Are Thermostat Jiggle pins essential?

I’m currently overhauling my old cooling system. New hoses, coolant, and thermostats. I did buy 82C ones, maybe I should have got 88C ? Anyhow they do not have the jiggle pin. I have read some people drill a small hole in them instead. Or will it be ok to fit them as they are. Am I worrying too much about this ? Any opinions on this ?

No, not essential, but it helps when purging air out of the system when you refill with water, so it is a useful idea.

kind regards

I’ve tried 88c and 82C, they both run fine on the V12. I wouldn’t go lower or higher. I prefer the 82C because i like seeing the bottom end of the N while driving. Although my mpg went down by -1 with the 82C.

Jiggle pin is essential for proper bleeding once car is off. I’ve simply drilled a small hole in mine when there was no pin. Only down side, may take an extra few minutes to warm up.

The purpose of the jiggle pin is, as I recall, to keep build-up from accumulating in, around, or cover the hole, blocking flow through the hole.

Eh? No. When coolant is flowing, jiggle pin is pushed to close tiny hole in thermostat. When engine is shut off, jiggle pin falls out, so that any air that finds it’s way will get out thru the tiny hole. That’s why it should be installed at 12 oClock.


*mFor my two cents or pence in your case, fore the price of a proper set of t’stats why not just get the recommended ones with the jiggle pins. I don’t like to mess too much with cooling , especially on a V-12 Jaguar. When I first purchased my car my mechanic told me he replaced the t’stats. Eventually when I finally got around to changing them myself. I’couldn’t remove the housing and low and behold no jiggle pins , no drilled holes etc.
I just ordered up a lower temp ones c/w pins. There’s always a work around but why bother. Do the job properly. They’re not much money.

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Once again we come back to where you live… hot or cool climate. After testing the 88C with the 82C thermostats it was a no brainer to go with the 82. My regular cruising temp is between 85 and 87 in the summer and 81 to 85 in the winter on average.
Another point is that it must be the double action thermostat. A couple of years ago at a Jag show I was parked with a row of XJS’s and I found when discussing cooling systems that two owners in the group didn’t know of double action thermostats and had fitted single action units.

What I read quite some time ago was that a simple one-way flap valve would do that, but they get stuck.

The jiggle pin helps prevent the valve from getting stuck.

I’ll see if I can find anything on what I remember reading, then either post it, or post that I must be imagining things.
I agree: must have a hole. The pin isn’t so important.

Also must be the correct legth with the correct bypass disc at the rear (no “s” at the end)
Now here’s an interesting one. I spoke with David Manners on this and they double checked the thermostats they have. Again, they supply genuine Jaguar at 88 degrees and there is no jiggle pin. It would appear that Jag no longer saw them as necessary and did away with them. DM said they sell quite a few of these each year and have never had any complaints. I guess it may take you a little longer to shift an air lock, however it should clear after running up to temp and getting the thermostats open?

Ray White
There is no reason why you shouldn’t drill a hole. The jiggle pin is only there to stop rust or break up gunk that might otherwise block the hole.

Some thermostats don’t have a bleed hole in them because there will be a special bleed valve elsewhere; either in the housing or radiator hose.

Which is right/wrong? Jiggle pin necessary, or just the hole is necessary, or is the hole not even necessary?

I’ll be going with jiggle pins, regardless of what their actual intent was for.

In my books, yes.
The engine likes the 88°C better.
If your cooling system is good you will have no problems.
If it’s not, it will overheat the same with either thermostats.

I guess everyone fails to mention, holes or jiggle pins make sure they are installed at the 12:00 o’clock position in order to bleed out the air. I believe I read that somewhere.

I install mine at 11:58 o’clock.

I keep seeing this mention of the twelve o’clock position - does this mean the center point of the front of the car or straight up to the sky - as you may realize, i have not changed gotten to checking the thermostat - Tex.

Straight up, as if you are looking at a clock face. So that the pin is high up, which will coincide with air bubble

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Well Greg. The obvious question is do you install at 11:58 a.m.? Or p.m.?

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PM in the winter, AM in the summer.

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I use a 24 hour clock. Anything between 2355 hours and 0005 hours is good on my opinion


Of course, every April 1st, you have to put them back an hour when the clocks change…

kind regards


Greg, I was just searching for ‘jiggle pins’ I purchased 2 82 C thermostats from SNG Barret and they have the jiggle pins, you say orientate them a 12 o’clock but where are you looking from? confused, Geoff

Just make them the topmost (closer to the bonnet versus the pavement) so that as air trys to escape, the rising air and the pins can meet each other.