Turbocharged motors are under lots of pressure and stress, which leads to heat and often times overheating when your cooling system is inefficient or fails. Consequently warped heads and blown head gaskets can be unfortunately common when you start modifying your turbo car.
Many turbocharged engines are iron cast for strength and durability reasons. To improve weight and heat dissipation, most cylinder heads are aluminum which can become warped at high enough temperature.
When this happens one of the first modifications that’s first on the list to add to your turbocharged engine is the multi layer metal headgasket and ARP headstuds. ARP hardware is rated over 200,000 psi of tensile strength, making them the perfect fastener for your boosted engine.
The engine we’ll be showing you how to change your headgasket in is a Nissan RB25DET, most famously found in the Nissan Skyline R33. Our vehicle in question suffered a warped head during a drift event when the cooling fans failed, subjecting the engine to temperatures above 220ºF under load.
Let’s get started on how to change your RB25 headgasket by raising the front of your vehicle and safely securing it with jackstands and chocks. Always operate with safety in mind and make sure to follow the safety guidelines.
Drain the engine completely of oil and drain the remove the radiator along with your fan assembly.
Remove the turbocharger assembly by disconnecting the oil lines first, and then disconnecting the downpipe from the turbocharger. Once you have these disconnected, remove the exhaust manifold and turbo as a whole from the engine or remove the turbo first.
Now with the exhaust manifold and turbo removed, disconnect your intercooler charge piping from your throttle body. Disconnect the vacuum lines from the intake manifold and your throttle cable. If you are still using the stock intake manifold, you can disconnect the throttle body to clear the spark plug cover first.
Unbolt the intake manifold and remove the entire assembly from the engine, make sure to remove the intake manifold gasket carefully to prevent bending.
Now you are ready to remove your spark plug cover by unscrewing the hex bolts located at the top of the engine. With the spark plug cover removed, unplug your ignition coils by depressing the locking pin and gently pulling. Never pull on the wires to disconnect your ignition coils in your Nissan RB25DET.
Unbolt the spark plug brackets and remove all your ignition coils.
Now remove the 12mm bolts that hold the spark plug valley guides, and remove the spark plugs to make rotating the engine much easier.
Unscrew the bolts that hold your valve covers in place, exposing your cylinder heads.
With both sides removed, you should now be able to access the camshaft caps and your front timing cover bolts.
You are done for now on the top of your engine, now it’s time to undo the front of your engine to access your timing belt. Start this process by either undoing the tension in your alternator bracket, or simply removing the water pump pulley. Now unplug your cam angle sensor in preparation of removing it.
Removing the top of the front engine cover is the next part of our how to change your RB25 headgasket article. There are long hex bolts that go through the front cover and into the back of the timing plate. Undo these top bolts and make sure to get the tricky one in the middle.
The RB25DET cam angle sensor is the next part that must be removed, undo the 12mm bolts that hold this part in place.
Once you have these bolts removed, the cam angle sensor can be taken out.
With the cam angle sensor removed you can now remove the top half of your front timing cover. You should now be able to see the cam gears and the bracket that the cam angle sensor mounts to.
Take out the bolts that hold the bracket in place and get ready to turn your engine to TDC. This can be done one of two ways, and both ways have their drawbacks and benefits.
You can turn your engine to TDC by removing your harmonic balancer and lower timing cover. You can access other parts of your engine, including your tensioner easier or simply slide off the timing belt by pulling up on your tensioner to relieve it.
If you are interested in removing the lower cover, continue on and if you intend on sliding the timing belt off skip ahead to pulling the camshafts out of the head.
You will be using a 1 1/16 drive to remove your RB25DET crankshaft bolt, and your harmonic balancer should come off with some wiggling. You may need a gear puller to remove your harmonic balancer.
Rotate the engine to top dead center (TDC) and check the marks on your cam gears to match your harmonic dampener. If you need to check our How To Change your RB25 Timing Belt Article, click here.
You are just about there in your quest to learn how to change your RB25 headgasket, but now it’s time for you to remove the camshafts out of the engine next in our guide. Using the camshaft service points, hold the camshafts in place while loosening your cam gears.
Remove the camshaft caps and line them up orientated in the same manner in which they were bolted onto the head. Keep these arranged the same direction and in the same position, if you get lost or mix them up, there are castings on top of each camshaft cap to identify orientation.
Now that the cam caps are out, you can remove the camshafts with the exhaust camshaft first. The camshafts will require you to remove the cam gear to access the bolt located behind the cam gear that holds the back of your timing plate in place.
With the cams and backing plate removed, you can now turn your attention to the cylinder head bolts.
There are small placement bolts located around the lower base of the cylinder head that are 10mm bolts, remove those before you get started on your cylinder head bolts.
You will need a 3/8th hex wrench to remove the cylinder head bolts.
Remove all the cylinder head bolts and now you are ready to lift and remove your cylinder head. If your cylinder head does not come off easily, make sure to check for those pesky 10mm locating bolts that are on the intake side of the head.
Take shop rags and clean up the coolant and any oil that might still be leaking out from the ports in your RB25 block and the cylinder head. Take a razor blade and clean the surface of your engine block, it’s iron so no worry about damaging it, but try not to gouge or scratch the surface.
You want to clean as carefully and as thoroughly as possible, scraping off all the old headgasket material until your block is clean. Run over it with acetone and a shop rag to clean the ports, spray brake cleaner down the oil and coolant ports with the oil pan off to clean things up.
Now that you have your head off, it’s time to have it serviced by a machine shop. Have the head cut to eliminate the warpage that occurred when the engine overheated.
You now know how to change your RB25 headgasket or at least remove the head, it’s time to select the headgasket you’ll be replacing it with. Stock thickness on this Nissan 2.5 liter engine is 1.1mm and you can opt for a thicker unit of course to lower your static compression.
There’s quite a few manufacturers to choose from here, HKS, Greddy and Cometic to name just a few. Take some time to browse our online catalog for more information on RB25 headgaskets available for your vehicle.
While you have your cylinder head removed, it’s a good time to service the shims and check the tolerances in your valvetrain. Check here for our How To Service RB25 Valvetrain article.
Checking the tolerances in your valvetrain is a great idea with your head off, and also time for any additional upgrades, like camshafts or a RB25 oil drain.
This cylinder head drain is meant for RB25 and RB26 engines, and it aids in bleeding off oil pressure that can build up in the cylinder head. This problem is most prevalent in the Rb25, and this piece helps the oil find its way back into the sump.
We’ll be removing the Welch plug in the back of the head and installing this unit in our How To Install a RB25 Oil Drain Fitting guide.
That does it for part 1 of our how to change your RB25 headgasket. Join us in Part 2 of this writeup, where we show you how to re-install the RB25 cylinder head and install your timing belt.