Take pictures of your setup prior to disassembly and note the positions and orientations of all accessories that are bolted to your exhaust parts, or any that need to be removed. Shift and other accessory brackets are commonly bolted to the exhaust parts. It is also good at this point to double-check that the replacement parts have all the same mounting bosses and other provisions needed to reinstall accessories (this should also be done prior to purchasing replacements, by viewing the product photos).
Exhaust manifolds are heavy, so be prepared prior to unbolting them. This is especially important for manifolds that are fixed using bolts (and not studs), since upon loosening and removing the bolts, the manifold may drop. Some cast iron big block manifolds can be 50 lbs or more each, and inline 4/6 manifolds can be 60-75 lbs. Building a rig with 2x4's and rope/chain to support the weight of the manifold can be very helpful.
Be sure to fully drain any water/antifreeze from the system, which will help prevent messes and reduce the overall weight of the exhaust parts.
Prior to unscrewing any bolts or nuts, it is a good idea to soak them with a product like PB Blaster or other penetrant, and then wire brush them with a stiff metal-wire brush. This will reduce the chance of stripping or breaking.
The actual removal steps will vary based on engine, but usually you want to remove any accessories first, then the exhaust hose, bellows, and water lines (usually attached via hose clamps), then the risers and elbows if applicable. The manifold is usually the last part to be removed as it is the heaviest. Removing everything that is bolted to it, including any pipe plugs, can help reduce the weight as much as possible.
Installation Surface Prep
Remove all paint from the gasket mating surfaces of your new replacement exhaust parts prior to assembly. Use acetone or lacquer thinner if available. A razor blade or gasket scraper can also be used but care must be taken to not damage the surfaces. Once the gasket surfaces are clear, it is a good idea to use compressed air and blow out any debris from both the water and exhaust passages. Some manifold manufacturing processes can leave behind metal debris and particles.
With the old manifolds removed and prior to installing the new manifolds, inspect and clean as necessary the mating surfaces on your engine head(s). Ensure there is no debris in the exhaust ports. Now is also a good time to inspect the exhaust ports and valves to make sure they are in good shape and do not show any signs of rust or corrosion. If your previous exhaust parts were failing and allowing water into the exhaust passages, your engine may need repairs.
Bolts and Hardware
In many cases you can reuse your previous bolts, studs, and nuts, if they’re still in good condition. You should replace lock washers where appropriate. Refer to the exhaust manifold bolt guide if you need new hardware.
Steps will vary by engine, but generally doing the reverse of the removal steps will be all that is necessary. In some cases with newer electronic engines that have sensors in the exhaust, you may need to reset them after installation (or replace them as needed). In those cases prefer refer to your owner's manual or a service manual prior to starting this project.
Unless otherwise stated in your product or manufacturer manual, gaskets should be installed dry without any gasket sealer, silicone, or compound. When installing riser and elbow gaskets, make sure they are in the same orientation as before (for gaskets that are not symmetrical).
Refer to your manual for proper bolt torque. If not available, use this chart:
Plugs and Fittings
When installing pipe fittings or pipe plugs care must be taken to avoid over-tightening. Most pipe plugs and fittings are tapered and an overly-tight pipe plug or fitting can crack the casting. Do not use thread sealants which contain Teflon , and do not use Teflon tape. Use a high-heat sealer designed specifically for exhaust manifold plugs, like the Permatex High Performance/Temperature Thread Sealant. Do not try to screw tapered plugs in flush with the casting; they only need to be snug.