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Nathan M's Avatar
 
Join Date: Sep 2002
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Angry Machine shop woes

After much waiting and chasing I've finally got my heads back from the machine shop. I sent them to a supposed reputable company used by one of the most respected Porsche Specialists in the North of England. They went in for new guides, recut seats, valve grind etc. When I've got them back I just had to have a look to satisfy myself they've done what they said (they assembled the heads).

First problem - after everything I've read about head to cam housing oil leaks why have they gone & done this...



In case you can't tell, they've stamped the head numbers on the mating surface! Will I ever achieve a leak free seal? I've expressed my unhappiness but it hasn't got me anywhere. There also appears to be a paint like finish over the surface which easily rubs off (don't know what this is).

Second problem - the seats look to have been re-cut but not the valves. Again, my "friendly" machinist insists they "lapped" the valves in, as the condition meant it was not necessary to grind any material off the valve. There is no mention of valve lapping in any of the books I have. Will this have a detrimental effect on the seal of valve/seat, or is it acceptable?



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Old 12-02-2003, 03:15 PM
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Hi Nathan,
In my opinion, whenever the guides are replaced the valves and seats both should refaced. I'm assuming the guides were the main problem with your 89 since this is not unusual on the 87-89 3.2 engines. When the guides start to wear the valve never comes down on the seat in quite the same place which causes the seats to get a bit oval......also with age, the face of the valve which contacts the seat starts to sink a bit. You might not be able to see this with the eye, but if you put the valve on a valve grinder and start to reface it, you can see this. In the best of all possible worlds, if the new guides went in EXACTLY like the old ones and the valve faces and valve seats were both perfectly flat it might work, but this is too much short cutting to tolerate when you probably paid for a proper valve job, and you consider the amount of effort you are going to removing, disassembling, reassembling and reinstalling the engine. You are reinstalling heads with worn seats, worn valve faces and new guides. As far as the stamping on the cam tower mating surface, if you lightly sand the stamping with fine emory and use loctite574 anerobic sealant between the 2 surfaces you will probably be ok. I don't understand them stamping there though as there are many other places on a 911 head to stamp. Actually, i dont even stamp, i mark them with a sharpie pen once they are cleaned.

YMMV

Regards,

Pat
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Old 12-02-2003, 04:41 PM
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I would remove a random intake and exhaust valve, paint the sealing surfaces with a sharpie and slide the valve into the seat, by hand several times and a witness line should appear where the sealing will rub off the sharpie and tell you where the sealing will take place. make sure its uniform all the way around the valve.

If you paid for a complete valve job, you didnt get one. Technically, onyl a half of one......
Old 12-02-2003, 05:09 PM
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inre: The first problem.

The area between the valves springs isn't so critical. The major sealing area is the area that surrounds the springs.

You could lightly run a file over the number area just to make sure that the area is flat. It shouldn't be a problem.
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Old 12-02-2003, 07:24 PM
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Thanks for your comments. Pat, just to confirm, I've been told the seats were re-cut using the new valve guide as a guide for the seat cutter. The problem is the valve head itself.

One other question, when new valves are used, are these simply fitted as supplied, or is other work performed on them? I can get my my hands on a set of new valves for next to nothing (didn't do this as mine all measured up ok), but if it will help the seating I'll consider it. Going back to the machine shop is not an option...
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Old 12-03-2003, 04:04 AM
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Pour varsol (paint thinner) into the heads, the valve seat should hold the liquid and not leak down.

I would not use a file on the stamp, you might damage a bigger area than necessary. Use something like a dremel tool to carefully clean that area only. Just make sure the stamp is not proud of the rest the surface.
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Old 12-03-2003, 08:29 AM
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that seat looks pretty damn wide. and not cutting the valves is just plain stupid. the stamp marks can be dressed with a smooth file, and shouldn't be a problem.
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Old 12-03-2003, 01:26 PM
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Email or call Walt at Competition Engineering (www.competitioneng.com). He should be able to give you a brief thumbs up or down on issues like these. Sometimes he surprises me when I call him and ask him a question about something like this. Sometimes he goes off on how it's bad, and sometimes he says it's okay. He's the expert.

As for stamping the surface there, many shops do that. I don't understand why, and I asked Walt about this when writing the book. In reality, it doesn't really hurt the sealing, but indeed, it is stupid in my mind.

-Wayne
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Old 12-03-2003, 01:53 PM
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The valves indeed look lapped. The dull finish is the area where the lapping slurry made contact. The seats look lapped too.

I think this technique may work just fine and may be acceptable for a shade tree mechanic or someone that was on a budget.

If you paid good money for the work, this isn't how it is done. It isn't that hard to grind valves. E.g. a decent $300 job on a pair of american V8 cylinder heads will include cutting the seats, valves and installing new guides from a fancy (e.g. silicon bronze) material.

Lapping valves is something you'd do if you put an engine back together that had a problem elsewhere and you just want to attempt to freshen up that valve seal.

Lapping valves is frowned upon by folks because it can change the geometry of the angles, the contact area and the location of the contact area. The contact area is real important for valve cooling. If it gets too narrow or located too far towards the end (the wide side of the valve), you can get problems. I actually feel that the sealing area (the dull area on your valve) looks like it may be a bit too far towards the end of the valve. That may not be good, especially on the exhaust side.

Take this all with a grain of salt, I am still learning myself and some of the know how is not pure flat 6 knowledge.

George

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Old 12-05-2003, 11:53 PM
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