Engine part compatibility question.

Just so I understand…..the engine was rebuilt and the valves aren’t sealing?

In regards to the heads it comes down to what the base engine is. A part number off the head or the block serial number would be helpful.

The differences to understand are the intake port (oval versus rectangular) and combustion chamber size.

 

Like Playdate said there are a half a dozen different cylinder head arrangements for the BBC, so it's essential to match.
All valve installations will have differing installed heights. The machinist measures the installed height then measures the spring tension and installs shimming to make all valves the same open and closed spring tension. The valve stem heights will vary to some extent.
The rocker arm / pushrod / hydraulic lifter are adjusted on the heel of the camshaft where the lash is zeroed then another 3/4 turn on the adjusting nut which sets the valve train mid-way in the lifter's range. It almost sounds like the valve lash wasn't done correctly.

 

So you also say you only rebuilt #7 and not the rest?

To answer your question the biggest difference is brass or stainless freeze out plugs. Also marine heads are typically not a triple face valve. Assuming the castings are the right ones that's basically it.

To adjust the valves there isn't an adjusting nut, it's done by the rocker center nut. The old school way is to loosen until clap and tighten until basically no clap and then 1/4-1/2 turn.

 

I think you will find that nut in the rocker arm is the adjustment for the valve lash. All Gen 3 Big Block Chevy's (7.4/454) are the same for hydraulic camshaft lifter adjustment. Mercruiser configuration is the same as any Chevy Big Block. Where did you hear it needs to be "torqued down"?
If you tighten that nut all the way down there is a good chance the piston will hit the valve.... Or if you get off easy spring coil bind and a bent push rod.

 

Do you have the original block serial number?

 

Ah - so this setup requires specific pushrod lengths or possibly shimming under the bolt head. PITA. I didn't know there were any of these in the Merc BBC lineup that year (being Mark V or Mark VI) - learn something every day. So, you are right if the valves were ground and/or the head milled all of the pushrods need to be measured and most likely changed. You will need a pushrod length checker tool and the procedure for that engine/camshaft. Then order the pushrods. My engines require specific length pushrods so the rocker is centered on the valve stem rather than a height requirement. I can say it is a long arduous process.
You might want to consider installing a more traditional setup.

 

Sorry, didn't know that setup was available in 96'. Thought that was only way into the 2000'.

Given that, then you can't use the aftermarket head. The rocker seats are a set height.

 

Is this the same engine?

 

I dunno about you, with all this hassle you are having, I would install studs where those bolts go and use the old school rocker, rocker ball and adjustment nut, like 99% of the Chevy's out there.

May have to have the bolt / stud hole machined to 7/16 / NC to accept the studs.

Then you would have adjustable rockers and not need to go thru all this crap of shimming and such.

Side note, where as the valve seat was ground too deep, the seat can be replaced and reground correctly.

 

The inexpensive way out is to convert to adjustable rockers. Comp Cams makes an adapter stud kit to do just that.
Check this out for your situation; I'd call Comp to make sure -
https://www.jegs.com/i/COMP-Cams/249/4514-KIT/10002/-1?parentProductId=764287
Just make sure the rocker sweeps across the valve stem correctly on that cylinder in question.
Here is a blog on the subject -
http://garage.grumpysperformance.co...ck-chevy-gen-v-vi-to-adjustable-rockers.4564/

 

There's no info on that replacement head. You need a replacement head of the correct generation. What gen. is your engine?

 

When you change valve stem height in the head 2 things have to change. Rocker arm centerline (fulcrum bolt shims) AND push rod length ( longer). This keeps the geometry, correct.
I’m not 100% sure but your machine shop should have a chart to calculate both for you