MTFCA Forum

I removed several head bolts from my 27 coupe. I noticed that only about 1/2 inch of the threads were engaged into the block. I would think there should be more engagement. Any comments?

About normal. That is why good new bolts and holes carefully cleaned to full depth, with those block threads chased with a bottom tap, so threads are clean and sharp for best result.


Note the bolt in far right distance, the amount of threads still in the cylinder head, and only the end threads do the chore.

That's another reason I use headbolts from a local GM Holden 6 cylinder motor. they are about 5/16" longer and will pick up more of the thread in the block. i have yet to pull one of those. they are specially needed when fitting an alloy head with washers under the bolthead.

Allan from down under.

There must be a significant difference in early heads compared to the later. I always have the concern that the head bolt bottoms out before it can torque properly. I find that the bolts can stretch enough and/or enough rust/dirt gets into the threaded hole to not allow torquing. After a lesson on not sealing a head gasket once, I frequently go for new bolts to be sure.

Here is what I did on my car. I used a bottom tap as Dan suggested and blew out the chips with an air gun, being careful not to leave any chips in the cylinders. I measured the thickness of the head. I then screwed a bolt into the block all the way down, finger tight, using some oil, without the head on, and marked the bolt where threads were at at the top of the block. I then added the thickness of the head to the depth of tread engagement and ordered bolts from McMaster Carr that were that were slightly shorter. I ordered grade 8 bolts but grade 5 will do. They come with different hex sizes so if you are not using washers get the larger hex. When the bolts came I placed the head on the block without a gasket and screwed down all the bolts finger tight using some oil. This is to make sure that none are bottoming out in the block. The holes in the block can be slightly different depths so it is important to check all the bolts. If some are slightly too long the ends can be ground off or slightly shorter bolts can be ordered.

I used the standard torque sequence and checked the torque after the first engine warm up and then in about 100 miles and again at 500 miles. If you are using an aluminum head use thick stainless washers, not the ones that come with the head, and follow the torque recommendations from the manufacturer.

I too don't like the short thread engagement of the stock bolts. I wish someone would make exact looking thicker heads from the stock Ford on a longer bolt.

I too have cleaned the threads (7/16" x 14 for those that don't know) and then measured the depth with the depth portion on a digital micrometer. If you don't want to do all the math, put the head on without a gasket and measure from the head bolt surface on the head, through the head and into the block.

If using a stock head, reduce that measurement to the next common size of bolt. When you get them, do as Grandpa Ford does and run them in with no head gasket to make sure they all seat., by the time you add a head gasket there should be no worries of them bottoming out.

If using an aluminum head with proper thick head bolt washers, set a washer on the head before measuring (still no head gasket), and again, reduce the measurement to the next common size.

However, while standard bolts will work, real head bolts have a slightly larger shank portion at the top which properly locates the head. so if you have high compression pistons or bigger valves, this could cause issues if you are already close and require perfect head alignment for things to fit. This is where Allan's use of a different application of headbolt comes into a better use.

Also, for those that don't know....because I see this alot in my line of work, bolts diameters are measured on the diameter of the threaded or shank portion---not the head size, and the lengths are measured from under the head of the bolt----the head is not included in the measurement.

To prevent rust chips, dirt and debris from getting down into the cylinders and lodging between the pistons and cylinder wall on top of the rings, it is best to totally seal all four cylinders with masking tape. It is much better than using rags, which will not prevent debris from entering the cylinders Through the wrinkles and folds of the rag. Be sure to remove the intake manifold and blow out the intake valve port from the top to ensure no debris remains that can be sucked into the cylinders after reassembly. Jim Patrick

PS. As you clean each bolt hole, be sure to place a piece of masking tape over the cleaned hole, so no debris from the holes you are cleaning fall into the newly cleaned holes.

In this video, at 5:10, I talk about head bolt engagement, length, stretch, etc. It might prove helpful to this discussion. https://youtu.be/WLcaqM89iQE?si=u0V_VW2cYYcy5xSn. GrandpaFord's advice is very good.