I have a '94 Honda VFR and two of the coolant hoses are 18 mm ID.

18 mm is a very unusual ID, and it would be easier to for me to go to the auto parts store and get a size they have in stock, which is 3/4" or 5/8".

Is it at all possible to successfully replace the hoses with a readily available ID?

  • If you can get the hose on there, yes. Too small can be stretched over or too large (a little bit) can be pinched with a good hose clamp. If it fits, it ships. I don't know what the difference between 18mm and a 3/4" or 5/8" would be ... I think a 3/4" should be pretty close (just checked, it's a shade large at +.041"). You should be able to take that up with a hose clamp ... use some grease on it (inside) to help it seal. – Pᴀᴜʟsᴛᴇʀ2 Jun 6 '19 at 1:00
  • That's an answer @Pᴀᴜʟsᴛᴇʀ2, but I'm not sure the math is right. 3/4" in mm is 19mm. – GdD Jun 6 '19 at 7:58
  • 3/4" = 19.05mm (exactly) so in practice it is identical to 19mm. Or equivalently 19mm = 0.748." But 18mm = 0.709 which is significantly smaller than 3/4." – alephzero Jun 6 '19 at 9:05
  • 39 thousands of an inch is significant? Not when it comes to rubber hoses. – Moab Jun 6 '19 at 15:48
  • @GdD - A 3/4" is .750". 18mm equates to ~.709". That's only a difference of .041", as I stated. Divide that in 1/2 to get the difference (0.0205") between the hose and the fitting around the circumference. That's a difference of a little over 0.5mm ... that can EASILY be taken up with a hose clamp while using a rubber hose. As Moab stated, not sure that's significant. – Pᴀᴜʟsᴛᴇʀ2 Jun 6 '19 at 20:32

18 mm is closer to 3/4 inch (roughly 19mm), 5/8 of an inch is closer to 15.5 mm. 1 mm is .03937 inches you can't beat that. Go for the 3/4 inch.

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