going to order parts for a valve job on my 25 this weekend. noticed some buggered threads on a few head bolts so I'm going to order a new set.
How do I tell whether I have got a low or high
Low heads were used 1909-17, so it's likely you have a high head on your '25.
With the head on, it's easiest to study the bolts where the water outlet is attached. On a high head, longer bolts for the water outlet would clash with the head bolts, while they would go free on a low head.
With the head off, it's easy to spot a low head by the ridges around the spark plugs inside the combustion chambers.
When you reinstall the head bolts, be sure to clean out the holes in the block. Just a little debris in the thread holes will bugger up the new bolt threads and ruin the block theads, necessitating drilling out and installing repair threads. I learned this the hard way.
It's a good practice to first test fit the bolts without any head gasket - if they can be screwed down without bottoming out, then they'll work with the gasket in place.
Since your going to pull the head I would clean out all the water passage ways in the block and head besides cleaning out the thread holes.
Use air pressure if you have it. Use a 1/4" drill carefully to clean out small holes in the block. You'll be surprised how plugged up they will be.
If you have one use a bottom tap to clean out the threaded holes. There usually is some debris in the holes. Be sure to get out as much of the debris as possible.
It may take a little longer but you'll be glad that you did it.
A Model T needs all the cooling it can get. This will help.
The bolt height on the high head is 2-11/16" and 2-1/16" on the low head.
Here are a couple of pictures from a previous thread that show that height difference. The "low" head is on the right:
The inside is also different:
The low head is at the top and has a small rim around the plug hole on the inside and a ridge at the edge. The high head is smooth inside and is shown at the bottom.
I hope this helps.
Ken in Texas