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964 Heads

Hi Guys

I am looking for a bit of advice on a couple of issues I have with my 964 heads.

I am rebuilding the entire engine using new late model cylinders and pistons (brought from our hosts, thanks Wayne). All machine work etc has been sub contracted out.

I have received my heads back from the machinist who has cleaned, fitted new guides and faced ready to receive the later model cylinders. Last night I checked the measurements of how much he has machined off as it looked quite a lot by eye.

The Porsche manual states a cylinder face to cam carrier face of 84.48mm +/-0.02mm. Two refaces are allowed, each of 0.10mm. My heads now range from 84.03mm to 84.28mm!

I have assembled heads 1, 2 and 3 onto the cam carrier and torqued up to 28Nm. By placing a straight edge across all three head faces, there are several discrepancies between heads, but the worst is 0.15mm. Now surly this is far too much is it not?

Even if I torque the heads to the cylinders first, then fit the cam carrier, I am going to transfer the gap to this junction. Even if I seal it up well I am worried that when torquing up or over time the cam carrier will distort and course me problems.

What do I do? I donít want to ask for more off as I think he has gone too mad as it is. I have not even thought about compression ratio and or piston to valve contact yet.

Apparently he does this work very often and for a couple of local reputable Porsche garages

One further point before I progress any further, does this Photo look OK? I presume that the exhaust port is coated in a ceramic coating, but it is badly cracked and a section missing. Three heads show crazing in this area, but this is the worst, God knows where that large chunk is.

I appreciate any comments/help.


Thanks

Old 09-03-2008, 02:06 PM
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I'd have my machinists neck in a guillotine if I received work back like this.
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Old 09-03-2008, 02:39 PM
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Mr Moots

Simple math tells me that one is allowed to remove a total of 0.2mm which leaves a total minimum height of 84.28mm (i.e. 84.48 - 0.2). The heads you received back at best only meet this minimum value of 84.28mm and the heads which measure 84.03mm fall below this minimum value. I would think they should all be the same height for each bank in order to have a level surface for the cam tower to mate to. Currently in the same process myself, I'm not sure there's a way to compensate for this - I can't imagine adding extra gaskets is recommended.

Bill
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Old 09-03-2008, 03:00 PM
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Quote:
Originally Posted by kenikh View Post
I'd have my machinists neck in a guillotine if I received work back like this.
Me, too,...

Personally, I replace the heads when the ceramic liners chunk like that.
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Old 09-03-2008, 03:58 PM
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Mmmm. Basically you have confirmed my own thoughts. This could become interesting.

Interesting about the ceramic liners. They did not have chunks missing when they went away, not sure about crazing as they were sooted up. Presumably the installing of new exhaust guide would have displaced anything loose, like that big chunk, but the first heating cycles when the engine is put back together could bring more down.

Thanks for your input so far.
Old 09-03-2008, 10:47 PM
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Mr Moots

I wouldn't be too concerned with the missing ceramic, afterall we're not dealing with something as critical as ceramic tiles on the space shuttle. I think it's there to insulate and there's already a lot of bare aluminum exposed around the valve guide that another small area exposed to heat isn't gona make much difference.

just a thought
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Old 09-04-2008, 04:09 AM
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Quote:
Originally Posted by Steve@Rennsport View Post
Me, too,...

Personally, I replace the heads when the ceramic liners chunk like that.
+1

Personally, when I have machine work done, I expect this work to be done within factory specs......across the board. I do not expect that machining done, creates new problems, i.e. cracked ceramic liners. "You break it, you fix it or buy it". It is obvious that the machinist did not have one iota of knowledge in the modular design of these engines. Bottom line, I would be pi$$ed.

mrmoots, I don't know what kind of resolution you can get from the machinist. I'm hoping you and the machinist inspected everything together before handing over the job to him. Otherwise, I fear he may come back and say, "the liner was broke when you gave it to me".
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Old 09-04-2008, 07:06 AM
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Not too bothered about the liner being broken, more concerned that further bits may break off and the inaccuracies with the machining of the heads.

I have photos of the heads as I was stripping them off the car so I have a fair representation of their original condition. Unfortunately the photos are not that clear of the exhaust ports, oh bother.
Old 09-04-2008, 10:31 AM
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mrmoots

What technique did you use to measure? Hopefully there's an error in measuring which is in your favor! Being on the exhaust side the chances of a piece entering the combustion chamber is unlikely plus you have gravity on your side. Even if it did get in I don't think it would do much more damage than a piece of carbon breaking off a valve or piston top. The valve or the guide in the photo don't appear to be new or they seem to have been subjected to heat?

Bill
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Old 09-04-2008, 01:09 PM
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The guides are new, you can see that in real life. Not sure what it is that on them, but it wipes off with your finger to leave a shiny guide, I am sure of that. Stupid really, I completely forgot the exhaust port faces down, so used to "normal" engines.

I measured three ways.

1. Simply with a pair of digital callipers in all four corners. Measuring head face to cam face.

2. By holding a straight edge across the cam carrier face and measuring the overall depth, before subtracting the depth of the straight edge. Recorded same measurements as 1 above.

3. By torquing down to the cam carrier and checking across three head faces with a straight edge and feeler gauge.

I know my callipers are accurate as I measured my crank journals the other day and had these double checked by a small engineering practice near my office. Their measurements were the same as my own.

Calling the machine shop tomorrow to discuss this. I hate confrontation, but sometimes you need to stand up and make your point.

Thanks
Old 09-04-2008, 02:06 PM
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mrmoots

Can you elaborate on procedure 2? I'm assuming you used the caliper as in procedure 1. Keep in mind that the edges should be beveled (i.e. as per Porsche service manual ) and area III (i.e. those wing shaped surface areas beside the bolt holes) stepped lower than the reat of the surface, which may be affecting your measurements. In any event, procedure 3 definitley indicates more than a +/- 0.02mm difference between heads and may be corrected if the issues I mentioned above are affecting your measurements. It's better you found out now rather than later.

Bill
Old 09-04-2008, 03:52 PM
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Its a very long and confusing story, but I have undertaken the following to verify if the heads can be refaced so they all match. My machinist thinks by simply refacing square should not cause a problem, I am not convinced, so I have decided to build up the engine dry and check all tolerances as I go. That way any problems can then be thrashed out before I rebuild properly.

All heads have been machined so the cam carrier face is not square with the cylinder face. All heads taper by approximately 0.2mm.

The lowest head measurement I have is 83.99mm. On that head I have a deck height of 1.06mm to 0.90mm (as the face is not square). Measured with crushed solder.

I have measured the compression ratio and its 10.59:1. I am using new Mahle cylinders and pistons, from what I can determine itís a misnomer that these will give a true 11.3:1 ratio.

Today I have finished the valve to piston clearance check using the most over machined head. I have done the valve clearances twice and reset the cam timing twice and on each occasion recorded the same valve to piston clearances. The minimum clearance I have is Intake 2.5mm. Exhaust in excess of 4.5mm.

Waynes book says clearances should be minimum 1.5mm intake and 2.0mm exhaust. Providing these figures apply to the 964 as well, then I am OK. Anyone know?

It also looks like the cam shaft to timing chain housing seal on the 964 will accommodate more removal of material off of the heads than on earlier engines. Again can anyone confirm this?

From what I have determined so far, I cannot see why I cannot have the heads refaced square then use them. This will give me a head height of 83.99mm, which is 0.29mm over machined from what Porsche recommend. Can anyone think of any other issues I may create in doing this?

Thanks
Old 10-19-2008, 12:26 PM
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Trying to follow this. Please clarify; regarding machined off "cylinder face to cam carrier facer"
I'm thinking you meant top of cylinder [sealing surface ] to mating surface on cylinder HEAD was cut. Yes?
Old 04-14-2011, 03:49 PM
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Any update about your build/ cylinder head...
I had also bought new mahle cylinders for 964 & send cylinder head for refacing to a reputed company, now I need to know if i have to do engine timing differently, as head has been machined ... My engine is 964, 91 Carrera 2..
Old 04-02-2021, 06:53 AM
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Quote:
Originally Posted by Rahul View Post
Any update about your build/ cylinder head...
I had also bought new mahle cylinders for 964 & send cylinder head for refacing to a reputed company, now I need to know if i have to do engine timing differently, as head has been machined ... My engine is 964, 91 Carrera 2..
Depending on the amount of material removed, your best option is to recover the proper stack with base shims.
964 do not ordinarily run base "gaskets" but the stack dimension is what's important.
LN Engineering makes 964 base shims.

BTW: stack is defined as crank center-line, to spigot base plus cylinder height, head measurement and cam cam tower to cam center-line.
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Last edited by Henry Schmidt; 04-02-2021 at 04:02 PM..
Old 04-02-2021, 03:59 PM
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I flycut heads on my cnc mill, Its quick and easy, and if you have a bad head that needs slightly more to clean up, its no big deal to put the the 5 back on my fixture and re cut. Very accurate and extremely nice surface finish.
Old 04-06-2021, 02:59 AM
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Those look end milled, not fly cut....
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Old 04-06-2021, 06:26 AM
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Quote:
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Those look end milled, not fly cut....
Correct. I used the term flycut, as it is typicially how one would face them .
Old 04-06-2021, 11:04 AM
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Generally speaking, 911 heads are cut on a lathe not a mill.
That said: there are many ways to skin a cat.
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Old 04-06-2021, 02:39 PM
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drgouk,
Curious.... What happened to the spot next to the valve guide on the far-left one?


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Old 04-06-2021, 03:31 PM
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