Here you can find information regarding the assembly of the Chevy 2.5L DOHC engine. In this guide we will
start from the inside of the engine including the
crankshaft, connecting rods, and piston ring installation and then move outwards all the way to the pulley belt
system. Along the way correct procedures and torque specs
will be given to aid in the assembly of the engine. Feel free to start from the beggining and work your way
outwards or skip ahead to your current position in the engine for what you may need.
Crankshaft Main Caps Installation
The 2.5L engine block main bearing caps should be inspected for any defects or flaws before installation.
Be sure to lubricate the bearing surfaces prior to installation. Once prepared you can place the caps onto the crankshaft
and begin tightening the bolts down in a multi stage process. The first being 10 ft-lbs starting from the middle and going outwards.
The second time around you can go to 15 ft-lbs and then for the third time you turn each bolt an additional 70 degree turn. Be sure to
go through each bolt during each step and work your way from the inside or middle caps to the outside or outwards bearing caps.
Once finished you should be good to continue with the rest of the engine.
Chevy Impala 2.5L DOHC Main Cap Torque Specs : 15 ft-lbs + 70°
Piston and Connecting Rod Installation
To install the pistons and connecting rods you must first install the piston rings into each piston. Be careful
not to stretch the rings or break them during installation. Make sure to put the correct rings in the correct
positions, this can be determined by looking at
the instructions given with the new rings. Each ring manufacturer is different so be sure to check for your
specific rings. Once the rings have been installed you can now fit the connecting rod bearings into the end caps
and lube them up with oil or lithium grease. The piston can now be lowered into the cylinder, make sure the dot
or mark is facing the front of the engine and that you don't scratch the cylinder.
Once installed you can match the connecting rod caps with the correct rods and start to torque the connecting
rod bolts to 10 ft-lbs and then to 18 ft-lbs followed by an additional 100 degree turn for each bolt. After all are done rotate the crank
to ensure all pistons move smoothly in and out of their cylinders and nothing binds.
When installing the oil pump be sure to use the proper sealant around the pump base if neccessary to ensure that oil pressure
doesn't drop due to leakage. Both surfaces should be cleaned and preped prior to installing. After preparing the contact surfaces carefully
install the oil pump onto the balancer shaft assembly. The oil pump drive system connects directly to the balancer shaft in order to run its system.
You can tighten down the oil pump bolts to 18 ft-lbs. The oil pickup tube can be tightened down to the oil pump using 10 ft-lbs of force.
The first thing you must do when installing cylinder heads is to ensure both the block and head surfaces are
completely clean from dust, oil, and any debris. The next thing you must to is
to set the camshafts to their correct positions to prevent any valves from hitting pistons during installation
and torquing of the head bolts. Much the same you have to set the pistons to their correct
locations, typically this means putting the #1 piston to TDC or Top Dead Center. Once everything is ready you
can install the head gasket onto the engine block by aligning the alignment dowels.
Something I usually do is spray down the head gasket with some engine copper spray from permatex which you can
find here, this ensures that any gaps that could be present between the 2 surfaces gets filled with the
spray. It
also helps to transfer heat between the 2 metals. With the gasket in place you can set the cylinder head onto
the gasket and block, if needed have someone assist with this process as the head can be heavy
and you don't want to scratch anything or drop it! Once the head has been placed you will want to start
installing the head bolts to make sure it doesn't move. Be sure to buy new head bolts as many manufacturers
use TTY or Torque to Yield head bolts meaning they stretch during torqueing and cannot be used twice. Also make
sure to lubricate the bolts in clean engine oil before installing them into the head. On Chevy DOHC engines there is occasionally
bolt holes on the engine block that cross into water passages. Any hole that does this should be installed with some silicon sealant.
A little goes a long way in this step and you can check holes by shining a flashlight down them and looking in the water jackets.
Once all the head bolts have been installed and finger tightened you can start the torqueing process, almost all
head bolts have a multi-step process for torqueing. The 2.5L DOHC engine uses a single size of cylinder head bolts with a total of
10 bolts being installed into the cylinder head. These head bolts should be tightened down to 22 ft-lbs and then an additional
190 degree turn. This degree amount is rather large so you can break it down into 2 different steps if you desire. However you do it make sure
to now overturn or underturn any bolt as this can cause an unbalance of surface pressure between the gasket and head.
On the Chevy 2.5 DOHC engine the balancer shaft timing system gets installed onto the bottom of the engine block and will need to be installed
prior to installing the oil pan. The oil pump on this engine is also driven off of the balaner shaft assembly and can be installed by following
the oil pump section for this engine on this website. The balancer shafts simply get installed and timed according to the timing dots on each gear face.
The balancer shaft assembly bolts tighten up the entire assembly to the bottom of the engine. These bolts can be tightened down to 25 ft-lbs. Be sure
to install the balancer assembly using a new gasket and some silicon sealant to ensure that nothing ends up leaking. From here you can install the
balancer shaft timing chain which should be installed prior to the camshaft timing chain. To install the timng chain make sure that the balancer shaft
sprocket is tightened down to 37 ft-lbs. From here the timing chain can be installed by aligning the chain painted marks with the dots on both the crankshaft
and balancer sprockets. Once aligned you can install the guides and tensioner and torque them down to 8 ft-lbs. Once complete release the tensioner and ensure
that the chain is tight.
On the Chevy 2.5 DOHC engine the timing system is ran by the use of a chain. This chain system consists of a sprocket on each camshaft along
with an internally installed tensioner, guides and a crankshaft sprocket. To remove the old timing chain system simply remove the side tensioner
and remove the chain from the system along with the guides and corresponding sprockets. When installing the new timing chain start by ensuring that
the camshaft sprockets and camshafts themselves have been torqued down properly. The camshaft bearing cap bolts get tightened down to 8 ft-lbs. The
camshaft sprocket bolts get tightened down to . Once all of the sprockets have been torqued down properly and the crankshaft sprocket has been installed
onto the crankshaft you can install the timing chain by aligning the triangle marks with the painted links on the chain itself. On the crankshaft side the
marks are simply a dot located on the sprocket. With all of the marks aligned you can install the timing chain guides and then the timing chain tensioner.
The bolts for each can be tightened down to 8 ft-lbs. From here you can double check the timing marks and then release the tensioners tension so that the
timing chain tightens up. If desired you can then turn over the engine to ensure that everything remains tighten and turns smoothly.
The timing cover on the 2.5L is used to cover up the internals of the engine and hold in massive amounts of oil. For this reason I recommend using
some silicon sealant along with a new gasket during installation. Be sure to clean all of the metal surfaces prior to installation and placing the silicon onto the metal.
With everything ready to install be sure to double check your timing and engine internals before placing the cover in place. Another tip is to lube up the
crankshaft seal with some oil so it seats better. If everything is good to go then place the cover
onto the engine block and begin to tighten down the bolts hand tight. Once all of the bolts have been set in place be sure to follow your sealants instructions by waiting the recommended
time. Once ready you can torque down the bolts, since this engine uses 3 different sized bolts for the timing cover each bolt size will have a different
torque spec. Staring with the smallest bolts you should tighten them down to 8 ft-lbs. Next the medium sized bolts can be torqued down to 18 ft-lbs. Finally
the 3 largest bolts towards the bottom of the cover can be torqued down to 43 ft-lbs.
Much like the timing cover on the 2.5L the oil pan plays an important role in keeping the engine oil inside the
engine. For this reason I recommend using a new gasket as well as some silicone sealant
during installation. Using the same technique as before with the cover you clean the surfaces of both the oil
pan and the engine block and then install the new gasket onto the block and then follow
it up with some silicone sealant. Be sure to follow your sealants instructions to ensure you get the best seal
from your application. The oil pan bolts torque to 18 ft-lbs in a cross pattern design.
Along with the pan is the oil pan drain plug, this gets removed and reinstalled quite frequently and can be
torqued down to 18 ft-lbs.
Chevy Impala 2.5L Oil Pan Torque Specs : 18 ft-lbs
The valve cover installation is rather simple, there are rubber seals for each bolt hole and a silicone gasket
that prevents oil from leaking out of the engine. It is recommended to replace both the seals
and the gaskets although if they are in good condition you can reuse them. If you do choose to reuse them I
would use some silicone sealant along with the silicon gasket to ensure it does not leak.
The valve cover bolts torque down to 106 in-lbs or about 8 ft-lbs in a criss cross order. Be sure not to miss
any bolts to avoid having oil leak out onto the exhaust pipes and cause a lot of smoking.
When installing the intake manifold you want to make sure you have all the surfaces as well as intake holes
cleaned out before placing the manifold into position. Once cleaned you can then place
the intake manifold gaskets onto the alignment pins. If you want you can also use a little bit of sealant on the
gaskets to help ensure there are no leaks. With the gaskets in place you can now put
the manifold down onto the gaskets and begin to install the bolts. The intake manifold bolts torque to 10 ft-lbs
and can be tightened in a cross pattern. The ignition coil assembly can be torque down to 8 ft-lbs and the spark plugs themselves can be
torqued down to 13 ft-lbs, be careful not to overtorque these as they can snap and cause big problems.
When installing the throttle body make sure that all of the gaskets are in good condition and will hold a seal as you do not
want to be leaking gas fumes around the engine. If everything looks good you can tighten the throttle body bolts down to 8 ft-lbs.
Chevy Impala 2.5L Throttle Body Torque Specs : 8 ft-lbs
Exhaust Manifold Installation
The exhaust manifold can be installed by cleaning the surface areas on both the head an the manifold itself and
then by using the exhaust manifold gasket and putting it in place. Once the gasket is in its
location you can put the manifold onto the heads and begin to torque it down to 10 ft-lbs. I always use some
copper spray from permatex on the exhaust manifold gaskets to ensure I do not end up with
any exhaust leaks once done. When you go to install the engine into the vehicle or if it is already in the
vehicle the torque specs for the exhaust manifold to the exhaust pipes or catalytic converter is 20 ft-lbs.
The water pump on the Chevy 2.5L engine gets tightened down to the engine block on the serpentine belt side and is ran off of the
drive belt. When removing the old water pump be sure that you drain out the coolant prior to removal otherwise the coolant will spill all over.
Once drained you can remove the water pump mounting bolts and remove the old pump from the vehicle. From here we can clean up the engine blocks
mating surface and prepare the new water pump for installation. Be sure to use a new gasket and some silicon sealant to ensure that no coolant
ends up leaking and causes further engine damage down the road. Once prepared you can install the pump onto the engine and hand tighten the bolts down.
Once all of the bolts have been installed you can then torque down the bolts to 18 ft-lbs. Make sure to follow your sealants instructions as they often
require you to wait an extended amount of time before torqueing down the bolts. From here you can reinstall the pumps outlet pipe if you happened
to remove it during the job. These bolts get tightened down to 8 ft-lbs.
Chevy Impala 2.5L Water Pump to Block Torque Specs : 18 ft-lbs
Most of what is left on the engine is just place and tighten objects such as the belt pulleys, belt tensioner,
throttle body, oil filter neck, crankshaft pulley, and motor mounts. If the items have a gasket and hold either
oil or coolant inside the engine then feel free to add some sealant along with the gasket. Always be sure to
inspect your gaskets and replace them if there is any deteriation or flaws with them. For the
belt system the crankshaft pulley gets tightened down to the crankshaft at 74 ft-lbs and then an additional 125 degree turn.
The drive belt tensioner can be tightened to 33 ft-lbs and the idler pulley can be torqued down to 16 ft-lbs. When installing be sure they
both turn smoothly and if they do not then replace them with new ones as the bearings can go bad. The engine
motor mount can be installed onto the engine block if it hasn't been already, the torques for
the mount to the block is 59 ft-lbs and then when installing the mount to the vehicle or frame it can be torqued to
35 ft-lbs. Finally the throttle body can be installed on top of the intake manifold
with a good gasket and some added sealant using 7 ft-lbs in a criss cross pattern. For installing the belt you
can use a tool to move the tensioner into its springed state and install the belt as shown in the
picture. The generator or alternator can be tightened down to the engine block using 17 ft-lbs of force.
The power steering pump can be installed and torqued down to 18 ft-lbs and finally the A/C compressor can be torqued down to
15 ft-lbs of force.
