatever. However, if you screw a 27tpi- threaded fitting into a 26tpi-threaded hole, you're going to need someting to fill the gaps. wink