Head bolts

[mbowen]

Which of these head bolts are correct for a 1925 engine? Also, is this a high or low head?

[George Mills]

How pure you want to be?

I can get dimensions and add them as a postscript but a 25 would be a high head…

And the bolt on the left is the more accurate!

Can I have some fun? Anyone care to comment why?

[Mopar_man]

Make sure you clean up the threads in the block and vacuum out all the dirt/rust. That way you won't have an issue when you tighten them.

[mbowen]

How pure you want to be?

I can get dimensions and add them as a postscript but a 25 would be a high head…

And the bolt on the left is the more accurate!

Can I have some fun? Anyone care to comment why?

Don’t need to be all that pure, just want them to work. 13 of the the ones on the left, and two of the ones on the right came off of my engine. I figured the ones on the left with the longer threads were correct but just wanted to make sure.

[mbowen]

Make sure you clean up the threads in the block and vacuum out all the dirt/rust. That way you won't have an issue when you tighten them.

Yes, I’m in the process of cleaning the bolt holes. According to my caliper, I’ve gotten as much as 0.20” of crud out of some of them. I have a 7/16-14 bottoming tap and a set of bore brushes on order to finish the job.

[Norman Kling]

After you clean out the threads, place the head on the block without a gasket and insert the bolts turn them down until they stop turning. Do not torque the bolts just stop when they stop turning. The heads of the bolts should touch the head. If they are above the head, they are too long, or the threads are not deep enough. If one is too long, grind off a little from each one so they will all be the same length, then in the future, it won't make any difference which one you use. After all the above, remove the head and install the gasket. Torque according to the diagram in the book, or other posts on this forum. Should be 50 ft lbs all around, but you need to start in the center of the head and work out toward the ends and sides evenly. Torque about 25, then go back again and torque to 30 and them all until you get them to 50. You shouldn't tighten one all the way before you tighten the others or you run the risk of cracking the head.
The standard head for a 25 is a high head, however some have been replaced with a low head to raise the compression.

[Dan Hatch]

Post a picture from the side of head to tell if high or low. You

[Oldav8tor]

Miles, how about a photo of the underside of the head? Low heads have a raised area around the spark plug hole.

Attached is a graphic from Model T Guru Dan Treace showing correct bolt length. Again, be sure they don't bottom out.

Follow the correct re-torque sequence and torque to between 45-50 ft-lbs. Too much torque risks stripping the threads.

[TFan]

George, I'd say the bolt on the right looks to be a domed head for the improved car which could still be possible on a late 25. Do I win. Jim

[George Hand]

I have always felt that new headbolts from a vendor is a wise move, if your head has the raised area around the sparkplugs as shown by Tim's photo that is a low head & would use the shorter bolt. A bottoming tap should be carefully run in each hole & debris cleaned (vacuumed as indicated), a bottoming tap can be made from a spare/common tap by grinding down to bottom threads, also chase the threads with a regular tap first. When taping or chasing a thread if turning the tap yields resistance or snag, stop & remove tap clean the hole lube the tap & reinsert. At the bottom do not, force a hole can be created into the water jacket if you try hard enough. In a "high" head that use the longer bolts the domed bolts came out for the later engines '26-7 as a dress update George

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[RajoRacer]

This is the best tool for cleaning out the block head bolt holes !

[Allan]

The bolt on the right is an earlier one. It does not have the reduced diameter shank like the one on the left. The domed ones used on the improved cars have a pronounced dome which is hard to miss if seen more directly from the side rather than from below.

Allan from down under.

[George Mills]

Ding - ding - ding -ding...we have a winner!

Ford went to rolled thread head bolts in 1923.....the one on the right is a cut thread version.

[mbowen]

Which of these head bolts are correct for a 1925 engine? Also, is this a high or low head?

70BE42EB-A2F8-451B-B308-4DFB32AB702C.jpeg

Thanks Tim. By your first two pictures, I do have a high head, as others have said is correct for 1925. My head does not have the internal bosses around the spark plug holes, and the length of the bolts are about 3-/1/4 from the bottom of the head to the end. Also, thanks for the tightening sequence; I now have it in two places, so I should be able to find it when the time comes. What is the source of that diagram? The Ford Service doesn’t specify it in the section on installing the head.

[mbowen]

I have always felt that new headbolts from a vendor is a wise move, if your head has the raised area around the sparkplugs as shown by Tim's photo that is a low head & would use the shorter bolt. A bottoming tap should be carefully run in each hole & debris cleaned (vacuumed as indicated), a bottoming tap can be made from a spare/common tap by grinding down to bottom threads, also chase the threads with a regular tap first. When taping or chasing a thread if turning the tap yields resistance or snag, stop & remove tap clean the hole lube the tap & reinsert. At the bottom do not, force a hole can be created into the water jacket if you try hard enough. In a "high" head that use the longer bolts the domed bolts came out for the later engines '26-7 as a dress update George

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I was thinking new bolts might be in order; now I have enough information to order the correct ones.

[John kuehn]

The tightening sequence is pretty much the same for any flat head engine or any automotive cylinder cylinder head.
Start from the center and work your way EVENLY out will do it every time. It’s a basic automotive procedure.

[Oldav8tor]

A couple more things to consider - A high head has a little more coolant capacity but has slightly lower compression than a low head. Since you have the head off, you might want to consider whether to replace your head with a high compression head from either Z (Michigan) or Prus (Ohio) to give you a little HP boost. My T has a Z and shows 25 HP on the dyno at 1600 rpm. From the outside it looks identical to a high head although it is aluminum, not iron.

As to the tightening diagram, I got it from this forum. Here's a thread from ten years ago <http://www.mtfca.com/discus/messages/17 ... 1308589982> citing some of the original sources.

[TRDxB2]

Just some pictures from old posts to show differences side by side

[John Codman]

The tightening sequence is pretty much the same for any flat head engine or any automotive cylinder cylinder head.
Start from the center and work your way EVENLY out will do it every time. It’s a basic automotive procedure.

I agree. The way that I was taught back when the world had corners was to start with the center head bolt at 1/3 of the final torque and work your way outward in a clockwise (or if you prefer) counterclockwise direction until you have torqued them all. Do it again at 2/3 of the final torque, then end up with the final torque. If you are really anal then check them again in the same way at the final torque. I have used this procedure on every automotive cylinder head that I have ever installed - including the last one, my '27 T, in 2014. I have never had an issue with any cylinder head that I have installed. BTW: Use a known accurate torque wrench, a click type would be my preference.

[Hudson29]

Ford went to rolled thread head bolts in 1923.....the one on the right is a cut thread version.

I have heard of cut or rolled threads before but never understood the difference. How are they different? Can I use my tap & die set of both?

Paul

[Norman Kling]

One more thing which I don't think has been discussed yet is the final torque. If you have an iron head, warm up the engine to normal operating temperature and then check the torque again while hot. You will usually find they loosen up a bit. If you have an aluminum head warm up to normal operating temperature, and then let it cool down completely, such as waiting till the next day. Retorque. After you have driven a ways, repeat. When it stays at the proper torque, you can just leave it as is until next time you need to remove the head.
Norm

[Allan]

Paul, if you look at the two bolts in the photo, the shank on the bolt on the right is the same diameter as the thread. That thread is made by the die cutting metal away. On the left bolt you will see a necking down under the bolt head to make a slightly undersized shank. That thread is a rolled thread, formed by a different type of die displacing metal rather than cutting it away. This raises the diameter of the thread somewhat, thereby making the undersized shank necessary. Rolled threads are stronger.

Each has its advantages. In model T's the rolled thread head bolts are less likely to rust up solidly in the cylinder head due to the lesser diameter on the shank. Cut threads on the six hub bolts, with their full diameter, are better in wooden wheels because they fill the holes drilled in the spokes.

Others will know more.

Allan from down under.

[Hudson29]

Thanks Allan, you did a pretty good nutshell explanation of the difference. I should be able to look at the threads for an expansion & understand which is which.

Paul

[mbowen]

In the aviation world we’re taught that rolled threads are stronger than cut threads. Here’s a video that shows why: https://m.youtube.com/watch?v=rwArBBcUNr4