Flaps & Thermostat range of movement

Discussion in 'Mech Tech' started by CJR, Feb 15, 2020.

  1. CJR


    I am putting some fan housing thermostatic flaps on my new engine (Type 1 dog house) that I "came across" without a thermostat and bought a new style "mexican" thermostat from JK.

    The flaps go from fully open to fully closed with 20mm (ish) of movement and the thermostat only expands 10mm (-10C to 125C, freezer to oven) which would only move the flaps about half way through their range

    I can't find any info on how far the flaps should move (do you go from closed to half open or fully open to half closed) or how far the thermostat should expand (is the thermostat I have a wrong one?) I tried JK and they don't know how far the thermostat I got form them is supposed to expand but did say nobody has complained so maybe there's something wrong with the flaps I have but they look like others I have seen on the web .

    the Ratwell pages say that the original thermostat on the type 4 engine (ones with a cable) go from 35mm closed to 55mm fully expanded and 46mm at 65-70C which looks liked there's a problem with my thermostat only expanding 10mm but maybe the Type 4 is different and I have the wrong flaps.

    Any additional info would be good
  2. Should move more than 10mm
  3. I understand that " nobody has complained " is JK's stock answer to complaints.
    However, I bet a lot of people would buy a new part, fit it and naively think that it's working properly ( as it should).
    Dubs likes this.
  4. Dubs

    Dubs Sponsor

    To be fair, I think I have had the "nobody else has complained" line from most of main suppliers.. I think it's a polite way of saying "yellow snow off mate, we have your money now" :rolleyes:

    To the OP, I would see if you can source a used original thermostat. I have never measured how far they open, but pretty sure it needs more than 10mm.

    If you must use the aftermarket one, set it so the flaps are fully open when hot, and only partially closed when cold. Then source an original..
    monkeyvanwestybike and lhu1281 like this.
  5. Doooplicate poooost.

  6. Ratwell.com has data on thermostat movement. I dont think its actually much more than 10mm .. he says 11mm over operational temperature range.

    Info and how to renovate some failed OG thermostats.

  7. The flaps should move from (almost) closed to fully open. The "new" wax thermostats won't operate properly with your original rectangular bracket. I believe the Mexi engines just had a plain L-shaped bracket (possibly also a different linkage - dunno).

    Best would be to find a bellows type - they are around, new or used. They should move the flaps through their full travel. The top of the rectangular bracket constrains their maximum length.
    Dubs likes this.
  8. The new style will work with a type 4 if you mount it on the outside of the bracket (ie facing the same way but closer to the flaps) - it rubs the case slightly but doesnt affect the movement.
    Mark Darby likes this.
  9. @CJR ‘s got a type 1, I think.
  10. CJR


    thanks all. On the bellows that 3901mick pointed to it looks (from the eBay pics) like there is only about 10mm of expansion space inside the bracket (on my bracket there is only about 50mm for the bellows to fit in and in the pics the bellows looks to be occupying 4/5 ths so only about 1/5th left for expansion ) which is the same as I have on the new thermostat I have so I am left with thinking it's something to do with the flaps.
    I don't fancy bunging £50plus for an old bellows just to find out I have the same 10mm movement because I have checked again and it is definitely 20mm on the push rod to go from fully open to fully closed. I think I'll just leave the flaps sprung open and wait until I can borrow a bellows stat and measure it hot and cold, If Ratwell's right it'll be 35mm cold, 46mm at 70C and 55mm hot but it doesn't look like that on any pictures I can find.
  11. That did look a bit big/expanded to me. Haynes has the measurements in. If it’s not peeing with rain tomorrow, I’ll have a look at mine.
  12. Actually, looking at those pics, that thermostat looks duff to me. I'm guessing, but I reckon you get about 30mm movement with a good stat.
  13. CJR


    snotty, thanks for getting back. I have had a look in my Haynes and it has the same test as Ratwell and Bentley which is to see if the bellows are at least 46mm when at 65-70C, I can't see any other dimensions in my copy. I can see that anything over 46mm is a waste because the bracket won't let it go any further

    Everybody says that the bellows are good 'cos they fail safe which to me means a broken one will be over 46mm so I can't actually see that the test is proving much because a non working bellows would pass it! Wouldn't it be better to cool the thing down and see that it went small? Any failure is likely to result in the escape of the contents of the bellows and if there's nothing left in the bellows to condense the bellows won't shrink. Do you get bellows that are less than 46mm at 65-70C? It doesn't seem likely to me unless the bellows have become stuck together or lost their springiness.

    The main problem I have with this is that millions of clever folk over many decades have been doing the Haynes/Bentley/Ratwell type test and think it's the correct thing to do so it probably is, and it's me that's confused.

    I thought air cooled would be simpler!!
  14. I think Ratwell has the cold length too.. 35mm going to 46mm at running temperature.

    The way I set mine I just make sure the top of the thermostat is touching the top of the frame when its hanging off the rod, held up by the spring on the flaps.. Then I pull the thermostat down and feel the flaps moving, and fix it to the bracket. Occasionally I get too enthusiastic and ping the rod out of the loop in the flaps mechanism under the fan housing.. I have now learnt how to lift the fan housing and re-thread the rod without pulling the engine.
    My thermostar is a strange length because I ended up soldering a new section of M4 internal threaded hex bar to the top of the thermostat (cutting off a section of the original top of the thermostat) as the old thread had completely worn away.
  15. Air cooled is simpler ;)

    I had a look at mine today. Haven't seen him for a while. On a cool-ish day, the body of the 'stat bellows was about 35mm long. I don't think mine is duff, as when they leak/crack they just get bigger, and touch the top of the bracket.

    Surprising...but it works. Lost a couple of screws from the cover plate, so when I'm under again I'll undo the stat and see what total movement is, from closed to fully open. I suspect it will be less than we think: 10 - 15mm? I never really paid attention to range of movement when setting the stat.
    Last edited: Feb 16, 2020
  16. I have a knackered type 4 stat at home, I can measure compressed and open if that helps you, although the extended length may not be the same as when it’s hot since mine is holed somewhere.
    Ratwell says that hotter stats have fewer ribs (10 as opposed to 13) but expand/contract the same, 35-55mm, which is surprising as it suggests the size is the same despite the number of ribs. Think mine is a 10-rib 85-90c.
    It also states: “When it hits its target temperature it measures about 46mm long and continues to expand until it's 55mm long. That's how much the cable that controls the flaps moves.” In your case I guess that’s ‘rod’ not ‘cable’.
    I still have a spare bellows bracket so could measure that to get the mechanical limit of expansion, but of course a type 1 may be different.
    Hope this helps.

    Sent from my iPhone using Tapatalk
  17. After a run today, took a squint at mine, and nice and hot it's fully extended, so flaps full open :thumbsup:
    Dubs likes this.
  18. :eek::eek:
    snotty and Gingerbus like this.
  19. Mine is a 10-rib type 4 85-90c and measures 27mm compressed and 38mm open (in its broken state) across the edge of the end plates.
    End-end overall that’s 44mm closed and 55mm open at the fixings.
    I reckon it could only open maybe another 5mm or so before the bracket would limit the movement, so broken seems roughly equal to the open length, give or take.

    If it were still fitted, being knackered I reckon the springs on the flap hinge rod would pull it open a little more (as there’s less resistance with a knackered stat) until either the flaps hit the limit or the stat hits the bracket.

    Of course as it’s broken a good one may measure up differently.
    And mine’s a cable type not a rod type linkage fixing.

    Assuming all things being equal - and they may not be - another 3 ribs on a lower rated stat would add 5mm or so compressed and 10mm or so extended, so that would make about 32mm closed to 48 open across the plates or there abouts.

    Not sure if that helps you much!

    Sent from my iPhone using Tapatalk
    Last edited: Feb 17, 2020

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