Chris Lockhart

Both heads are about the same. The worst spots were about 0.61mm deep. I need to get a more accurate measurement this afternoon with a better depth gauge. I can't tell from the info I read on machining the heads in the WSM if the specs they show are correct. It sure seems like very little room for any machining, even if there weren't issues. IIRC the "new spec" was 147mm +/- 0.1, and the refaced limit was like 146.8mm. Am I looking at the wrong info? How much is safe to machine, assuming taking the same off of both heads to keep them even? Thanks in advance for any help or advice.

Cosmo Kramer

I was told that if the pits aren't in the area where the gasket seals and not in the fire ring area, don't bother machining them. Just make sure you use good quality coolant and change it regularly going forward.

hans14914

I have seen worse. They can be repaired by welding and then grinding back flat. IF you go back in the archives, Jim Morton did reduce the head height on one of his engine builds in conjunction with modified 85/86 cams to get some really nice power (not really applicable in your situation, as you have the blower), but the amount he removed did not seem to effect the timing. You may be able to find that final number with some digging.

Good luck with the project.

GlenL

Right.

Unless I'm missing something, the ugly pits are where the head is exposed to coolant and there's no sealing. Look at the outside of the head and, especially, for the cylinder tops.

Rob Edwards

Porsche makes a thicker head gasket set (1.6 mm vs. 1.0 mm) for 'reconditioned' cylinder heads. I would measure a couple of chambers' volumes, get the pits welded, get the heads skimmed, then re-buret a couple of chambers and calculate if there's much change in the compression ratio. If not, just use a regular head gasket. If it's high (>10.5 or so) run a set of the thicker head gaskets.

ptuomov

I thought the thicker gasket height was 1.4mm. Maybe I remember compressed thickness? This matters to me because the difference in compression ratio between,say, the 1.1mm and 1.4mm head gaskets is meaningful.

Rob Edwards

Honestly I'm not sure, 1.4 mm may be the compressed thickness. I think the 1.6 mm is the uncompressed thickness on the 968 head gasket for reconditioned motors, and I'm extrapolating to 928, never having had occasion to need a thicker 928 HG. Unless Chris is planning on supercharging without additional engine management it probably doesn't matter too much since the post-weld skimming should remove very little material.

ptuomov

We relatively recently measured the compressed thickness of the second gasket to be 1.4mm or 0.055".

In terms of the geometry, there's some benefits from not moving the intake ports and the cam gears by skimming too much stuff off the heads. That, however, may be just hair splitting.

Forced induction will have enough turbulence and can benefit from lower compression. I'd use the thicker gasket regardless in those applications. Similarly, the normally aspirated 5.0L motor will benefit from tighter squish and higher maximum squish velocity, as well as from the higher compression, so in most cases I'd just use the thinner gasket in those applications.

Chris Lockhart

Thanks guys. I do feel a little better, and so does my pre-Christmas wallet!

Rob, I WILL be Sharktuning it, once I get the engine completed and reinstalled.

GregBBRD

If the pitting is close to the fire ring, the problem with not addressing the problem now is that the pitting will eventually eat under the fire ring. The damage caused by acid erosion is very difficult to halt. Not fixing the problem now means that this job will need to be done again, with the potential that erosion under the fire ring will cause catastrophic damage in the future (split cylinders, bend rods, etc.)

Your "side view" picture of the pitting is difficult to determine the extent of your damage, in relationship to the fire ring.

In my shop, it has gotten to the point where it is extremely rate to remove a cylinder head and not weld it to repair the damage...the years of neglect are catching up on almost all of these vehicles.

The repair process seems relatively simple, looking at it from an uniformed point of view, however many of these heads are turning to junk by doing this job improperly. The head has to be completely stripped and be properly prepared for welding....this requires the removal of all of the oxidized aluminum. This is typically done by bead blasting, which means that all of the oil galley plugs must be removed for very careful cleaning, when the job is done. The entire head must be preheated before welding, in order to not warp the head. And the welder must have the skill required to not "spit" tungsten into the welded area...otherwise when the head is surfaced, the surfacing machine will "jump" each time it hits a piece of tungsten (super hard material) and leave an irregular surface.

Chris Lockhart

Yeah it definitely needs to be done correctly. I've still been trying to locate a reputable automotive machine shop near me to handle the heads. The only recommendation I've gotten that I'm comfortable with is your shop Greg. Unfortunately it's on the other side of the country.
Still, could you possibly PM me with a rough estimate of what the cost would be to do a complete service on both heads assuming no broken or damaged components? TIA.

Cosmo Kramer

Can you please post a pic of the heads with the camera above the pitting?

Imo000

Once the acidic coolant is removed, what is going to keep the corrosion growing?

GregBBRD

The closest thing I can compare it to is tooth decay. If you don't get rid of every tiny little bit of compromised aluminum, it just keeps eating at the good aluminum.

I'm guessing that if you bead blast or soda blast it down to pure clean aluminum, you might be ok....but the pits generally get pretty big when you do this

Imo000

There's got to be a better explanation than this. Tooth decay is done by bacteria. When the bacteria is removed the decay stops. The head is eaten away by the acidic coolant, when that's removed, what else can keep the corrosion going?