Unusual Findings on Teardown
 

Hi All

Having disassembled my 1990 3.6L (i.e. for #3 spun big end bearing) I was happy to discovered a PO had the cylinder/head update and the DMF was replaced with a LUK. Cylinders 1-3 did weep oil when it was running and a #3 head gasket (i.e. the metal ring type) was found to be broken. One thing I found that I can't understand is the cylinder base seals (i.e. the ones with a copper ring vulcanized to the black rubber O-ring) appear have been sucked into the case. I found a good portion of each O-ring smeared against the outside of the cylinder wall. It appears the shreds of the O-ring had passed in between the inner edge of the copper ring and the outside of the cylinder wall indicating there must have been a large vacuum built up inside the case. On assembly I plan to replace with the green viton type seals and the graphite type head gaskets.

All responses welcome
Bill
90C2

Do you have a photo of exactly what you mean?

-Wayne

Wayne

I've since cleaned the cylinders but I've kept the seals. If you can imagine the shreds of the rubber O'ring you see in the photo passed thru between the inner edge of the copper ring and the outside of the cylinder and in turn was stuck to the outside of the cylinder where it enters the case spigot. I can only think there must have been a large pressure difference between the inside of the engine and the outside of the engine in order to force the rubber O'ring past the inner edge of the copper ring seal and shred it in the way I found it.

Thank you for all your opinions
Bill
90C2


http://forums.pelicanparts.com/uploa...1219754353.jpg

I thought the c2 cylinder used a viton o-ring inside the the spigot bore of the cylinder. I haven't of the base gasket/o-ring that has just been described. Are you saying this part has metal center?

Dave

If I'm not mistaken the green viton is the prefered seal and I'm guessing this the inferior OEM version. Never having seen an unmolested new seal I'm not postive as to the construction but it appears that it may have been a square profiled black O'ring which is vulcanized to one side of a thin copper (i.e. 0.3mm thick) ring about 3.5mm wide. Then again it maybe a totally incorrect application for this seal which explains why it looks and failed the way it did and why I had a base cylinder leak on the left hand cylinder bank. I'm also guessing that a previous owner had the head/cylinder update under warranty 18years or 230,000kms ago.

All opinions welcomed
Bill
90C2

Gentlemen

I've measured all the heads and discovered they range from 83.322mm to 83.726mm - not good! They've all been machined under size from the 84.28mm minimum limit as described in the Shop Manual by a PO but atleast each bank is within the +/-0.02mm deviation tolerance. Thing is, I've driven the car for the last 130,000kms/8 years in this condition (i.e. unknowingly) with only a slight weep on the left hand bank cylinder/cylinder head interface. This may explain why there was a copper sealing ring (i.e. as far as I've been able determine, this sealing ring was not meant for this engine but is a part for earlier engines) at the base of the cylinders - in order to compensate for the deck/cambox height and maintain the camshaft/crankshaft geometry? It may also explain the higher compression readings I've been measuring over the years during my yearly scheduled maintenance checks - I thought it was just my guage out of calibration. I always thought this car had an above average crispness in engine response and power compared to the other 2 or 3 964's I test drove (i..e. one of the reasons I picked this one) back then - could this extra compression be attributed to this?

Anyways the question now, is this a known (non)standard machining practice and can I put it back together this way or should I start looking for new heads to get it back to spec?

Thank you for your responses above Wayne and Dave - this place is great!

TIA
Bill
90C2

Bill,
The weeping you refer to had shown up in a number of C2's. I have a set of C2 cylinders in my 930 and have the same thing showing in two of the cylinders. Porsche ultimately replaced these cylinders. I believe Steve of Rennsport has been modifying these cylinders and heads with great success. I guess mine will be going out to Steve for this mod sometime in the future.

I guess if the heads are out of spec, your options are limited. Maybe Steve or anyone else can chime in on this. If you do get another set of heads, I would recommend sending the heads and cylinders to Steve and have them modified. This will take care of the weeping issue.
Referring to the copper cylinder base gaskets, I strongly discourage the of these gaskets to alter compression ratio. This would have a detrimental effect on the squish area of the combustion chamber. Base gaskets should only be used for deck height and deck height only.

Dave

I guess I didn't make myself clear. The heads and cylinders have been modified by a PO. There is a groove at the base of the cylinder where the base o-ring/copper base gasket pictured above was installed. A dealer lead me to believe they were used on older 911 engines. Maybe someone experienced in the older 911's will chime in and recognize the O-ring-copper gasket. To narrow it down, it will have the same outside cylinder diameter (i.e. 107mm) as the 3.6L. A groove was machined into the top surface of the cylinder to accept the steel head gasket ring and the head surface was machined 145mm in diameter but in doing so I suspect they machined too deep as the distance from the head/cylinder surface to the head/cam box surface is almost 1mm (i.e 83.322mm to 83.726mm ) less than spec (i.e. 84.48mm or 84.28mm if it was machined to the limit). Although comparing my heads to pictures I've seen of other heads which have had the machine work done. It appears there hasn't been as much material taken off my heads - so maybe the top of the heads were machined - I doubt it! The base gaskets I mentioned above with the copper ring I suspect were installed to raise the cylinder off the deck to compensate for the extra material taken off the head and in turn keep the crankshaft - camshaft geometry closer to spec otherwise the camshaft would be almost 1mm (i.e. the amount of extra material machined from the head surface) closer to the crankshaft. Resulting in the cam chains being looser.

Hope this isn't more confusing
TIA for your responses
Bill
90C2

No sily-con on older gaskets as bought, but we do put some on when we assy. the engine... this would be on the copper shims that go between the cyl and case. On the cyl to head there are a few different seals used throught the older cars, but again, and I am fairly certain of this, they were all metal and not a combo metal/rubber. I think either you have some kind of OEM seal or aftermarket seal.

Best regards,

Michael

Thanks Michael

Another possibility is that the vacume created inside the engine case which sucked the O-ring past this copper shim and the outside of the cylinder wall - (i.e. shreding it in the process), also caused it to fuse to the copper ring? I suspect they were 2 separate parts when they were installed in the engine. How wide and thick are these copper shims before and after they are installed in an engine - I presume they are meant to crush and flatten out to a certain thickness under a specific head torque figure?

TIA for your responses
Bill
90C2

Gentlemen

I believe I figured out the base cylinder seal / oil leak mystery. The O'ring seals which were installed in the groove of each cylinder base never had a chance to seal against the case spigot because the 4mm wide copper ring which was placed between the O-ring and the case prevented the O-ring from ever coming in contact with the case spigot and creating a seal. At least this was the case on the left hand bank of cylinders which leaked - the right side was dry so the copper ring was making a better seal on that side. I believe there is both pressure and vacume built up inside the engine case just from the pistons moving up and down and because the gap between the case spigot and the outside of the cylinder wall is so large in relation to gap between the cylinder base / copper ring / case interface that this was the path of least resistance for the seal, - hence it was pulled/sucked into the engine being shredded as it was pulled past the edge of the copper ring . The sealing in this area can be resolved by just putting in a new green viton O-ring and eliminate the copper ring but I believe the copper ring was installed to act as a shim in order to compensate for the over machining done on the heads. So my question is: How much below spec can be shaved off the heads before problems with piston to valve clearances arise? I know in the olden days on North American engines it was common practice to plane down heads in order to raise the compression ratio. Can the same be done on the 3.6L and if so how much? If I do the solder on top of piston method to measure the clearance between the valves and the piston what is the minimum distance recommended keeping in mind that metal expands when it is heated to engine temperatures especially in the combustion chamber.

All responses welcome

Bill
90C2

Bill,
The maximum machining on the heads is .25mm +/-0.02mm (0.01" +/- 0.001"). Generally, heads are machined once, after that, they're out of spec. You're right about adding shims to the base to offset any machining done on the heads. Measuring Deck Height should be sufficient to offset machining on the heads as long as you are using the same cams and pistons. Ideal deck height is 1mm. Anything less than that, you stand the chance of pistons kissing the heads. Any more than that, you lose ideal squish area and less than ideal combustion. Machining the heads is not the ideal way to raise compression on these engines. Generally, machining the heads will get you only a 1/4 to 1/2 a point increase in the compression ratio. Pistons are your best and safest route in this area.
If the machinist that milled the heads followed machinists' guidelines, there should be a "-25" stamped or scribed on the heads. Judging from your description of the crush o-ring between the head and cylinder, it sounds like the head was not machined properly either on the cyl/head surface or on the area that surrounds that sealing surface. I would recommend you have the heads and cylinders sent out to Henry or Steve and have them spec'd.

Thanks Dave

I suspect I may be asking questions beyond the scope of this forum,
but
I've already determined that the machinist, whoever it may be, did not follow machinist guidlines as the heads have been machined out of spec according to my measurments so there is no -10 or -200 stamped on the flange (i.e. as per the Official Porsche 964 shop manual) to indicate the machining done.
According to the Official Porsche 964 Shop Manual the maximum machining on the heads is 0.2mm and mine have been machined 0.65 mm and 1.16 mm on each bank but with the 0.5mm shim the net is 0.15 mm and 0.66 mm. The 0.66 mm is 3X more than the maximum of 0.2mm spec'd in the manual, yet it worked with no issues (i.e. other than a slight oil weep on that bank) for the last 8 years or 130,000 kms I owned the car. What I'm trying to determine, is there a general rule of thumb for minimum piston to valve clearance? Can I go beyond what the factory specs (i.e. I already have) - I'd be interested in hearing from anyone who has experience in modifying these engines in regards to head/valve clearance specs and gone beyond these limits. Are there shims commercially intended or available for the 3.6L engine or can/do machinists make them (i.e. I supect the latter)? As soon as I determine that these heads are worth machining I'll send them out but I'm not interested in incurring shipping charges and finding out they can't be machined. I'd like to learn to spec these engine parts myself and with the help of Wayne's book I believe I can - I've already determined that all the valves and guides need to be replaced so it may make more sense to buy a new set of heads - maybe those hi flo 993 heads!

Thank you all for your comments
Bill
90C2

I don't think this situation is out of the scope of this forum. I recommend you create a new thread on each subject regarding this engine. I think Henry of Supertec or Steve of Rennsport will gladly chime in on the 964 engines, though the principles don't differ that much from the earlier vintages.

Thanks Dave

Bill
90C2