I need to use washers under my head bolts.
What size are the the bolts?

7/16"

You should not need washers… Have you cleaned out the bottom of the holes. You should be able to turn the head bolts in to touch the head without the gasket in place if the holes are clean. Respectfully, John

I may be all wrong on this, but I'd be afraid the use of washers would mess up torque readings and thus screw things up.

I agree with John. You shouldn't need washers. Run a bottoming tap into the holes and blow out the debris with compressed air.

What if its a aluminium Z head ????

I think the alumin head is the exception.... spreading the load.
David.

I'm about to change a head gasket for the first time and I plan to order a Tap in advance, please confirm the size, thread and length I'll need for a "bottoming Tap" for my block?
Thanks

I used an extra head bolt as a bottoming tap - I just made two cuts on opposite sides along the screw across the threads with an angle grinder.

For limited use like clearing out old soot from the treads it'll do. Remember to scrape the bottom of the hole with something sharp edged and blow out whatever's there before and after using the shade tree "tap". And use safety glasses

Good idea Roger I'll have a look for a spare stud. Today I made an attachment to go on my vacuum cleaner with a 1/2 inch sucker hose. Hopefully I can suck out any crud after I break it up if not I'll blow it out.

I use an under size drill to loosen the junk in the bottom of the holes and blow out the debris before using the bottoming tap. The tap itself will bottom out on the crud if it is compacted by the head bolt.

Hope this helps.
Allan from down under.

I understand the need to check the depth of the thread and clean the bottom of the hold but I am trying to address a head gasket problem without having to remove the head.

See the thread where there is a discussion about my have to remove the motor for more info.

Fred, the size you need is 7/16" X 14 NC (national coarse), for cleaning out the head bolt holes w/the head in place you'll need an extra long tap. Roger's suggestion of using an extra bolt w/grooves ground into it is a good one.

You could pull one head bolt (actually a cap screw) at a time and measure the depth of the hole to see if'n you really need the washers.

That makes sense Allan, ta.
Thanks Ken, so if I buy a tap it will be a 7/16th x 14 NC.

Fred, I use washers under all my head bolts, most all modern engines use them. I use the hardened type and add a drop of oil between the head of the bolt and the engine head, usually use the T spark plug and head bolt wrench and don't have any problems ever. KGB

Cut slots on a 5/16 tube and turn it down in there and hitting it with a air gun. Wear glasses.