Making a common motorcycle High pressure Radiator cap

for high ambient temperature, high coolant temperatures and high altitude conditions

  ~5000ft - Pikes Peak Raceway in Colorado -  - it's 106f hot - hot and the barometer reads 24.66 inches (sea level barometer near San Francisco is ~30.00").
Colorado Springs IS at ~5000 ft elevation.  (that's about 1 inch barometer per 1000 feet!  Remember that tidbit to amaze your peers )

So - there's not many molecules in each handful of air up here - and even though you are ramming just as many "handfuls" of air through the radiator, the air, because there's fewer molecules per handful, it simply can't pick up the as much of the transferred engine heat from the radiator.
The temperature production/radiation ratio will equalize and stabilize at a higher than usual coolant temp. - like 220f to 230f on the track and usually - 250f when you shut off the engine in the pits.So.....

Your stock radiator cap will only hold back about 14.7 psi. An aftermarket cap that you can find in stock at Larry, Moe and Curley's in Colorado Springs will be 16 psi. The difference between 14.7 psi and 16 psi pop-off pressure is small. Don't expect the 16 psi cap to help much in retaining the high pressure of the 220f to 230f to 250f coolant.

You CAN make a higher pressure cap that looks stock.
(I'm giving away a secret)

1. Buy a "Stant" brand, #11233 cap. It has a BROWN spring in it that is just shy of 1'/25.4mm diameter. Actual measurement was .965"/24.46mm on the one that I'm measuring right now. It's the only "Stant" radiator cap spring that is brown AND ~1"/25.4mm diameter (if you forget the STANT part number and have to measure every Stant cap on the rack). It's almost always in stock in automotive supply stores.
2. Take the STANT cap. Get pliers, diagonal cutters, needle nose and attack the STANT cap. You WILL be amazed at how some parts can be designed to be assembled quickly and easily and are nearly impossible to have accidentally come apart.  I bow in awe at the ingenuity of the rad cap manufacturers.
Anyway - get the brown spring out.
3. Take your stock cap. Now take the brown spring and spread the "tail" of the spring by hand, just enough to work it around the spring retainer on the stock cap.  You'll have to lever the end tail over the cup to avoid damaging the rubber gasket.

A 21 to 25 psi cap - just like all car guys can buy and use.
And it looks stock!

Now - It won't make the bike run cooler, but it WILL keep the coolant in the cooling system and run as cool as it possibly can with a stock cooling system. It's WAY better than blowing out all your coolant during a race and running even hotter!
Funny thing is - when you don't have any coolant - most bikes don't read too high coolant temp - because there isn't any coolant to read the temperature of! Scary.

As always - YOU are responsible for whatever you do with this information!
We ARE NOT responsible - use this information wisely - and if you have ANY questions - ASK a reputable and experienced local person!

You - and ONLY you  are responsible for your own actions.




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