Mating Surface Prep


by Ryan King

Edited by Patricia Kalin

Have you ever wondered why a car doesn't run and drive as well as it did from the factory, after a repair?

Assuming the repair is otherwise performed correctly, the answer is usually pretty simple: poor preparation.

It happens all the time with amateur and professional mechanic alike...even at dealerships. Poor assembly doesn't just effect how the car runs and drives, but the reliability as well. The real question is, then: How do you maintain the way a car runs and drives, as well as its reliability after a repair? The first and most important step is, of course, preparation and there are very few places where this is more ignored than with the mating surfaces.

My personal approach to automotive repair – the thing that keeps my aging cars reliable mile after mile, and long after other cars with as much age and abuse are having problems, disabled, or gone, are the detailed steps I take to make certain that the repair I do is as good or better than when that piece was originally assembled at the factory.

When dirt, corrosion, foreign matter, or old gasket material "that won't come off" are left in between the two surfaces you are putting together, you get a result that's destined to fail.

When an engine or any other component is assembled at the factory, quality control is a factor that is carefully watched, assuring that there is nothing that will cause premature failure of the components, because it will ultimately cost the company more money.

Your repairs should be no different, because failure will cost you money, time, energy, and cause many other problems – possibly even death depending on the component you are working with and what you are doing with your vehicle.

Follow along with me as I prep an engine block deck for the installation of a cylinder head and show you the detailed steps to help your repair, restoration, or modification turn out just like it was new...or better. I'll include tips and tricks as well as the kind of result you are looking for from prepping a mating surface.

Prep Procedure

Block Mating Surface Before Prep

Block Mating Surface Before Prep

While preparation is time consuming, there are tricks that make the job quicker and easier. Knowing them will help to reduce your effort as well as improve the quality of the job you do. This is how the engine block deck mating surface looked before it was prepped. You can see old gasket material, residue, and corrosion all over it. Click image to enlarge.

Scraping the Mating Surface

Scraping the Mating Surface

Many people may be tempted to leave these contaminants, but that is a poor choice that may quickly lead to a leaking or blown gasket. It may also seem easier to use a brush or Scotchbrite pad in a drill, but my experience has shown me that this can easily lead to ruining the mating surface by eating away metal and making it uneven – again leading to gasket failure. Instead, the first step should be to use a cleaning method that won't damage the surface you are working on, followed by a more aggressive method meant to remove hard to remove gunk, and finished with a proper cleaning. In this case a putty knife/gasket scraper will remove the large deposits and chunks of gasket material without harming the block deck. TIP: Stuff cylinders and cover the lifter valley with paper towels to help keep them from getting filled with crud that will have to be removed before assembly can begin. Click image to enlarge.

Preliminary Wipe-Down

Preliminary Wipe-Down

Once you have cleaned off as much as you can with a gasket scraper (or another method for other surfaces), go over the mating surface with a rag that has a cleaner on it that will not damage the surface you are working on. In this case a strong degreaser that can also breakdown sealants will help remove the loosened gunk. Brake cleaner or mineral spirits will do the trick and will evaporate without leaving any residue that will need to be cleaned off. When using these chemicals, be sure to stay safe. The vapors from these are poisonous. I wear chemical resistant gloves such as nitrile or chemical handling gloves, as well as a respirator with the appropriate cartridges, and eye protection. Here you can see the mating surface already starting to look better, but it's not done yet, there is still corrosion, stubborn, stuck-on gasket material, and sealant. Click image to enlarge.

Stubborn Sealant Sealant Removed

Stubborn Sealant and Sealant Removed

Like brake cleaner or mineral spirits, gasket/sealant remover is a poisonous chemical and the proper protection (gloves, eye protection, and a respirator) should be worn when using these chemicals. Mask or protect any areas that may be damaged, such as painted areas or the lifter valley where a caustic chemical can get into the oiling system and break down the lubricant causing ring or bearing failure later on. Let the sealant remover sit for the necessary period of time and try to remove the stuck-on sealant. I was able to remove it with the gasket scraper I used before. Once the gasket material/sealant is removed, wipe the mating surface down with an appropriate cleaner on a rag (such as the brake cleaner or mineral spirits used above) to remove any excess sealant remover. Click image to enlarge.

Fully Prepped Mating Surface

Fully Prepped Mating Surface

Here's a close-up of the engine block deck surface after the final cleaning process. After cleaning with scraper and rag, use a wire detailing brush (powered brushes and cleaning pads are too aggressive as I mentioned earlier) to scour the surface until all left over corrosion, gasket material, and sealant are loosened. Once loosened, use a clean rag with cleaner to wipe it completely free of contaminants. Although time consuming, this process ensures a proper seal once you go to reassemble your engine. Click image to enlarge.

Corroded Dowel Bosses Cleaned Dowel Bosses

Corroded Dowel Bosses and Cleaned Dowel Bosses

Here's a unique problem with Ford block decks: dowel bosses. They corrode and have to be cleaned so new dowels will sit correctly in the bosses and line up the heads. In situations were the surface is tough but recessed in a way that a brush won't work, use a small flat blade screw driver (or similar implement for different surfaces) and carefully swirl it around, then wipe it out with cleaner on a rag, or bottle brush, or both. Click image to enlarge.


Although the techniques described in this story do not necessarily apply to all areas of automotive mating surface preparation, the detailed approach outlined here does provide a basic blueprint.

Different surfaces require slightly different techniques and tools, but the results should be the same: you want the surfaces to be as good or better then when they were assembled at the factory, without removing any material from the mating surface itself.

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Remember when you get impatient during any project, the result is only as good as the steps you take to get there.

Also, remember safety first whenever working on your car or you are in the shop. It's always a good idea to wear eye protection as flying debris or chemicals can cause severe and permanent ocular damage. Nitrile gloves are necessary, as well, to protect from dermatological problems and to keep harmful chemicals from being absorbed by the skin, which will enter the bloodstream and find their way to any place in the body. Even something that may seem harmless, with enough exposure, can potentially cause health problems.

Lastly, when handling dangerous, poisonous, or otherwise harmful chemicals, using any equipment or tools, or removing and installing any component, it's necessary to follow the safety instructions, and wear proper safety gear for those products and procedures. Please refer to any and all manufacturer literature such as current handling, use, and procedure instructions, as well as MSDS before starting any project. Those instructions should always take precedence over any provided here.