Replace the cylinder heads and pistons!

Sunroof · 02-06-2020, 04:34 AM

My car is in the shop, failed a leak down test and they are advising me that they might change out the jugs (head and piston combo) as replacing piston rings may not be a perfect cure! This seems odd to me as I was always under the impression that you can replace worn rings. What would warrant the replacement of the jugs, besides the huge expense. I just want to get some feedback on this.

Thanks

yahh · 05:47 PM

The cylinders could be out of round or the walls could be worn. That's not enough info

darrin · 05:01 AM

presume they're referring to the cylinders and pistons, and not the heads (which fit over the cylinders) -- worth asking what's causing them to want to replace cylinders. What model car/engine do you have? Believe some piston rings are hard to source/replace.

On my 3.2 Carrera (and many other 3.2s), my rebuild was caused by worn valve guides which allowed the oil lubricating the valve train to be sucked into the combustion chamber and caused high oil consumption(quart of oil every 500 miles). The remedy was to have the heads rebuilt (valve guides replaced and valve seats reground)-- my pistons and cylinders were fine and were reused. What's your oil consumption like?

If worn rings are causing leak down problems, I'd think there'd be contaminants in the oil (from gas, etc. making its way past the rings and into the sump).

r lane · 02-06-2020, 06:03 AM

Unless a piston has broken or something has grooved the cylinders. I have not had to build a motor due to that but on a couple of cases, but many due to valve guide wear. I would get another opinion. Is the car running poorly or consuming copious amounts of oil. In the case of a rebuild, the cylinders can be reconditioned to spec, a lot more economical than a new set. Bob

Sunroof · 02-06-2020, 06:04 AM

My engine comes out of a 1973.5T. Its the 2.4L with the first CIS. About 75K on a total rebuild. About a quart every 1,000 - 1,500 miles.

The cylinder heads comes as a kit (Mahle PS84-100) complete with pistons and rings). I thought replacing rings would take care of the issue. The exhaust gives off blue smoke even after warm up. Not heavy but visible.

Sunroof · 02-06-2020, 06:07 AM

Thanks r lane.

The set of Mahle cylinder heads/pistons are expensive (5K-6K). Do you have any idea what is involved in "reconditioning" and cost?

faapgar · 02-06-2020, 06:28 AM

The key to be able to replace just rings is the condition of the top ringland.You need to check the clearance of the ring to the piston.Cylinder ovality needs to be checked next.Trueing the cylinder can leave to much piston to cylinder wall clearance.If the piston is good I would replate the cylinder with Nicasil to restore back to stock.All Goetze rings from this era are cast iron which seat very quickly with Nicasil.Have done this many times and total cost is less than $1500 to get back to a new spec.Also the Nicasil creates less drag and adds about 12-15 hp.over cast iron & lower oil temp through less drag.Ciao Fred

GH85Carrera · GH85Carrera's Garage

When my engine was taken down due to a "wet" head that was leaking after 150,000 miles the cylinders looked like brand new. They still had the hone marks on them from manufacture. The head needed a total refurbish. Then the "while you are in there" started. In the end I was happy I did a full rebuild as some of the rob bearings had some hot spots, and the oil pump intermediate shaft bearings were worn.

I would ask them for specifics, and ask to see it in person and take pictures.

RedCoupe · 02-06-2020, 06:56 AM

Sunroof, I believe the Mahle parts you quoted are cylinders and Pistons, not cylinder heads and Pistons. Unlike air cooled aircraft engines, the cylinders and cylinder heads are separate parts and the heads can be re-machined for a tight seal to the cylinders. The valve seats are typically reground and the valve guides (which are a wear item) are replaced. Unless they're cracked, there is no need to replace the heads. As discussed earlier, worn cylinders can be replated and machined back to specs. Only the Pistons need to be replaced if they show wear, and of course the piston rings and possibly some valves.

Sunroof · 02-06-2020, 08:25 AM

Thanks for the opinions so far. I have to place my trust in this shop; however, I also want to be armed with the knowledge as limited as that might be that I am not dropping a ton of money into a repair of which a portion of that may not be necessary.

What's the worst case for three cylinders failing a leak down test?

al lkosmal · 08:30 AM

Sunroof,
One of the benefits of the leak-down test is that you can tell how much it is leaking and where the leak is located before tearing down the engine.

Listen for where the air is escaping :::

Intake valve : Air whistling out of the intake, carburetor or throttle body indicates a leak at the intake valve.
Exhaust valve : Air heard hissing out of the tailpipe means an exhaust valve leak.
Piston rings : Whistling or hissing out of the oil filler cap hole means the air is leaking past the rings indicating ring or cylinder wall wear.

The pistons and cylinders have a size specification and measuring them will determine if they are in spec or not.

i'd be asking for the specific leak down results.......what was the % per cylinder and...where were they leaking from?.....then you'll have a better idea of the seriousness of the problem

Trackrash · Trackrash's Garage

As mentioned, you need to know where the leak down failed? I am assuming rings, from what you said.

If it is from the exhaust valves, that could just be carbon particles that came loose and got stuck on the valve as the motor was turned over. AHIK

The bottom line, on your motor, is the cylinders are iron. Those are usually done, meaning worn, by 100k miles or so.

Then, do you recondition the cylinders and get new pistons? Or a whole new set?

Perhaps the previous "rebuild" they in fact only replaced the rings and now the cylinders are done. Just a guess.

Burning a quart every 1000 miles sounds fairly normal to me by the way...

r lane · 02-06-2020, 09:56 AM

I think that engine had the cast iron cylinders still and if so, they were very durable. A quart every 1000-1500 miles is acceptable, so wondering if the smoke you see is oil smoke. Do you leave a trail under acceleration? What compression and leak down numbers did they have and was it done hot. Number one decision is do you require a rebuild, if so, then making appropriate assessment after tear down of parts to keep and which to throw away. EBS can recondition cylinders. Bob

Jeff Higgins · 02-06-2020, 10:44 AM

It sounds like there may be some misunderstanding of the names of the various components at play here. While it is common to replace pistons and cylinders as a set and, yes, they are still available as new from Mahle (and reconditioned form other sources), I don't believe that new heads are available from anyone, much less Mahle. As others have said, we rebuild heads, but rarely would one need replacing. All of the wear items in the heads are easily removed and replaced, the basic casting is pretty much good forever.

Pistons and cylinders:

Cylinder heads:

One thing no one has asked yet is just how much use does this car get? If it sits a good deal, it's not uncommon for it to smoke for a good long time after startup. It is also not uncommon for it to fail a leakdown test. Your oil consumption is, as other have stated, perfectly normal. It is well within the range specified by Porsche when these cars were brand new.

You may be able to "fix" your motor with a good old fashioned "Italian tune-up". Take it out and drive it, and drive it hard. Up to redline through the gears, repeatedly. Hammer on it. You can't hurt it. Doing so may very well blow that little chunk of carbon off the valve seats, may very well "re-seat" the rings, and cure all of its ills.

If nothing else, it will make both of you feel better...

Sunroof · 02-06-2020, 12:06 PM

Thanks Jeff for clearing up some things for me. I already did an "Italian tune up" as recommended last year and it was not successful at remediating the problem. The car over the years has been a weekend driver (Saturday drives) never raced, never over revved and well taken care of. The onsite of blue smoke well after warm up came on fast, but its not a huge plume, rather a faint stream.

I plan on going over to the repair shop tomorrow to see what's going on and ask about my options.

I appreciate everyone's input as stated earlier which reaffirms to me the value of this brotherhood of Porsche enthusiasts.

Bob

Jeff Higgins · 02-06-2020, 01:24 PM

One more thought, Bob, since I just repaired this problem on my car. I had a pretty good oil leak coming from the right side camshaft thrust plate that is installed in the chain box. It was dripping right on the flange where the heat exchanger bolts to the muffler. For those following me, it looked for all the world like it was smoking from my right hand exhaust outlet. It turned out that the smoke was being generated by that drip, in front of the muffler, blowing around it and fooling folks into thinking it was coming out of the pipe.

So, just a thought. How does the muffler look? Any oil residue on it? Single, stock outlet or some kind of "sport" dual outlet? Is the smoke really, really obviously coming out of the tailpipe(s), or do you just see a haze in the rear view mirror? Have others commented on the smoke? Like I said, in my case, even the guys following me (all in their own 911's, all of whom really know these cars) were all mistaken regarding the source of the smoke. These things will really fool even the best of 'em sometimes...

yelcab1 · 02-06-2020, 01:25 PM

No Italian tune up is going to solve high oil consumption or leakage past the rings

Jeff Higgins · 02-06-2020, 04:24 PM

Quote:

Originally Posted by yelcab1

No Italian tune up is going to solve high oil consumption or leakage past the rings

He does not have "high oil consumption". 1,000 -1,500 miles per quart is, again (as a few of us have pointed out), well within factory specification when these cars were brand new.

He does not yet know where the leakage is occurring.

yelcab1 · 02-06-2020, 05:03 PM

Quote:

Originally Posted by Jeff Higgins

He does not have "high oil consumption". 1,000 -1,500 miles per quart is, again (as a few of us have pointed out), well within factory specification when these cars were brand new.

He does not yet know where the leakage is occurring.

He doesn't know but presumably his shop does, otherwise they would not make the recommendation. Unless, we all assume that the mechanic is crooked which is a terrible assumption.

Jeff Higgins · 02-06-2020, 06:45 PM

"Crooked" is a rather poor assumption, and not the first one I would make. Unsatisfactory leakdown results represent, to me anyway, no more than a point at which to begin the investigation and discussion. The shop has provided a recommendation, one that I see as a "worst case". We do not know if, just because they made that recommendation, if all (or any) of it is really necessary. Even the best shops would have no way of knowing without disassembly and inspection. I believe they are preparing Bob for the worst, which many shops will do. I'm trying to prepare him for the best, by offering alternatives that, in my experience, have proven effective.