flycut head but nothing done to pre combustion chambers???

[vdubspeed]

How many thousandths are prechamber cups supposed to stick out above the head surface?

Regards,

Jason

[RabbitJockey]

the prechambers do stick out a certain amount from the factory, i don't have the numbers tho, maybe someone with a bentley or knowledge on the subject can chime in.  i'm surprised they actually took them out... usually they just cut them down in the head like what happened to mine.

[rallydiesel]

They should have cut the head with them in. You should get your money back. There's few places that know how or have the tools to deal with pre-cups.

[vdubspeed]

They should have cut the head with them in. You should get your money back. There's few places that know how or have the tools to deal with pre-cups.

No. Doing them in will result in messing up the machine that does the cutting because of the steel and aluminum AND you will have a completely flush head. You need the combustion chambers to stick out a little.

He can keep my money.

[BigVWman]

He said the steel of the prechamber was too tough for his machine unlike the aluminum. They are about .015 out of the deck.

And what is the deal with everyone saying decking the head puts stress on the cam journals. I've had 20+ gas heads decked without an issue. and that changes all the geometry too.

Its not just a matter of "decking the head" being bad. When a head is warped and you mill the head its still warped you just now have a flat surface to seal a gasket to, the cam is still being stressed and prematurely wearing in the cam bearings(causes oil pressure problems later on)! On a diesel the additional detail of the depth of valves for headgasket thickness needs to be considered when you start removing alot of material. Just a light surface grind for a good sealing surface is fine! When warped they should be baked at high heat in a special oven to remove it, then a light grind. I also do not like to surface my heads with the prechambers in , i have seen both but IMO you end up with a much nicer job doing the surface without them and then finishing the prechambers later. You want them to stick out slightly but the spec eludes me at the moment.

[vanbcguy]

For what it's worth I went through the Mk III Bentley as well - nothing there about maximum protrusion for the precups (or really anything other than about 2 lines talking about installation).  By the same token the Bentley also says that head skimming is not possible, so take it as you will.

I could have sworn my Haynes manual had something about measuring the precup protrusion, but I wouldn't have read that in over 3 years... It's somewhere in the trunk of my car probably half decomposed by now.

[vdubspeed]

Well thanks for looking. As for the Bentley saying do not resurface the head...that is laughable. I understand why to not grind sodium valves but that sounds like an excuse to buy new VW heads. Hell...it's why VW switched to TTY bolts on damn near every bolt in the engines and suspension parts these days. Mo money mo money.

[kevinm]

The bottom of my head was machined with the precups still in from a very reputable shop that has done it numerous times... I notice no problem and would think it would actually help seal better around the head gasket, especially if you are using a metal gasket.

[Mark(The Miser)UK]

The bottom of my head was machined with the precups still in from a very reputable shop that has done it numerous times... I notice no problem and would think it would actually help seal better around the head gasket, especially if you are using a metal gasket.

EDIT for 'only'  read often. Never say never etc
Exactly, Kevin.
Protruding precups are like walking around with your flies[zipper] undone. Not good [generally ].
How can it seal on the stepped interface for either fiber [also with metal regions]  or metal. A head bolt or even stud is not capable of extruding the gasket in the metal regions.

Removing intact precups is just shoddy work skills that are inviting trouble. Use lubricant if neccessary. If the tool is knocking them out then job is being rushed. Unmoved precups are best. There can be no better holder of a precup that a purpose built aluminium home.

My hand-abraded head is clearly done with skilled arms, [even if I did overdo it slightly]  Dewarping a head is a complex issue. But my own opinion, is that with  a failed head gasket, whilst still on the  engine the warpage is in the interface and not carried through to the cam carriers. Only when released from the block and removed from the intake and exhaust manifold does further shape changing take place. 
Leave the manifolds on and tight if possible. 
Much warpage is in the surface, where it has turned to jelly [jello] and reset. Machine minimal, but enough to allow contact round  the cylinders, and not as I did as I watched the symetrical abrasion patterns emerging.
Here is my hand  abrasion setup,; a bit crude, but fully functional and calibrated with newspaper to remove natural sag in 3 sheets of plate glass on  bench:
http://imageshack.us/g/222/tenthabradeka0.jpg/

[BigVWman]

When i say they should protrude slightly i am thinking order of .0005 or less not flush but slightly protruding. I must enjoy the breeze from my zipper being down but have had no problems and  had engine with my machined head go 300k and still climbing! Also as for machining with them in, frankly i wouldn't let you near my tools or machines! Iconel or stainless machine differently than aluminum. When milling the cutter reacts differently to the harder material and screws up your finish, especially if your auto feeding, when grinding you should be using a different wheel for the aluminum than the steel. I like to pop them out put a nice finish on the aluminum surface then pop them back in and take the prechambers down as close as i can to the aluminum surface without touching it with that wheel. They even protrude slightly on fresh from VW heads if you can find one! I would suspect the reason is the different rates of expansion of the materials! just my 2c 

[Mark(The Miser)UK]

When i say they should protrude slightly i am thinking order of .0005 or less not flush but slightly protruding. I must enjoy the breeze from my zipper being down but have had no problems and  had engine with my machined head go 300k and still climbing! Also as for machining with them in, frankly i wouldn't let you near my tools or machines! Iconel or stainless machine differently than aluminum. When milling the cutter reacts differently to the harder material and screws up your finish, especially if your auto feeding, when grinding you should be using a different wheel for the aluminum than the steel. I like to pop them out put a nice finish on the aluminum surface then pop them back in and take the prechambers down as close as i can to the aluminum surface without touching it with that wheel. They even protrude slightly on fresh from VW heads if you can find one! I would suspect the reason is the different rates of expansion of the materials! just my 2c 

Grinding better than milling for sure. So how do you knock out and refit, without encouraging slackness? Punch-peen?

Yes, I don't use auto anything, including CAD... With hand abrading I did have to keep the grindings  clear, as indeed the Inconel grit could snag and scour the ally.
Of course my hand job  cost me time, a bit of abrasive, and a head gasket or in otherwords basically nothing.

Not seen a head fail but then it will take  me 10 years to do 300'000 miles, and I'm not sure my Quantum will survive, due to lack of small rubber parts, but then I do have two rust free one's in cold storage...

[vdubspeed]

TONS of reading on this. Everyone has an opinion. I have read...BUY a new engine, NEVER resurface, resurfacing is fine, pull the prechambers, don't pull the prechambers, etc etc. I am assured now there is no definite answer.

As for getting them to stay in place...they will be peened. I would do the same if I were buying brand new ones.

Lastly, when I get with the machinist today they will be installed with .002/.003 above the surface.

No more opinions...just time to get it running.

Regards,

Jason

[92EcoDiesel Jetta]

Maybe the reason the swirl chambers has to stick up a little more is because it has a different thermal expansion rate than aluminum? What is your swirl chambers made of? Inconel, steel?  Look up their thermo properties.

[Mark(The Miser)UK]

TONS of reading on this. Everyone has an opinion. I have read...BUY a new engine, NEVER resurface, resurfacing is fine, pull the prechambers, don't pull the prechambers, etc etc. I am assured now there is no definite answer.

As for getting them to stay in place...they will be peened. I would do the same if I were buying brand new ones.

Lastly, when I get with the machinist today they will be installed with .002/.003 above the surface.

No more opinions...just time to get it running.

Regards,

Jason

Definitely

I suspect that they sink over a period of time due to the extreme heat.

Has anyone ever damaged a head by peening?
Optimum peening from edge, and from last peen?
One could enter a grave early thinking about it.
Good luck