Hi Chris,

If you perform a compression test with spec results once and on repeat, simply planing (milling) the head so it is flat and true, you'll ensure a good seal between the block and head. That's fine for some and that may work okay.

Once you remove the head, you'll see the valve seat conditions on visual inspection. That'll help you decide what you need to do.

If you've performed a compression test on the engine and find cylinder(s) with low compression, squirt oil in the spark-plug hole and the low compression reading does not change, you have valve and not questionable piston compression rings that do not seal on the compression stroke for that engine. So, you'll have some cyl head work to do.

If you can remove and reinstall the cylinder head yourself and can rebuild it your self or use a reputable machine shop and:
- Pressure test the head to find fractures in the light aluminum alloy and and if they can repair those fractures.
- Check for true and plane (mill) the cylinder head. I've not ever removed an aluminum and some iron alloy (steel) heads that were not warped.
- Press out the old valve guides and install new if they are out of spec. Use valve guides from Volvo or the OEM manufacturer. Do not let them talk you into "knurling" the existing valve guides. Knurling valve guides is a questionable practice.
- Inspect the valves for true and the seating lip. Rarely do these need replacement.
- Verify the valve spring health for strength, compression, and rebound. These can fail, yet so rarely. This step may be optional for some.
- Inspect the valve seats and check with the corresponding sealing "lip" on the valve. Lap (minor wear), grind (more wear, perhaps pitting, regains optimum valve seating angle and seat width), or replace the valve seats (rarely likely) as necessary. (May need lap or regrind using original valve with new valve seat).
- Inspect cam journals and the cam itself for true and damage and scoring.
- New rubber "hushers" under the tappets (for that 1979 B21 sound in your low friction B230)
- New rubber valve guide seals.

If you must replace some or all of the steel valve seats, and a good machinist knows this, they may give a bit of extra clearance on the valve adjustment. New valve seats can seat a wee bit more as you operate the car with your fresh cylinder head, narrowing the valve clearance. You may need to check valve clearances after some miles, though more an issue with older machining methods on older cars. A good machinist knows the material and can mathematically figure this out pretty well so with the valve seats seating a wee bit, the clearances wont narrow up below the minimum valve lash spec.

I think that's it. Other folks? I probably missed something.

Comments? Questions? Thanks.

Fuddy Duddy