I recently bought a 1997 850 T5 with a bad head gasket. The car still runs without any engine knocks. The head gasket will need to be replaced, of course. I have replaced several engines in cars before as well as head gaskets. My biggest concern would be that this vehicle had its head gasket done before, about 75k ago. Does anyone know how much can be shaved off the head before its best to replace with new?

 

Unless the head is warped you shouldnt need to shave it, just make sure it is level...

The Bigger question is why did the head gasket go after 75K?
Was it overheated, was the last job not torqued properly...?

Thats what I would look into, I went 400K before I did my head gasket, and really didnt need to but I pulled my head to port and polish it.

 

Why not ask the shop you'd be taking it to as they should be able to answer that question. After all, what you are looking for is someone who will only skin enough off to make it flat and give it the proper finish to grab the new gasket. If they can't tell you how much they might not be the right place to take it to.
The other problem is you don't know if or how much was taken off the last time.
If it failed in 75K something is wrong. Either they didn't get the head surfaced and there was a bit of bow to it or they didn't get the surfaces clean and properly finished or they didn't torque the head down right. There is always the chance they did do all those things right and just never changed antifreeze but I'd sure double check everything when you have it apart and again when you're going back together with it if you plan on keeping it for a while.
Try to find the place where it failed and see if there is any damage to the head. I've seen heads where there has been a leak for a period of time and it's actually eaten a channel into the aluminum. It's not much of a channel but it has to be machined out and we're only talking about thousands here.

 

Ill pull the head off this weekend and get it to the machine shop for analysis. The previous owner was informed that the head was leaking in June and continued to drive it until last month when gasket failed completely and blew the cap off the overflow tank as well as water into the exhaust. My hope is that the head will require minimal machining however, the shop that did the head gasket repair in 2005 probably didn't stamp the head indicating how much they removed from it the first time. If need be i could always get a custom head gasket from Cometic Inc. in Ohio. I suppose this would ensure proper valve clearance and combustion. BTW just to be clear, is 0.012 or (.30)mm the maximum allowable machining on one of these heads?

 

If you need the cometic gasket let me know I am a dealer and I can get you a discount

 

Valve clearance is important but a better reason for spending the extra money for the thicker gasket is cam timing. Milling the head brings the cams closer to the crankshaft and advances the cam timing and distributor timing since the distributor isn't adjustable. Normal engine it really isn't a lot but if you're running a turbo and you're pushing a high boost then it does become more critical as a couple degrees can either lose you some power or push you over the top as to what your connecting rod or head of the piston can handle.

If you haven't done it already you should drain the radiator so nothing leaks into the engine and pull the plugs and spin her over to expel any coolant that might be in there. Then squirt an ounce or two of oil into any cyl you think took some coolant or just do all of them and spin it over a few times so the rings, piston, walls and valves get coated so they don't rust or don't rust further. Especially if you're not planning on pulling the block out of the chassis.
Last thing you want is to put that head back on and find out you have no compression because the rings rusted in the piston.

 

Okay, I removed the head from the block and found no signs of a broken head gasket anywhere. More disturbing was that the cylinders were all in good condition with only normal wear and carbon build up. The same cannot be said of the cooling jackets surrounding the cylinders; they looked like they were salvaged from the Titanic same can be said of the backside of the water pump, disgusting rusty goopy sludge. I noticed that the overflow nipple on the thermostat housing was completly plugged. So...any suggestions why there would be coolant in the block if it wasnt getting there via the cylinders?

 

What signs of a blown head gasket did it show?

 

That goopy stuff is probably oil/water mixed and a by product of running a car with a blown head gasket. The real question is what is the condition of that head. You HAVE to get it tested for warpage. Its possible the previous owners blew a head gasket and replaced the head gasket doing nothing else. Get that head tested.

 

+1000:d

 

Could be the result of running it with only water! Someone may have run it with straight water because it started to go through a lot of coolant. You may not have seen an obvious sign of leakage but that doesn't rule it out. When you take the head in also make sure you have them pressure test it on top of checking to see if it's straight. There is always a chance you have a crack in the head or block. I'd very carefully check each cylinder wall with the piston all the way down. If they don't find anything wrong with the head I'd go back and check each cylinder with a really good light and a magnifying lens.
You bought it recently?
Did you drive it much, miles?
You said the cylinders were in good shape. Are any of the cylinders or top of the pistons looking cleaner than the others? Any sign of corrosion of some of the pistons that might be related to coolant leaking?
Same with the valves. Any look noticeably cleaner or rusted from the others?
Who was it that said the engine had a blown cylinder head gasket?
What were the vehicles symptoms?

 

The engine had milky oil in the crank and when it was running, water would come out of the overflow cap. The overflow cap was broken, I'm assuming from the plugged up cooling system. Somehow antifreeze got in the crank but not through the head gasket...

 

You bought it recently?
*Yes, about two weeks ago.
Did you drive it much, miles?
*Towed it to my house. Didn't want to make things worse. I only started it to get it off the tow dolly and pull in the garage. The car has about 180k.
You said the cylinders were in good shape. Are any of the cylinders or top of the pistons looking cleaner than the others?
*All the pistons look good, only normal carbon build up. None of the cylinders look as though water was introduced into the combustion chamber.
Any sign of corrosion of some of the pistons that might be related to coolant leaking?
*Only corrosion is in the water jackets surrounding the cylinders, it looks horrible and very plugged up.
Same with the valves. Any look noticeably cleaner or rusted from the others?
*All the valves are clean with no visible signs of rust. The water ports around the head are also very corroded and somewhat plugged.
Who was it that said the engine had a blown cylinder head gasket?
*The previous owners mechanic.
What were the vehicles symptoms?
*Water in the block. Also the expansion tank cap was fractured and when the car was running, water would come out of the top of the tank. I believe that it came out of there because the thermostat overflow nipple was corroded shut.

 

Well then .........................................
I think the mechanic might have been mistaken. At this point I'm going to quit guessing as it's not going to help.
You are going to do the head gasket, hopefully the head is flat and sound. You'll need to flush the cooling system as it sounds terrible in there and with enough crap coating the coolant passages it could by itself cause overheating and force the system to expel coolant.
You'll need to do an oil change after the head is on and depending on how bad it is you might want to just drain and pour four or five cheap quarts in without a filter and run it up to temp and then do a full lof so you know it's clean in there and you get the crud out of the crankcase.
Now would be the time to check your flame trap or just replace it in case it's plugged and what you are seeing in the oil is severe condensation that isn't getting pulled out of the engine. I would think there is a way to check a flame trap I just don't know what it is.
If that crankcase is really milky, very light brown the only other way a lot of coolant can get in there is if the engine oil cooler inside the radiator is starting to fail.
Yep, I'm guessing again.