Adding an Oil Thermostat to Your BSA/Triumph Triple

Are you looking to add a transmission thermostat to your older BSA or Triumph triple? We show you how to install two different units.

Reader Contribution by The Motorcycle Classics Staff
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by Keith Fellenstein
Two readily available thermostats for your BSA/Triumph triples: the Madigan (left) and the Tru-Cool (right).

The Triumph Trident and BSA Rocket III were among the first production motorcycles fitted with an oil cooler. Due to the extra cost, no thermostat was fitted. For long, fast runs, that probably never mattered.But as these bikes have become used more for pleasure than utility, shorter, slower rides are more the norm. Because of that, the oil often doesn’t reach high enough temperature to boil out the condensation that occurs when the motor is warming up. It also takes longer for the motor to warm up with the oil constantly circulating through the cooler. Neither of these factors are good for the long term life of the oil or the engine. Fortunately oil thermostats are in widespread use in the trucking industry as regulators for transmission coolers. Also, triple specialist David Madigan has designed a thermostat specifically for the BSA/Triumph machines that incorporates a special feature, a feed pipe for the rocker boxes. We’ll show you how to install either the commonly available transmission thermostat or David’s specialized one in this How-To. A detailed explanation of the function of these thermostats is well beyond the scope of this article. For a good report of the function of both of these designs, refer to, where you can find a report written by Richard Beard, who was present at the creation of the original cooler for the English triples. The Tru- Cool was sourced via Amazon, while the Madigan thermostat was purchased directly from David Madigan. The Tru-Cool was about $47 shipped, and the Madigan was about $80 with Priority Mail shipping paid by check. PayPal adds about $2.50.

1.) As always, disconnect the battery. Iput a rubber cap over the positive connector to prevent any accidental connections while working on the bike. Disconnect the gas lines from the petcocks to the carburetors too.

2.) We begin by removing the gas tank; the oil lines we need to accessare under the tank, along the top tube. You’ll need a 1/2-inch socket and ratchet to remove the bolts holding the tank in place. An extension will be useful in removing the front tank bolts to avoid interference with the oil cooler.

3.) Keep track of the rubber buffers that insulate the tank from vibration and remember their placement.

4.) Once the tank is off, locate the feed and return lines. Most bikes I’ve seen have the engine to cooler feed on the left, return on the right. Double check to be sure. It’s easiest to trace the line that runs to the top of the oil tank; that’s the return line.

5.) The feed side of the cooler has a tee connector running a 1/4-inchdiameter hose to the valve train to feed the valve rockers. If you’re using the Tru- Cool thermostat you’ll need to keep this in place.

6.) Disconnect the oil hoses, beginning at the cooler. Unless your bike has been sitting for a while, you will get some oil drainage from here.

7.) Disconnect the oil hose at the cooler side of the tee.

8.) Remove the section of hose, and remove the hose clamps for reuse.

9.) Loosen the clamps, beginning at the cooler. This hose won’t have much if any oil in it as it drains back to the oil tank.

10.) Loosen the clamp at the oil tank.

11.) Disconnect the return line from the cooler and the tank. Keep the clamps for reuse.

12.) This is the Tru-Cool thermostat with what I hope is a helpful diagram of the flow. C is for cool temperature, showing the oil path through the body of the thermostat bypassing the oil cooler. Once the thermostat’s operating temperature is reached, the oil passes through the H for hot side and through the cooler before returning to the tank.

13.) Cut a segment of 7/16-inch oil line to connect the thermostat to the rocker feed. Use the existing rocker feed for this thermostat since it doesn’t have one of its own. Keep the feed on the cold side of the thermostat to avoid starving the rockers while the oil comes up to temperature. Tighten the hose clamp at the thermostat a little tighter to compensate for the smaller spigot on the thermostat. Use an existing clamp to hold the other end of the short hose to the tee.

14.) Connect one end of your 7/16- inch hose to the hot side of the thermostat and gently bend it toward the inlet of the oil cooler. When you have it situated to your satisfaction, mark it and cut the hose.

15.) Using an existing hose clamp and one of your purchased clamps, attach the hose from the hot side of the thermostat to the inlet of the cooler. Don’t tighten the inlet clamp just yet, we’ll use it to prime/fill the hot side just before we finish. The clamp at the thermostat will require extra cinching to compensate for the smaller spigot on the thermostat.

16.) Attach one end of your 7/16- inch hose to the hot side of the thermostat on the return side and gently curve the hose as you did on the inlet side. When you’re satisfied with the placement, mark and cut the hose.

17.) Using an existing and purchased hose clamp, attach the hose to the cooler and the thermostat. Tighten both clamps. Again the clamp at the thermostat will require extra cinching to compensate for the smaller spigot on the thermostat.

18.) Attach one end of your 7/16-inch hose tothe return spigot on the oil tank. Use an existing clamp to cinch it in place.

19.) As you did with the other lines, measure the hose length needed to reach the thermostat and mark and cut the hose. Once again, extra cinching is needed to seal the hose to the thermostat.

20.) The finished installation of the Tru-Cool thermostat.

21.) As we mentioned in the introduction, the Madigan thermostat is purpose-built for the Triumph/BSA triples, incorporating a dedicated tap for the rocker feed that removes the need for the tee in the feed line. In addition, the spigots are the proper size for the 7/16-inch oil lines, so you don’t need to cinch the hose clamps down as hard.

22.) For the Madigan installation, follow steps 4-20, but omit step 13. Remove the tee on the feed side to the rockers.

23.) Disconnect the hose at the rocker feed.

24.) Remove the tee and hose and set aside.

25.) Attach the feed line to the spigot marked engine, rotate it into position and tighten the hose clamp.

26.) Attach one end of the 1/4-inch hose to the rocker output spigot on the thermostat and measure the length needed to reach the rocker pipe. Once you are satisfied with the length, mark and cut the hose.

27.) As you did in the Tru-Cool installation, measure the hose length necessary to reach from the tank to the return side of the thermostat and cut and clamp.

28.) The same procedure as before will lead you to measuring hoses for the inlet and outlet of the cooler. Tighten the clamps at the thermostat and cooler.

29.) Regardless which thermostat you picked, the last step should be priming the hot side of the thermostat to the cooler. This is probably not really necessary, but doesn’t take long and avoids any big air bubbles.

30.) Here’s the finished installation of the Madigan unit.

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