Cylinder head work questions
01-12-2004, 08:23 PM
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Cylinder head work questions
Due to a timing belt incident a while back 
, my 968 is in need of some cylinder head repairs. I finally had time this week to do the belts and get it back in time. The compression check showed: 180, 80, 205, 80 psi from cylinders 1-4, respectively. Not so good.
I don't think I'll have any trouble pulling the head, but I have some questions about what exactly to do with it afterward. (I don't want to sound like a complete idiot when I start calling around!)
1. Should I take the head to a Porsche mechanic to be checked out, or should I just take it straight to a machine shop? In other words, will a good machine shop have all the knowledge (porsche specs) to tell what exactly should be done to it? I'll probably supply them all the parts needed.
2. Will I be fine just replacing the damaged valves, or should all 16 valves be replaced while it's out? (My dad seems to think they should all be changed. I don't want to throw away money on needlessly replacing valves, though.)
3. Are valve guides normally all changed out, or just the ones that are out of spec?
4. Are springs normally replaced, or are they generally fine?
5. Anyone have a recommendation for a good machine shop anywhere in southeast michigan (detroit, ann arbor, monroe)?
6. Any thing else I should be aware of?
BTW, the car has just over 100k miles. I'll also be changing the variocam pads and chain (if the pads ever come off backorder).
Thanks in advance,
Jeff
, my 968 is in need of some cylinder head repairs. I finally had time this week to do the belts and get it back in time. The compression check showed: 180, 80, 205, 80 psi from cylinders 1-4, respectively. Not so good.I don't think I'll have any trouble pulling the head, but I have some questions about what exactly to do with it afterward. (I don't want to sound like a complete idiot when I start calling around!)
1. Should I take the head to a Porsche mechanic to be checked out, or should I just take it straight to a machine shop? In other words, will a good machine shop have all the knowledge (porsche specs) to tell what exactly should be done to it? I'll probably supply them all the parts needed.
2. Will I be fine just replacing the damaged valves, or should all 16 valves be replaced while it's out? (My dad seems to think they should all be changed. I don't want to throw away money on needlessly replacing valves, though.)
3. Are valve guides normally all changed out, or just the ones that are out of spec?
4. Are springs normally replaced, or are they generally fine?
5. Anyone have a recommendation for a good machine shop anywhere in southeast michigan (detroit, ann arbor, monroe)?
6. Any thing else I should be aware of?
BTW, the car has just over 100k miles. I'll also be changing the variocam pads and chain (if the pads ever come off backorder).
Thanks in advance,
Jeff
01-12-2004, 10:34 PM
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Jeff-
Albeit I am new to the 968 scene (looking for a stable mate for my 928 S4), I would believe that there are numerous similiarities between the 2 vehicles.
If a timing belt slips/breaks on a 928 motor - we generally advise the following-
1) Replace all bent valves - as well as any other valves which exhibit the need for replacement.
2) Unless the machine shop has access to the specs for the 968 valves - take the head to a P shop - not necessarily a dealer, but a shop with water cooled P car experience. Note I said water cooled experience. In many instances, we 928 owners continue to challenge the typical P car mechanic when we ask for work to be performed, as not all are familiar with our cars.
3) Besides the valves, you should consider replacing the chain and tensioner as well. There is alot of stress put on these 2 items when the pistons commence smashing/bending the valves as these items are in the direct path of the force.
4) In as far as the valve guides, I would personally replace them while I was already in there for thoe valves replaced. Depending on the milage on the motor, I would consider replacing all of them.
5) The valve springs should be okay. Again, the Shop Manuals have more direct information on this, and a check is well worth while.
6) You might consider checking to see if a 928 Chain Tensioner pad would work - I don't know if they would (haven't read/found this - yet). If they do, check with one of the 928 Vendors for a replacment.
hth-
(p.s. If I've screwed up any of this, someone please let me know)
Albeit I am new to the 968 scene (looking for a stable mate for my 928 S4), I would believe that there are numerous similiarities between the 2 vehicles.
If a timing belt slips/breaks on a 928 motor - we generally advise the following-
1) Replace all bent valves - as well as any other valves which exhibit the need for replacement.
2) Unless the machine shop has access to the specs for the 968 valves - take the head to a P shop - not necessarily a dealer, but a shop with water cooled P car experience. Note I said water cooled experience. In many instances, we 928 owners continue to challenge the typical P car mechanic when we ask for work to be performed, as not all are familiar with our cars.
3) Besides the valves, you should consider replacing the chain and tensioner as well. There is alot of stress put on these 2 items when the pistons commence smashing/bending the valves as these items are in the direct path of the force.
4) In as far as the valve guides, I would personally replace them while I was already in there for thoe valves replaced. Depending on the milage on the motor, I would consider replacing all of them.
5) The valve springs should be okay. Again, the Shop Manuals have more direct information on this, and a check is well worth while.
6) You might consider checking to see if a 928 Chain Tensioner pad would work - I don't know if they would (haven't read/found this - yet). If they do, check with one of the 928 Vendors for a replacment.
hth-
(p.s. If I've screwed up any of this, someone please let me know)
01-12-2004, 10:54 PM
#3
Good advice. To follow up here, I'll add a couple of things.
1. If the valves are bent or stems below spec, OD size, or margins very thin, replace.
2. Consider shipping head via Fed Ex or UPS to "Porsche" machine shop. Expect to pay more as Head has 16 Valves etc. I keep banging this same old drum, but Performance Developments, 949 646 7461 is one of the best. They know all the specs and specialize in Porsche race engines and machining.
3. Change the chain and the Tensioner. Absolutely.
4. Guides will all be checked for size when repair is done. If some need to be replaced, consider doing them all, and at the same time they will upgrade the material, and these will last forever.
5. The Springs will also be checked before the Head is re assembled. They will check the overall spring rate, and the Installed rate. If any are low, they will replace. At your mileage, you should expect to change them.
6. Consider a MLS Head gasket. PD has them for your engine. Far better than stock. Surface the Head. Most have some sort of corrosion so this needs to be fixed. As long as it does cause a problem, a skim will do. This ensures the Head is perfectly flat. They will either lap or surafce your head.
Also have it pressure checked. It is probably ok, but have it done to be sure.
1. If the valves are bent or stems below spec, OD size, or margins very thin, replace.
2. Consider shipping head via Fed Ex or UPS to "Porsche" machine shop. Expect to pay more as Head has 16 Valves etc. I keep banging this same old drum, but Performance Developments, 949 646 7461 is one of the best. They know all the specs and specialize in Porsche race engines and machining.
3. Change the chain and the Tensioner. Absolutely.
4. Guides will all be checked for size when repair is done. If some need to be replaced, consider doing them all, and at the same time they will upgrade the material, and these will last forever.
5. The Springs will also be checked before the Head is re assembled. They will check the overall spring rate, and the Installed rate. If any are low, they will replace. At your mileage, you should expect to change them.
6. Consider a MLS Head gasket. PD has them for your engine. Far better than stock. Surface the Head. Most have some sort of corrosion so this needs to be fixed. As long as it does cause a problem, a skim will do. This ensures the Head is perfectly flat. They will either lap or surafce your head.
Also have it pressure checked. It is probably ok, but have it done to be sure.
