How to Replace Head Gasket

So, how do you replace a head gasket? Well, there’s no real answer, at least not in a single article. There are tens of thousands of different engines they’re all different. You’ll need to use a service manual, whether you’re a professional mechanic or a talented DIYer.

This article contains tips on replacing head gaskets. A fellow shop owner was having some difficulty with comebacks on head gasket repairs on the Gen3 Prius and asked for some advice. This article covers the advice I could think of at the time. So here it goes…

Head gasket tips


Clean everything thoroughly. Cleaning offers an opportunity to find problems.

Short story. After failing tech inspection at the track because I forgot to safety wire a caliper bolt, a friend recommended wiping/cleaning the motorcycle before tech. It worked (and still works) great.

I’ve noticed that mechanics with this habit don’t have comebacks and that the inverse is true; mechanics with messy habits (both on car and in work area) make more mistakes. Slowing down and cleaning up is a powerful tool to prevent comebacks.

Inspections before ordering parts

Once everything is disassembled and cleaned, do all the inspections before ordering parts (except maybe a head gasket set, but don’t open it).

This is more of an efficiency thing. If you keep needing to stop and start because you’re finding new problems and needing additional parts, you’ll waste a lot of time. It takes some time to “get back into it” every time you start again.

However, early inspection is also important because some findings will derail the project. If you find a serious problem your customer may decide that the car isn’t worth fixing. It’s better to find that out before needing to return a bunch of parts.

Ideally there will be only one break in the job. The flow should go: disassemble, clean, inspect, approve/order, reassemble. The approve/order step should ideally be the only break for the mechanic.

Stopping several times to do other work isn’t ideal either. Service writers should allow mechanics to stay focused until the job is complete.

Momentary interruptions

Distractions can lead to problems. If other people need things from you, tell them to come back in a bit if you’re in the middle of something. Anything short of a shop fire can wait for a few minutes for a good stopping place.

Visually inspect the cylinder head

Always visually inspect the cylinder head early in the process. The head is the second most expensive part next to the bottom end. If it’s damaged beyond cost-effective repair, it’s important to know that early in the game. Things to look for:

  1. Cracks around the valve seats radiating to the spark plug hole or between seats (or anywhere else).
  2. Damaged threads
  3. Deep dings or scratches that can’t be repaired
  4. Valve heads that are sitting low compared to the others

Visually inspect the cylinder block

The deck surface should be carefully scraped with either a wide carbide scraper (search Goodson Super Scraper for an example) or a new high-quality single edged razor blade. Be very careful not to damage the surface either way. Use light at different angles and bare hands to find any residue, nicks (high spots), or other issues. The final step should be wiping with a lint-free cloth and then with a bare hand.

Never use a Roloc or any form of abrasive. Using fine Scotch Brite by hand once the surface is nearly clean is OK though.

Block inspection

Check the block with a straight edge. Block warp specifications are usually around .001” per cylinder as a rule of thumb (a 4 cylinder would have a spec around .004”, an inline 6 would have a spec around .006”), and a V6 .003″, etc.

Block warp is usually not an issue, but “pockets” are. Use a .001” feeler gauge to search for these. If there’s also a warp, then you’ll likely need a 6” straight edge to find pockets. Inspect especially carefully in the areas where the head gasket failed. This is where pockets are likely to develop.

Turn the engine so that 1 and 4 are at TDC and check that the piston height is equal. Turn again to 2 and 3 are at TDC and check again. If any piston is sitting lower than the others then a connecting rod is bent.

Leaking rings

You can use transmission fluid as a quick and dirty check for ring sealing by setting all 4 pistons to mid-stroke and adding ½” of transmission fluid to the cylinder. If the level sinks on one and not the others, there’s likely an issue.

This has the added benefit of potentially freeing sticking oil control rings.

Obviously, the ATF must be removed before assembly or it will bend a rod.

Cylinder head inspection


Check the cylinder head for warp just like the block. However, I’d recommend always machining the cylinder head. It’s too hard to be sure there are no pockets.

Cam bore warp

It’s still important to check for warp though, especially in the cam bores. If the cylinder head curls up when unbolted from the block, and is then the sealing surface is machined flat, the cam bores will retain the warp when bolted down. Oil clearance on a cam journal is typically around .001” so very little cam journal warp is acceptable.

Typically heads bend like a banana with the pointy ends down. When you check for gap on the bottom it will be in the center.

When checking warp in the cam bore, the straight edge will rock because it’s high centered, making it hard to take an accurate reading. You can hold one end down and measure the gap on the other, then divide by two.

Leaking valves

Solvent check the combustion chambers. Simply level the head with the combustion chambers up and fill completely with solvent. Leave the head to sit as long as practical. If a valve is leaking the fluid level will drop and leave a trail down the intake or exhaust runner.

You can also use a Sioux vacuum tester at the intake and exhaust ports for a quicker check.


Ideally the cylinder head should be pressure tested. We have a pressure testing set up that’s very nice, but it’s also not inexpensive. AXE Equipment

It may make the most sense to sublet this work, especially if you don’t have the means to machine the cylinder head yourself since you’ll need to send the head out anyway.

Cylinder head resurfacing

MLS gaskets offer superior sealing, but only if the mating surfaces are perfect. Ideally the cylinder head would be surfaced to 20 RMA or less. The smoother the better. “Some roughness is necessary for sealing” is a myth with all gasket types. This was a statement directly from the engineering department at Stone/Ishino, so I’m sure it’s true.

We resurface in house, and we don’t use a profilometer because we don’t own one, but we did have Hasselgren Racing measure one of our first heads and it was 13 RMA. Ideally any sublet machine shop would be able to do roughness testing or have the experience to visually/tactilely confirm an appropriate finish.

Cylinder head bolts

Most modern engines are using “torque to yield” head bolts which are designed to work in the plastic range. This style of head bolt is more likely to need replacement than traditional head bolts.

Some engines require new bolts with every installation, but most require measurement of bolt length. There is a specification for maximum length in the service manual.


Head bolt threads

Clicking and popping while torquing the cylinder head is a bad sign; it means the head bolts are binding. Binding will lead to uneven clamping force and may lead to failure.

The head bolt threads should be cleaned on a fine wire wheel and then oiled.

The threads in the block should also be cleaned. This can be done with an old head bolt with flutes cut in it. Run the bolt in and out with the cylinder head off and plenty of lube. Use spray solvent and compressed air applied to the bottom of the threads to blow debris out.

Cleaning and inspection

The deck surface and cylinder head surface should be absolutely clean. They should be pretty close to clean already since the head was machined and the deck surface was scraped prior to inspection, but any oils should be removed with a lint free cloth or paper towel (Kimtech, Goodson, etc).

Spend some time making sure everything is ready prior to setting the head down. Dowels in place? FIPG at the timing cover joint? #1 TDC? No rags or fluid in the cylinders.

You can use two old head bolts with the heads cut off and slotted for guides, or just use caution when placing the head onto the dowels. The dowels are important, so be sure they’re in place.

Rocker arms

It’s easy for a rocker arm to work out of position. Look carefully for any out of place rocker arms before putting the valve cover back on. A misaligned rocker arm can fall off a day or two after the repair resulting in an engine speed knocking noise.

Post assembly quality control

Add a lube sticker for the follow up check and heat tab for evidence of overheating. Clear all trouble codes if the battery wasn’t disconnected during the repair (but it should be disconnected during the repair).

Run the car in inspection mode until the cooling fans cycle on and off at least once. Listen for any abnormal noises, misfires, etc. Lift the car and check for oil and coolant leaks. Allow the car to cool and check the coolant level.

It’s a great time to clean your area and put away all of your tools and find any bolts or parts that didn’t make it back on to the car. Or any tools that you might have left on the cowl.

After making sure that the oil and coolant are both right on the full line (which is important for the follow up check in 500 miles), take the car on a test drive and note any issues, including issues not related to the work done. This can help with customer phone calls later.

Follow up check

Check the oil and coolant levels and look for external leaks. Optionally change the motor oil. Regardless, add a new lube sticker. This is your chance to find and fix small issues that may become large issues.

Other reading

In this article about MLS gasket sealing from Victor Reinz, the author suggests shining light under a straight edge to find pockets in the sealing surface. Since deviations over .0005″ (1/2 thousandth inch) over a short distance (~2″) can cause sealing issues, and the smallest feeler gauge is typically .001″, the light method has merit.

Alternatively, draw filing or stoning to identify and knock down high spots can also be used to find/eliminate pockets. However, this is an art/skill and shouldn’t be learned during a repair. You can also research scraping (an even more difficult to learn but very precise flattening method) if you’re interested, but I’ve never heard of it being used for deck surface repair.

When I was studying to become an ASE Master Machinist (back when the certification still existed), a lot of what I read was written by Larry Carly for Engine Builder magazine. Many of his articles including this article titled How to Prevent Repeat Head Gasket Failures can be found on his website, which is frozen in time somewhere around the year 2000, but with modern ads.

Here’s another article on cylinder head sealing. I got a chuckle out of this because it’s a couple of Larry Carly’s articles on a machine shop’s website. The reason I thought that it was funny is that when I started back in 2001, I borrowed (stole) one of Larry Carly’s articles on clutch repair. With credit, but without permission.

Further rambling

In one of the articles the specification for pockets is given as “no sudden irregularities that exceed .001″ in any direction”. What’s sudden? I don’t know. I don’t think anyone really knows what the tipping point is.

Ideally the surface would be completely flat and glass smooth, but nothing will ever be ideal. The farther away it is from ideal, the more likely the head gasket will fail.

You just need to be aware that if there’s a pocket, and even if only a .001″ feeler gauge fits, it’s more likely to fail. Total warp isn’t the only issue. That’s why we always machine the head. Pockets are normally two sided. On the deck and the other on the head. Machining the head will eliminate 1/2 the problem.