Temp sender

Hi folks I'm thinking about installing a temperature sender in the blanked hole between cyls 2 & 3 on my '69 Merc800,1/8"-27NPT I'm thinking.
I have a Mercruiser gauge which I would like to use,so could anyone tell me the spec for the sender I need,resistance and temp. range,I'm also at a stage where I have no qualms about tapping the hole bigger if need be.And I have trawled the net looking
The markings on the gauge read SW 479 J 12V NEG GND 07.12 V NEG GND, I understand,the rest ????????
Or is there any chance a standard sender will work?
Any advice,as always greatly appreciated,except taking the black beauty and dumping her over the end of the pier,

Got a pic of the gauge? I think i see a problem.

You need a low reading gauge as the motor will hardly ever get over 120 degrees, better off installing a water pressure gauge 0-15psi...

I use one on my 850. The lowest temp gage set up I could find barely moves when at operating temperature. It can be an indicator of over heating, however, should the needle suddenly jump up.

Jeff

Here are a few pics of the panel.
I figured I would need a low temp sender but didn't think it would drive the existing gauge and I did think about the pressure and the thought crossed my mind that I could use the oil pressure gauge,but again would the water pressure sender drive the gauge?
Though as Jeff said if it was installed and the indicator made a sudden rise I would know it was over heating.One of the gauges has to go to make way for a fuel gauge,decisions decisions
There is a trim gauge in the panel as well but as I have no trim sender I was wondering if it was possible to retro fit a working sensor on the old two ram trim system from the '70's?
Why ?? I can hear people asking.Well why not?

I'm hooking up a head temp gauge on a 1997 30hp 2cyl 2stroke. What temp range should I expect to see? Thanks.

Under 150 or even less.


This discussion has prompted me to look into using a gage head other than the 'correct' temperature one to get that needle higher in the range. If I find the right gage head, this should work for all it's doing is looking at resistance.

Jeff

The gauge has 140 in the middle so maybe that's considered an average operating temp? Guessing.

Nice gauges.
Operating temperature seems to vary from outboard to outboard and from two cycle to four cycle or on this side of the Atlantic,two stroke to four stroke.And inboards run hotter again,hence the higher temperature required to move the hand up the gauge as opposed to the cooler operating temperature of an outboard would barely move the hand out of the cold mark.That's my understanding of what I have read on the www.
What I'm wondering now,is it the resistance in the sender,be it head or fitted in the water jacket that drives the gauge or resistance in the gauge itself or a combination of both.
If the sender for a Merc outboard,with say 450ohms,shows cold.When resistance drops to,say 200ohms,normal operating temperature of 120F,wouldn't the unmarked gauge,like mine,rise to the top,indicating normal working temp. Or have I misunderstood some or all of what I have read about water temperature gauges and senders

Thanks here's another view of them in they're mounting cups.

I think the lower the sender resistance the lower the gauge needle rise to a max point. Correct me if I am wrong.

I feel a bucket of hot water and thermometer experiment coming on...........after the new year.

A couple of things to remember about a inline... they run warmer at idle that at WOT,sender needs to be at highest point on block as the middle of cylinder block can give false results, that why merc plugged em and moved discharge to top of block as a weak impeller would only fill block 2/3 full and discharge would show it pumping.Also different water temp in different lakes will effect temp reading, a 70 degree lake may read where you want it but a 85 degree may show its hot at idle.This is why water pressure guages was offered for inlines and no temp guagesmade for them.