Engine I'm building is a 26/27 block. I installed high compression piston. I just put the head on without gasket to assure myself there is clearance between pistons and the head. This is where I found my fancy dancy domes 26/27 head bolts are too long by about 1/2 inch. Do I assume I have a low head?
How can I tell if the head is a low head?
If I decide to go with the low head, is there going to be any issue with running the high compression pistons?

Tyrone
I run a low head with high dome pistons on my '13. No problems

You can look at the area near the water outlet. If you have two different types side by side you will notice that the gap between the head bolt hole and the water outlet hole is greater on the low head.

I will check that Herb, does this gap mean the water outlet in the block does not line up with the water port in the head?

This engine was in a running car when I got it but owner was deceased when I got it. So could not ask any questions.

A low head will work on a later engine, as Les S says, but sometimes the high-dome pistons will hit the spark plug. And you would have to have shorter head-bolts.
There are so many little details different between the high and low heads, and minor differences through the many years. Easy to tell high and low apart when you have one of each side by side. Not so easy without both.
One quick thing to look at is inside the combustion dome. Where the spark plug goes in, it is essentially smooth (but rounded slightly and rough cast) in the later high heads. In the earlier low heads (up to about 1917), the hole the spark plugs goes into is raised a bit on an otherwise flatter combustion area. If the area is raised? You can't miss it.
I will go out in a minute and see if I can find a couple to measure.

The low head I was able to reach quickly was just over two inches through the bolt holes according to the caliper I could reach it with (about 2 1/8 inch). I couldn't reach a high head well enough to get an accurate measure (it was under and behind a bunch of stuff), but the caliper indicated that it was somewhat more than two and a quarter inch through the bolt holes. From that, you should be able to confirm which era head head you have.
Drive carefully, and enjoy, W2

Wayne reading your post in regards to the spark plug bore inside the chamber. If I read you right, this head I have, the spark plug bore does indeed extend into the chamber, off the roof of the chamber. The bolts I took out of the head were not head bolts but grade 8 bolts about 2 1/8 long. So either way, it is a low head. I will put in spark plugs and see if the pistons hit. However, I believe I would better off in the long run staying with what should be on there. Thank you all.

I could only use a high head with my HC pistons. They would have hit the head if I tried to install a low head.

It looks like the head on top of photo. So yep. its a low head. Darn.

Tyrone

High head on left and low head on right.
Ron the Coilman

Bolt holes on a low head are 17/8" high. On a high head they're 25/8".

Tyrone, I'll trade a good high head for your low head and pay the shipping both ways.
Dale