DIY - Do It Yourself Projects

Topic: Marine V8 Valve Job Part II
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Posted: 10 December 2008 at 11:55am | IP Logged

GM 305/350 Mercruiser 230/260 valve job

Day two
A week later and Lorelei is still afloat, a good sign. Bart has a van full of parts, tools and as official gofer; I help haul it all down to the slip.
Reassembly procedure

The next few steps are all about achieving an accurate torque on the head bolts.

"Chase out" head boltholes in the block using a 7/16 14-tap secured in a hand drill.
Liberally use WD 40 or equivalent like CRC Formula 6-56 for lubrication and flushing
Pour motor oil into block valley and lube cylinder walls with your fingers.
Dip into the block valley for more oil.

Finish lap (optional) and thoroughly clean the block surfaces with Carburetor Cleaner. Stage the head gaskets and heads for installation. Final clean block and head surfaces with carburetor cleaner (no residue).

Tap in the four head locator pins.
Prepare the head bolts with grease where the bolt's surface contacts the head's surface. Apply (recommended) Quicksilver 'Perfect sealing compound'to bolt threads.
Place both head gaskets on block. Be sure of proper orientation.
Install Cylinder heads and loose fit the bolts.

Install Cylinder heads

Install heads and loose fit the bolts. The locator pins will hold the heads in place until the head bolts are on.
a) The four medium length head bolts are for holes in the head corners.
b) The long bolts are for holes on the middle of the head (under the valve covers).
b) The shorter bolts are for the head's perimeter boltholes.
Tip: Use a power drill and socket driver combination to loose-fit head bolts.
Loose install the bolts using inside to outside pattern.

Next step first! Cover lifters with rags.

Note how Bart has the torque pattern info in front of him while doing the torque. This is a time to be Careful and thorough! Add 2qts motor oil and 1qt Dextrom III transmission oil through lifter lube ports.

Complete two-step torque following G.M. or Mercruiser service manuals torque pattern specification.
a) 1st torque: 45 lbs
b) 2nd torque: 65lbs- double check pattern and torques!

If the dipstick oil pan tube was removed or is loose, install now. Assure they're aligned with where they are secured.

Grease the top of the valve stems
Install pushrods and arms.

Add 2qts motor oil and 1qt Dextrom III transmission oil through lifter lube ports. The transmission fluid acts as a powerful cleaner. Spin the lifters to check for smooth action.

Prime pushrod tips & pushrod bores.
Fit the rocker arms and rocker nut on top of the pushrods and prime with oil

See how rocker arms are not yet fitted to the top of the pushrods when doing the oil prime. Then, fit the rocker arms and rocker nut on top of the pushrods. Be sure the pushrods are properly seated onto the lifters and into the rocker arm cavity. Spin the pushrod with your fingers to check straight, smooth rotation. See how rocker arms are not yet fitted to the top of the
pushrods when doing the oil prime.

Valve adjustment procedure:
Positioning valves preparatory to adjustment: First, align the two valves on cylinder #1 (portside, forward) so they are both closed and pulley notch is at the T.D.C. mark on the block The easiest way to do this is with what is called an 'auto-start'. This useful tool is a simple momentary switch with two wires and alligator clips at their ends. The alligator clips are connected at the starter solenoid lug and the main12 volt lug to the battery. When you press the momentary switch, it allows you to activate the starter which turns the engine. In the correct position, this allows you to adjust half the valve sets (four pairs) without having to rotate the engine. Remember, both valves must be closed. To adjust the other half of the valve train, tune the crankshaft ½ a turn (180 degrees) to the TDC mark, and adjust.

Adjust valve lash

Adjust valve lash

With your fingers, rotate the pushrod to feel rotational friction. Tighten the rocker arm nut until the pushrod tension just stops finger rotation. This takes a bit of practice, but you will get the hang of it. Then, turn the nut one more full turn (360degrees) clockwise (tighten). That's it- valve adjusted! Repeat until all valves are adjusted. Prime the oil filter with one quart of motor oil and install it on the remote filter flange. Ultimately, Two oil filters will be needed, one for startup and flush, one for the final oil fill. Using the Auto start, turn the engine over for about 30-40 seconds to lubricate the internal parts of the engine. Stage hoist flanges for installation. Stage Quicksilver 5900 Sealant. Final-clean intake manifold and the mating head surfaces with carburetor cleaner. Prepare intake manifold bolts same as head bolts, except no grease is needed.

Apply Sealant beads to valley edges.

Apply 3M glue or equivalent to head surface (left intakes seen) where the intake manifold gaskets are to be located- we do not want the gaskets to move when we lay the intake manifold on them- Remember, there are no locating pins like on the head gaskets. Position the gaskets to align with the intake manifold’s boltholes and press the gasket onto the glue.
Next on the head surfaces, position the valve cover gaskets using glue.

Carefully! Lay intake manifold onto heads
Loose fit manifold bolts include the hoist flanges.
Cover carburetor and distributor holes with rags.
This (gasketed) surface mates with the intake manifold holes.

Torque intake manifold bolts per the service manual’s torque specification and bolt order in a two-stage torque process.
1st torque-20Lbs.
2nd torque-30Lbs. Double check bolts for torque!

Install Valve Covers

Note coolant bypass ½- hole and the bypass sleeve properly positioned Install thermostat, bypass sleeve, gaskets (use gasket sealant), housing and housing cap.
Install Distributor
Secure distributor cap self adhesive gasket

Install Distributor

Using the Auto start, first find T.D.C. Air pushing out #1 cylinder exhaust port as the crankshaft’s pulley TDC mark comes to bear on the block’s TDC notch. Note: secure distributor cap self adhesive gasket to distributor body, not the distributor cap- this guarantees a good fit.


Secure distributor cap self adhesive gasket to distributor body Spray WD-40 or equivalent to both sides of gasket.

Install gasket onto intake manifold for the Wedge Plate. Tighten down the two aft nuts which partially secures the plate. The other two forward nuts are put on when the carburetor goes in. Spray WD-40 on both sides of gasket and place on wedge plate.

Install the carburetor. Spray paint the intake manifold and engine as needed.

Install the carburetor

No sealants needed. Note: because of the wedge's triangular geometry, check that the wedge plate is seated flat on the intake manifold and that the carburetor is seated flat on the wedgeplate!

Install the engine block's coolant drain plugs. Use a thread sealer. There are drain plugs on both sides of the block, under where the exhaust manifolds go. Spray paint the intake manifold and engine as needed. Gap and install spark plugs. Apply dielectric compound to plug's contact lugs.

Install Exhaust manifold

Install Exhaust manifold gaskets and manifolds Install heat exchanger and hoses.

Install Exhaust manifold gaskets and manifolds. Note: see the two mounting rods? These rods allow for easy installation! All the pros use them. Once the manifolds are in place, unscrew the rods and replace with bolts…slick. Mount remote oil filter holder on exhaust elbow. Install alternator and all belts. Install Carburetor linkage and springs except link to Carburetor arm. This is so you can use the throttle on 1st startup.

Install heat exchanger and hoses.
Note the oil filter holder at right in front of the exhaust elbow.


Install spark plug wires and all other engine wiring

Install spark plug wires and all other engine wiring. Put a dab of dielectric compound
on the distributor cap contacts.

Tip- Remove brass plug from the thermostat housing cover so air escapes during the fill process. Tip-If the raw water hoses have reinforcing wire, pull the hose wire out a bit and cut off.Then, when dealing with the hoses, the hose wire will not bite you! Fill the engine with coolant. 3 gallons coolant and 3 gallons water in a 50/50 mix. If you don’t have premixed coolant,don’t bother mixing- it’ll get mixed in the engine. Check for leaks. Tip- The cooling system Burps air after the first startup. Check coolant level for the next couple of weeks.Check engine oil and coolant levels. Replace the thermostat’s cap plug.Open raw water thru hull valve.Open fuel valve.

The Big Moment!
Use the Auto Start for Engine startup. Set timing at 8 Deg BTDC. Start Engine. Tune to smooth idle. 1980s era Mercruiser engines used Thunderbolt Electronic Ignitions. These units control timing advance of some 23 to 28 degrees of advance during RPM increase and tops out at about 42 degrees (8degree manually set plus the 23-28 degree electronic module advance). During adjustment, you can see this action with a timing light. Run engine- check oil pressure at console gauge. Run for 20 minutes at varying RPMs, check for leaks, read console gauges, check belt tensions, weird noises etc. Stop engine. Drain and replace oil and oil filter and water separation filter on the fuel line.

To optimize engine performance: With someone competent piloting the boat, put it through its paces: W.O.T., cruise speed. Idle, gear engaged, trim tabs up and down. Adjust timing and carburetor underway and find the ‘sweet spot’.


Mr. Bart Girard
Bay Area Mobile Marine.
Castro Valley, Calif.


The cylinder heads were replaced. They were found unusable due to water intrusion. As part of the rust mitigation I did two years ago, I purchased a pair of rebuilt auto heads. Auto cylinder heads are identical to marine heads. As long as the head part number is specified for that block. To be sure of quality after being on the shelf for the two years, we had them inspected by the machine shop. The shop ended up replacing the seals and springs for new. The rebuild company apparently used old springs that upon further testing, indicated a spring tension to be around 45 lbs. each- barely within tolerance. 70 lbs is standard for new springs. Also, we had the head resurfaced to be sure of flatness.
Oil recommendation: Mercruiser 25/50 weight marine grade motor oil. It's the only available heavy oil (50 weight) for marine gas engine loading.

Use 'Freeze off' penetrant for stuck bolts. This stuff brings down the bolt temperature, reducing its size slightly and the lubricant component really penetrates. Good stuff.

Use a fuel system cleaner with a water removal component in concentrate if available. Pours into the fuel tank.

Check the oil and water separator filters for rust before using them. When we pulled replacement filters out storage, we found the insides had rust, rendering them unusable.

Use a quality 3/8" ratchet set; they are strong enough for a valve job. 1/2  drives are ok, just bulky.
Have a good 12vdc pump, tubing and Mr. bucket to suck out coolant, oil, messes and the bilge.
Get an auto start switch.
Have small torque adjustable drill handy. Adding a socket extension makes things go fast and easy.

A word on electronic ignition.
A quick way to check a Thunderbolt Ignition Module: With the ignition switch "on" unscrew the gray/white colored wire coming out of the module at the distributor screw. See if it sparks to ground. On Thunderbolt ignitions, the rotor sensor is powered from the module: Color code: red/white= 12vdc, gray/white= ground. As mentioned above, the ignition module also creates the timing advance, typically around a total of some 45 including the 8 degrees manually set. You can see this action with a timing light when the engine is revved with max advance of 45 degrees at around 3800 rpm. If you see this action, the module is doing its thing.
Sorry, Mallory Ignition systems are not recommended due to install complexity, inappropriate material for a marine environment and thus, questionable reliability.

That's about it!

Thank you for reading my article. I hope its helpful.
Fair Seas!

Roy Yates

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Posted: 10 December 2008 at 12:41pm | IP Logged

Outstanding post.

You make it look so easy...and I know it isn't.
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Posted: 11 December 2008 at 9:22am | IP Logged
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Posted: 11 December 2008 at 8:18pm | IP Logged

Diver, the techs at the dealership said that you should be the teacher on the gm training classes. awsome job! happy cruising!


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