re tapping the hole would make the threads larger or is there a way to do a helicoil since the head is cast iron? also I am trying to add a picture but not figuring out how to get it added?

Hmm. My first question is where are you looking to put the water temp sensor? Is it the factory location just below the thermostat?

If that's where you intend to install it, then the factory thread size for this hole will be 5/8 UNF. Now if the previous owner did something to screw that up, I can't really help you there. Some other OEM and a few aftermarket water temp senders are going to be 3/8 NPT (Chevy, Ford, etc). Most new temp senders (most senders for that matter) are 1/8 NPT. To make it fit, you'll need an adapter. Or you can make one yourself like I did out of the old sender.

Cut the end off the sender, drill it through, and then put the shell into a vise and tap it for 1/8NPT.

I hope that helps. If you're having a different problem, please post up some pictures. I work better visually.

Unless someone has re-tapped the head, it will be 5/8-18 (UNF).  It's not a common bolt size that you are going to find at the home center.  A local nut and bolt store may have one.  

Electric aftermarket temperature gauges typically do not have that thread size on their sending units.  They may have a similar looking but larger 3/8 NPT.  Many aftermarket and Smiths mechanical temperature gauges (capillary tube type) temperature gauges will have the 5/8 UNF expansion bulb to screw into the head.

You need to use a matched sending unit and gauge regardless of the thread size.  You cannot use the Mini sending unit with an aftermarket gauge.  Likewise you cannot use a Smiths (Mini) temperature gauge with an aftermarket sending unit.  The mixed parts are not calibrated to work together.

If you are trying to use the previous owner's aftermarket gauge on your Mini, make sure you have a matching, smaller sending unit and use a thread adapter to the Mini cylinder head.  Many aftermarket gauges will have either a 1/8 NPT or 10mm threaded sending unit.  That will be much smaller than 5/8".

Hey guys I am new to the form. I am having issues with my 87 mini 998 I purchased. A few days ago I decided I was going to hook up the temp gauge sine the last owner installed an after market gauge and it looked like an easy job. They had the sensor plugged with a bold, and when pulling it out found out not correct bolt. It is too small to fit the threads so was filled up with silicone to fill the hole. So I cleaned it all out and started to put the sensor in it and it is too big to fit? I asked tech support for minimania to see thread size to get bolt size to plug while ordering one and they said 5/8 fine thread? I chased to find a bolt and is too big. Got sensor in mail ant it is too big also. Call 7ent.com and the also said 5/8 fine? Not sure where to start now?