Resurfacing heads and cylinders

[anol]

Hello, my first post here. Have been reading for a year and got all excellent tips but now I have some questions that I don't have heared of before.

I live in Sweden so please have some understanding if the english is a bit odd.

I am rebuilding my 1981 SC which had broken head studs.
I have had the heads resurfaced (see picture) but now I am going to have the cylinders also resurfaced. The question is how are the surface supposed to look like?

At mine 3 of the cylinders have the inner mating surface higher than the outer and on the other 3 the outer is higher than the inner surface (of the ring gap)

Should the surfaces be equal? 3,2L tapper to the outside have I read but how about the 3,0 SC with sealing ring?

My mechanic is soon to machine the cylinders so I appreciate some comments.

[euro911sc]

Anol,

Do not machine the tops! The inner plating wraps over the edge of the inner sealing surface and must be maintained, preventing any machining of the top sealing surfaces to simply level them out. This is not like the heads that you have already done.

If you do machine them the cylinders must be completely re-plated. This is just fine and several places will do it, but make sure that the sealing ring grove is returned to the correct depth and shape as well. I believe, that most people who machine the top are also boring out the diameter for bigger displacement so they have to re-plate anyway.

Cnavarro on the board does plating I think. Though he is on this side of the pond... I'm sure he can explain the plating details better

The taper on the sealing surfaces is interesting and I have not read of it. Others with more experience need to comment there. I'm at work, but it is something I can check when I get home on my parts I just got back.

Best regards,

michael

[cnavarro]

Cylinders cut for the CE groove did not get the .0015-.003" taper across the sealing surface, as found on the 3.2 Cylinders.

You can resurface the cylinder tops by surface grinding them, setting them all on the table at one time off the cylinder bases that seal against the case. That said, its not something every shop can do accurately.

It's almost impossible to resurface the cylinders in a lathe without chipping the plating, not to say it can't be done without the right tooling and fixture again, in a pinch, as I have done. But I prefer the surface grinding method myself.

In your case, I would just resurface the heads and lap the cylinders into the heads. From the pictures your cylinder tops look fine as long as they all look like the single closeup.

[anol]

Thank you very much for the replies, this board i excellent

Yes the cylinder tops looks fine, but when I measured them there is a difference of approximate 0,1mm (0,003") between the inner and outer sealing surface. 3 are higher at the inner and 3 are lower at the inner surface, very confusing. I discovered this when I tried to lap them together with the heads and only one surface was in contact! So I think I must machine them.

Is it right to machine the tops to the same level across the hole surface then?

Do I need to machine the bottom of the CE groove equal much as I removed on the tops? Or is this overkill?

How important is the surface of the bottom of the CE groove? I have some small dimples there from crushed and smashed spring belonging to the broken CE-rings. (Hard to tell from the picture)

If I need to machine the CE-groove, do anyone have the factory dimensions of the groove (depth/width)?

And final question (at this time ) : My mechanic have a lathe, what is the right approach to not destroy the plating

[cnavarro]

Anol,

When you torque the heads, the .003" taper will crush and seal, like it did on the 3.2 Carrera cylinders without sealing grooves. What I find odd is that half are tapered one way and half the other way - maybe it didn't matter. In any case, I think you should be fine if you did want to resurface the tops to make them flat granted you don't take much more than .003", which is hard enough to do without chipping the plating.

When I am in a pinch, I use a fresh carbide insert for each cylinder and basically you machine from the bore outwards, taking as small as a cut as possible, in our case .0002", until all cylinders have fresh sealing surface AND all cylinders are the same height. You will need to use very good measuring tools and measure everything at one constant temperature.

This will take LOTS of time to get right. It will also help to "break the edge" by stoning the intersection of the bore and the tops of the cylinders at a 45 degree angle by placing a small chamfer on the surface, which reduces the chance that the cylinder will chip. Once a cylinder chips, you're done for, as the chip will propogate as you try to machine more to fix the damage already done.

You also should use a fixture that centers off the bore rather than the cylinder base and you should do a skim cut off the cylinder bases (where the cylinder seals against the case) to ensure that after everything is done, that the base and top are parallel to each other and that the top and base are both perpendicular to the bore. If you don't do this, you will end up with runout and uneven cylinder heights.

It is best to have a machine shop that is set up to do this AND has experience doing this kind of machining before allowing your cylinders to become training pieces.

Surface grinding is the ideal way to do it. You might be better off leaving it alone.

[anol]

Charles,

Thanks for sharing all your knowledge,
I understand that its very easy to destroy the plating of the cylinders, maybe surface grinding is the way to go then.

But how about the CE-groove?
When you use surface grinding, then you don't machine it? and the ring just compress a bit more? and that works?

----------------
I am very accurate to do this right because a already had this engine rebuilt once.
Some background info, the engine was rebuilt because of broken head studs. Split case, new bearings, rings, valve guides, machined valves all wear parts replaced etc. The heads was not resurfaced just bead blasted. All 24 head studs was replaced with 993 style.

But after about 300 miles the heads/cylinder started to leak and when I inspected it all CE-rings was broken. I don't really trust my Porsche mechanic so I turn to all Pelicans for help. I suspect bad surfaces as a fail couse
-------------------

Second tear down in 6 months

[cnavarro]

That's the thing, I normally don't recondition cylinder with CE rings, usually the 3.2 cylinders are more desireable, so no, I have never had the need to resurface cylinders that have ce rings. I would imagine that an additional .003" of crush is nothing, anything more and I would try to remachine the depth of the groove to ensure you have close to the depth of the orignal cylinder.

[euro911sc]

Anol,

Bummer that you do not trust you p-car mechanic!

I used to design metalic sealing rings that competed with CE. Ours were plated though and .003" made a difference if it was on the + (bigger) side, but not on the other end. If you grind it down say .0015"-.003" I really do not think you will have any issues as far as the CE ring is concerned as it is not a plated seal and it has a spring core for support so it will probably stay elastic as far as material is concerned.

My concern would be grinding off the cyl plating layer by going too deep, but that's Charles' domain and if he says it can be done right he knows. I wonder if you even need to grind it much though... AL deforms pretty easily and I bet your sealing surfaces will deform under torque and seal just fine if they are all w/in .0015".

This is merely conjecture and I have no data/experience to back that up...

Charles: If he does grind them, should they be mounted in a fixture that is like the case spigot? or just on the bottom lips as if you planted them on a table? Also, how would you clamp them down to the grinding table and still be able to reach the entire surface with out un-mounting them half way through the process? I assume you do all of them at once to ensure same height across the board?

Best regards,

Mihael

[cnavarro]

Michael,

To be honest, I don't have a surface grinder large enough nor the expertise when it comes to surface grinding, so I farm it out, when needed.

It's only when there is a crunch that I do them by hand in the lathe, and even then, sometimes I chip plating and we're not any better off than having sent them out and waited to have them ground.

They have fixtures and setups that simulate the case when doing the process to ensure the cylinder tops are parallel to the what would be the case deck.

You do not want to set them up off the cylinder bottoms(bottom of the skirts as opposed to the lip that normally sits against the case), since they are far from what I would consider a true surface.

Like I said before, if there is .003" of variation across the cylinders, then probably taking that down to make them just flat, by taking no more than .003", will not change how the CE ring fits. Anything and I can't say for certain. I too think that the cylinders, when clamped down, will seal completely regardless of taper, since they do on the 3.2 cylinders with the factory specified .0015"-.003" taper. It's just unnerving.

[anol]

Michael & Charles,

Thanks for your interests in my questions, but I haven't quite follow your advises. Here are how I did:

As I said before I had my cylinders measured and there was some rather big differences between them, so I thought I would need to do some maching on them. But I was a bit scared of ruining the plating. But I also needed to refinishing the CE-groove.

After feeling with the steel from the lathe on the edge of the Nicasil we thought it would be possible to machine them. The chamfer on the Nicasil looked like 1mm x45 degrees so cutting 0,2mm wouldn't be any problem.

I held my breath when my friend took the first cut on the top, very gentle and it worked perfect. We leveled out all cylinders to 85,20mm (original 85,40) and cut the CE-groove depth to 2,10mm on all of them.

I think as long as you have a well trained mechanic and take it easy it works find to machine the tops in a lathe.







I have now machined 0,2mm on the cylinders and 0,3mm on the heads, will I be fine with a 0,5mm shims under the cylinders?
That would meen some more compression, but how much, I calculated some, approximate 1% per 0,1mm so 9,8 x 1,02=10,0. Does it make sense? (I run 964 cams, CIS, SSI)