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I have read with some amusement some of the posts about rebuilding Porsche 911-cylinder heads. The loyalty to certain shops is apparent and commendable.

I am the owner of Performance Developments and we have rebuilt many of these heads for many well-known shops and customers for over 20 years. Most of our work would be called 1/3 party work for independent service and repair shops. Lately we have gotten back into the 911 air-cooled work with Jeff’s 911 project “saving the un savable”.

I thought I could give my opinion on what I feel a good head repair is and what you should expect to have done.

Firstly, if you are the builder of the engine, you must set an overall budget and an acceptable level of workmanship. No point installing Timex parts in a Rolex repair. This should then determine the costs and level of repair each part of the engine will require. An engine is the sum of its parts. An engine should be looked at as a collective mass of individual parts. Each part must do its worth of work to collectively produce whatever result you require. Cylinder heads are the most important part of an engine when it comes to the transfer of air in and out of the cylinders.

The performance all comes from the heads.

Nice clean shiny heads do not mean that the repair is good. But, if time was put into the appearance, it generally tells a story of the repair shops dedication to quality. You cannot inspect a head for damage and cracks if its covered in dirt and grime. Heads should be crack checked (zyglow) before any repair work is started along with retainers mag'ed or zylow checked. There should be an order of sequence when repairing these heads. General Inspection upon delivery, some scope of the work required, a quick test of the sealing capabilities of the valves could also be done now.

Disassembly tells a lot. You can immediately tell if there is a guide issue. You can see any defects in the seats, and the valve faces. Valve margin widths can be seen where the valve has ended its life and any repair to the valve can no longer be done. You can also get some idea of the spring forces as the number of shims under the spring base can tell a lot.

Cleaning takes the time, removing all the carbon and dirt takes time. Any nondestructive crack checking should be done now. If the guides need to be replaced, often a second crack check inspection should be done post guide installation as old cast guide bosses can crack. Removing the old guides should be done with care. Some hammer them out when others do it in a more careful manner. This is where your choice of shop and the cost of the repair starts to show. Hammering out the guides and hammering them back in is quick and time is money. The guides should be sized to obtain the clearance to the Valve stem. After the valves are cleaned and polished, the stems should be measured for size and the valve head checked for concentricity to the stem.

Guides very rarely go back in concentric to the existing valve seats, so any seat work typically requires the seats angles to be re cut deeper into the chamber. The top angle that transitions onto the chamber roof often must be changed to suit. This is when the chamber gets cut and from here on out things start to go south on the performance. There is no other way but to replace the seat, so again, the cost of the repair dictates the work done. I have seen seats sunk so low that the air flow was severely compromised. For engines that make little HP, a loss of 5-10 horse power is huge.

Most of the time street heads will use a 45° contact face to the valve. These can be stretched wider for longevity and typically the Intake will be 1.00mm – 1.30mm and the exhaust at 1.50mm – 1.70mm. Seat width has a lot to do with the installed seat pressures as well. The wider the seat the less seat pressure is requires as the transfer of heat from the valve happens quicker. Conversely, narrower seats require less closing force so they don’t wear out quicker.

Once the seats are all cut and the heights are checked, valves should be installed and the stem heights checked to ensure the correct rocker geometry is held. Any adjustments need to be done now. Then the heads are probably faced for height and sealing surfaces to the Cylinders. Final cleaning which may include ultrasonic cleaning will happen now.

The retainer heights are measured, springs checked for equal or almost equal rates and the installed heights calculated to obtain the same installed pressures. Over the nose pressures and a coil bind checks are done at the same time. Then the heads are assembled with new seals, bagged and shipped to the customer. We log all measurements and work done on every set of heads so that in the event we have to replace a head or something similar we have a blue print of what is required.

This takes time. I know that a clean shiny head gets people all excited. I have read where they want to make a display out of them. Whoa, it doesn’t take much to make them all excited! Like it has been said, the devil is in the details.

Machine shops are like watch repair shops. You own a Timex, you would never take it to a Rolex repair shop. The opposite applies as well.

Old 07-23-2017, 04:35 PM
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If i look to the pics you post here, heads are looking nice but valves seem to be cut much too deep. Could be an indicator that there is no good precaution to concentricity when changing valve guides or generally a bit rude approach.
I had lots US reimports here and motors rebuilt (which were usually redone min once in US) and in any case valve seats were set too deep to allow good valvetrain geometry. (and causing the need to replace all the valve seats, which usually is not necessary).
At least one of the big differences between US and EU Porsche engines if i am looking back on 25 years doing these.
There are possibilities to work as concentrical as possible when changing valveguides andor valves and this minimises necessity to "rescue" concentricity by cutting valve seats too deep.
Just my 2 to consider.
Old 07-25-2017, 05:42 PM
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Originally Posted by crummasel View Post
If i look to the pics you post here, heads are looking nice but valves seem to be cut much too deep. Could be an indicator that there is no good precaution to concentricity when changing valve guides or generally a bit rude approach.
I had lots US reimports here and motors rebuilt (which were usually redone min once in US) and in any case valve seats were set too deep to allow good valvetrain geometry. (and causing the need to replace all the valve seats, which usually is not necessary).
At least one of the big differences between US and EU Porsche engines if i am looking back on 25 years doing these.
There are possibilities to work as concentrical as possible when changing valveguides andor valves and this minimises necessity to "rescue" concentricity by cutting valve seats too deep.
Just my 2 to consider.
Agree on most of what you say. The photos often posted do not lie. Many show the seats sunk well back into the head.

There are shops here in the US that do great work and do it correctly. Your experience unfortunately is of the receiver of work not done by them.

I know that some of the best work done on Porsche engines is done here in the US. We strive everyday to be among them.
Old 07-26-2017, 10:39 AM
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Oh sorry, i admit to have sounded a bit generalising.

Just my experience of what i received and here its 100%.

BUT i absolutely do NOT believe that ALL US work is bad, not at all!

Maybe it is mainly badly maintained cars which go back to Europe for cheap.

However the posted photos here seem to show to deep valveseat "corrections" even if looking nice visually.

Plus i wanted to say one can work concentrically (using the valve seat as guide) to minimise the need of correcting the seat too much to avoid resulting into coming too deep for accurate valve seating.
Just my observation and not meant to offend anyone.
Old 07-26-2017, 06:17 PM
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Agree 100%. You are dead on with your observation.

Its a shame that these basic errors are not understood. Education is the key. Knowing what is right and what is wrong.

I intend to write a full rebuild paper on how we rebuild these heads and put it up on the PD web site. If this inspires others to follow and copy how we do this work then everyone benefits.
Old 07-26-2017, 07:29 PM
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Quality Air-Cooled Porsche Machine Shop

Going back to the original question (but with a specific locale), I'm searching for a top-notch shop in the Colorado Front Range area.
Back some 30 years ago when I was in the Norcal area, Elgin's Machine Shop in Redwood City did outstanding work on air-cooled 911 engines. Everything from top-end head work to line-boring cases to balancing (crank/pulley/flywheel/pressure plate assy) to recon work on rods to re-bushing the rockers. That's the sort of shop I'm currently looking for in the Denver-to-Colorado Springs area. Any of you Front Range folks have a strong/confident recommendation?
Thanks.
Old 08-06-2017, 06:34 AM
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Contact via pm, Pelicanite "Walt Fricke". He's in your locale and may be able to offer some assistance. Good guy, technical, experienced, he'll be able to offer solid recommendations. Walt, if you're reading this......sorry .
Be forewarned.....finding qualified air-cooled engine craftsman is getting more and more difficult as I have found out the hard way. Be prepared for a long distance solution.
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Old 08-06-2017, 08:26 AM
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Originally Posted by Neil Harvey View Post
Agree 100%. You are dead on with your observation.

Its a shame that these basic errors are not understood. Education is the key. Knowing what is right and what is wrong.

I intend to write a full rebuild paper on how we rebuild these heads and put it up on the PD web site. If this inspires others to follow and copy how we do this work then everyone benefits.
The rebuild paper I wrote is up on our web site. We will put more photo's up over the nest few days.

I hope this helps give a better understanding of what is involved and what to expect when rebuilding these air cooled heads.

If anyone has any questions or a better way, I'm always happy to learn.
Old 08-06-2017, 11:03 AM
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Another plug for Craig here. He did some heads (lots of porting, twin plug etc) for me recently and they came out perfect. Outstanding work and communication.
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Old 08-07-2017, 04:21 AM
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I think ya'all got trolled, as the OP never replied with his location or a simple thank you.

BTW I do quality head work, twin plug, 911 and 996 engine rebuilding, etc in Canada.
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Old 08-07-2017, 09:28 AM
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Quote:
Originally Posted by Neil Harvey View Post
The rebuild paper I wrote is up on our web site. We will put more photo's up over the nest few days.

I hope this helps give a better understanding of what is involved and what to expect when rebuilding these air cooled heads.

If anyone has any questions or a better way, I'm always happy to learn.
Neil, I read your article. Very informative. Thanks for sharing.
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Old 08-23-2017, 08:25 PM
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So, where is everyone sending their cam towers out to be cleaned, checked, and refaced?
Old 08-28-2017, 07:37 PM
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So, where is everyone sending their cam towers out to be cleaned, checked, and refaced?
Youshould start a new thread for this question, but I wouldn't reface the towers, every cut you make heads, towers you affect your timing chains and you don't want too much slack. You can't just remove a link because you have to remove two links and too much slack on the chain will cause more wear on the idlers and adjusters.

Cleaning the towers is actually quite easy, drill out the factory plugs remove the bolt galley plug, note the plug has a point and corresponding alignment hole. The tube now slips out. Tap both ends for 1/8" NPT, you must tap till the plug sits flush in the hole. Now clean everything. Install tube and aline for the bolt with point and install bolt. install the plugs with locktite or thread sealant, but first try dry to make sure the plug doesn't bottom out on the tube. If it does bottom out carefully grind the backside of the plug till it doesn't bottom out. install plugs and your good to go.
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Old 08-29-2017, 07:13 AM
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I ultrasonic clean cam carriers and do not remove the spray bars anymore unless you are going to seal the bar where it passes through the cam bearing. The bar feeds the cam bearing and if not sealed you loose a lot of oil between the spray bar and carrier. I assembled a carrier and it was a substantial amount of oil loss when tested.
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Old 08-30-2017, 08:57 PM
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Originally Posted by cgarr View Post
I ultrasonic clean cam carriers and do not remove the spray bars anymore unless you are going to seal the bar where it passes through the cam bearing. The bar feeds the cam bearing and if not sealed you loose a lot of oil between the spray bar and carrier. I assembled a carrier and it was a substantial amount of oil loss when tested.
Do you touch the bearing surfaces at all (light pass with SB etc) or suggest leaving these alone if within spec? Resurface only if absolutely necessary?

Curious as these essential engine parts (cam towers) seem to be overlooked most of the time.

Thanks!

Old 08-31-2017, 02:55 PM
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