To Skim or Not To Skim a Head

The skimming of the head is generally used to do 2 things: a) re-establish compression, b) make the thing flat enough the head gasket seals.

When the valves are done and the seats recut, the combustion chamber has more volume and will thereby have less compression. A small skim, here, re-establishes the desired compression ratio.

A skim of 0.003 can make the head flat enough to hold a gasket. You can get away with this kind of skim several times in the life of a motor.
A skim of 0.030 is used to change the compression ratio. This, on the other hand, it a one time event.

So it all depends on how much was skimmed off.

 

What finish quality did they put onto the surface. The factory surface finish is very near polished? Many machine shops will not have equipment to put a fine enough finish on the heads and that can lead to premature head gasket failures. A properly done valve job should not change the volume of the combustion chamber or the installed height of the valves. If a shave of the head is required to correct the volume of the combustion chamber after a valve job it was not a god valve job. If you had a reputable shop do the work I am sure you will be fine in all these areas if not good luck.

 

Well I don't know but I suspect if resurfacing the heads was not allowed by Ferrari then they wouldn't have published the specifications to be maintained during resurfacing in section B3.03 of the shop manual.



Does anyone think to refer to those anymore?

 

Really? Many machine ships can't make the same cut as ferrari? Ferrari must be using unicorn teeth to do their machine work...

 

Actually, the specific type/material of head gasket used dictates the finish smoothness on the head. Granted the factory finish is quite smooth, but that has little to do with the perceived quality of workmanship.

Regarding skimming the head: Any competent shop (or machine shop) will measure the head thickness prior to any machine operations and compare "that number" to the nominal head thickness, as listed in most WSMs, even Ferrari. A 0.003" clean-up cut to ensure flatness and/or the lack of corrosion "craters" around the coolant passages makes great sense if a quality outcome was the intention. To infer that a machine shop is incapable of doing this and achieving the correct surface finish merely says that the incorrect machine shop was chosen. Quality workmanship takes effort and isn't always easy to find...

 

Winner winner chicken dinner.

 

this is the gradient scale used by a "profilometer" (Mitotoyo for example)used when evaluating surface finishes-be they a cylinder head, bores in the crankshaft for compression alloy plugs with SS balls to seal oil galleys, or the finish on a cylinder wall.

The surfaces required for an "MLS" head gasket is typically at 10 or less-usually less..the "mirror" finish to which you reference.
This is ONLY obtainable with a "CBN" head, and cannot be obtain with a "stone" as is used on everything from a 250TR to a 512BB. Those head gaskets require a surface which has some "bite" capability into the head gasket materials -especially those of the era which had asbestos in them. One stone in my Kwik-Way 856 lasted for 22 yrs
This ALL presupposes a liner protrusion level of, ideally, .09mm but can be as low as .07 or as much as.11, but they ALL must be the SAME.
WITHOUT a proper Ru finish on a LATE model head, you have ZERO hope of a long term life expectancy of cylinder head sealing to block by the gasket-period.

This must be done with equipment which is nearly impossible to buy new anymore, and vintage machine shop equipment-like the vintage cars which they service-typically are worn out, and can't hold specs after 30 yrs to the degree to which they were designed and intended therefore they too must be "re-scraped" in their ways, resurfaced, and otherwise "restored" to their former capabilities of +/-.0001"

On the other hand, "NEW" machines-all cost tens of thousands of dollars-are designed and manufactured with CBN heads as MOST "modern" auto designs REQUIRE this as "old fashioned era" head gaskets are not made anymore-see problems with Daytonas/C-4s, etc etc-as the materials required are NOT available or even LEGAL to be used anymore...

The principle problem with belt drive mills is this: the dimensions alter the ability to maintain accurate cam timing specs(Daytonas, for example: allow for 1.5 degrees of adjustment with EACH hole in the vernier on the end of EACH camshaft) as the geometry has changed, combined with the LIMITED degree capability of precision-cogs on belts, adjustment holes on cogs are gross compared to vernier of chains-once altering this geometry, it can become an issue of only being able to get with "so close" to specs, which in turn: provide less efficient combustion, which in turn: now wreaks havoc with closed loop dipoles capabilities in the feedback circuit for the fuel management system.
Less precision in event horizons means: less precision in the flame front, resulting in less complete combustion, dirtier exhaust, mitigating much heavier lifting for the computers, and a potential for a power loss AND a substantially shorter life cycle for the catalyst bed's substrate as well as excessive coating by said detritus upon the aluminum oxide wash coat which is the attachment layer for the catalysis sol.(increases surface area of substrate by 600X)...
Remember: this is a finely particulated colloidal platinum sol, in the range of between 10-25 angstroms....

 

You are seriously misinformed about the adjustability of cam timing on the belt drive motors.

None have less fine adjustments than your example, the Daytona but in the case of this motor the adjustability is infinite.

 

Gimme a break.
Image Unavailable, Please Login

 

What he said^^^^^^^ I think. My head hurts.

 

I'm pretty sure you don't want CBN tooling but you do want PCD for Aluminum heads that we are really discussing here. (J & M Diamond Tool: PCD and CBN Information)

The topic of head gaskets is very interesting to me especially in light of all the issues DHelms reported in colorado. I have never had time for a discussion and lesson on that from him. Maybe that's a topic we can drift to here. What material is Elring currently using on Ferrari gaskets? Why is there all the reported sealing problems with Ferrari motors and Elring gaskets? Is this a problem of the wrong surface texture for the available gaskets from Elring and people trying to stick to Ferrari specs with really incompatible gaskets. Ferraris don't leak from the factory so I'm not sure why some people are seeing a head gasket problem during rebuild.

I'm a fan of MLS. Some say they don't work on Ferrari motors. I don't see a problem with them and would not hesitate to use them on a Ferrari motor. On non-MLS gaskets I will admit to 30+ years of cheated using copper spray with 100% success but I rebuild very few motors and I don't do this for a living so luck over intelligence goes a long way sometimes. Just about everyone will tell you not to use copper. But copper is available everywhere with lots of turnover on the shelves. Someone is using it and it can't all be garage hacks like me. There are just not enough of us. I'm thinking more people are using copper that care to admit it.

 

The timing change on a Daytona cam gear vernier is 2 crankshaft degrees per hole, not 1.5. I just spent the better part of an afternoon today setting up the cam timing on one. Read the manual.

 

And I can get a degree anytime I need it or want it on a belt drive car. Not as easy but its there.

Daytona with dual pins.....no way.

 

Why are we discussing valve timing when the question was in regards to skimming a cylinder head?

Skimming a cylinder head makes absolutely no difference to valve timing if you can adjust the camshafts pulley position, which with my simple Alfa Romeo Nord engine I can, so surely with a Ferrari engine you can too. Even with an engine that does not allow simple and quick valve timing adjustments you can simply machine another key groove (or drill another pin hole) in the camshaft pulley to restore correct valve timing.

BTW: I could take 2 inches off the top of my Alfa block and the valve timing would be unaffected ... would have a big problem with conrod length, etc. though

Pete
ps: Yes the Alfa Nord uses a chain to drive the camshafts but the same principle applies.

 

Assuming that the head can be successfully surfaced:

At what point does the cam bore(s)
have to be align bored ?

 

When the cam does not turn well.

A head surface that needs renewal or repair does not mean automatically that the upper surface is warped or misaligned in some way. Heads are hollow for one thing, then there can be erosion problems.

Alfa 4 cyl heads are famous for distorting so bad in an overheat that the valve seats move and quit sealing yet the cam bearing alignment is almost never an issue.