Freshly rebuilt piston slap
 

Hi.

I had a rebuild on my 1980 911SC engine recently due to a broken head stud.
I kept the same pistons and rods as they were great but had all studs, bearings, valve guides and rings replaced.

I have done about 2000km since the rebuild and sometimes I would hear an uncharacteristic knocking. Mostly when cold. That knocking seems to be more the usual than the exception now.

I returned the car for a general checkup, new oil etc and asked that they investigate the noise.
The dealer (They are very good Porsche specialists and have been in operation since the 1970s) told me that the noise is some piston slap and that I shouldn't worry as it doesn't matter.

Now I generally trust the guy as he is a real expert, but this was a $10 000 job and I need to be damn sure that this is not going to cause problems later. I am not going to pay for even bigger work if something is wrong and I don't own a Porsche to drive it like a Volkswagen. So I am going to drive it properly and sometimes hard.

So the question is. Is it unacceptable to have some piston slap on a rebuilt engine with new rings but original pistons/sleeves? It did not have this before.
If there is some piston slap, is it something that needs to be looked at immediately or is it indeed not something to be unduly worried about?
Is it something that I should monitor and deal with if it doesn't go quiet when warmed up? Obviously it shouldn't get worse.

Its not terrible, I have sensitive ears, but it is a definite knocking on a specific cylinder. Power feels great after the rebuild at low speeds. High speed still feels a bit tight and I struggle to go over 128-ish mph where I should be easily doing 135.

Any comments?

In the end, you need to ask the shop who did the work if they are going to warranty the work. If there is an issue then the question is are they going to stand by it? If they are a good shop as you say then the answer will likely be yes. That being the case, act on their advice and drive her in good health knowing the shop who did the work says it to be a non-issue armed with the knowledge that if it is an issue they will stand by their work and correct their mistake.

Ask them what pistons they used.

Mahle pistons run dead quiet unless there is something else wrong.

Aftermarket 2618 pistons can make some noise when cold depending on fitment.

It sounds like the shop used the original pistons supplied with the engine before rebuild.
If there was no noise (sometimes new engine owner hear noises they didn't hear before) prior to the rebuild, I would suspect an intermediate shaft knock.
Very common on early SC engines.
It's possible they replaced the gear. "-1" gears are harder to find. Perhaps they replaced a "-1" gear with a "O". This will create a knocking noise that is annoying but not a death rattle.

1: If it is piston slap you will be able to tell without a doubt by placing a stethoscope on the cylinder fins... or use the "screw driver trick" if you don't have listening tools.

2: Steve, it's interesting that you say the Mahle pistons run dead silent, when I was doing some research post build (on my 964 3.6) I seemed to find people saying that the stock pistons would slap if they were any looser than center spec on the piston to wall clearance. I have some piston noise on mine, again only when it's cold, and when I went back and checked the data all the pistons were out to the loose side of the clearance, and one was actually in spec due to rounding at the .000X" decimal place. I assume this is why I get some cold piston slap, and I will end up tearing it down at some point for P&C work.

3: If they reused your P&Cs I don't see how you can really come back to them for warranty work on the used parts, IF it is a piston slap noise. It doesn't make sense to me that the noise is new, unless they botched the cylinders up trying to recondition for new parts.

4: Ask them what the piston to wall clearances were, and what they would accept for reusing parts. Technically what you paid them for in this are was their expertise in part fitting, You might have paid for bad advice, but unless they were negligent, I'm not sure you have recourse to try to make it quieter. The fact is if you wanted nee P&C the rebuild would have been MUCH more costly. You could possibly ask for help with the added labor, but I think you'll be on the hook for the parts cost.

5: since they did do bearings you want to make darn sure it's not a rod noise. There have been Glyco rod bearing problems on these engines. Ask if they were Porsche OEM bearings, or Glyco, and ask what clearances they targeted. Double check for piston slap noise with your stethoscope and make sure its not coming from the case!!

6: I'm not pretending to know more about these things then steve... he's built a LOOOT more of these engines than I ever will!

I agree with henry about the IMS... did they replace those bearings?

Thanks for the ideas.

I can't find my invoice now to check items, I think it is still in the car since the rebuild.
I was told that my original pistons and rods were in extremely good condition and the bearing were replaced as a matter of course. The only issue was the one head stud.
The car has only 180 000km and I have had it since 140 000. I am always careful when I drive to have the engine heat up correctly. about 10-12 km or so.
I'll have a more careful listen when I fetch the car later this week and ask the questions. Perhaps I am just paranoid. I can always record the engine for reference!
They would have used Porsche OE items as they always do. I also insist on that. It may just be later items from 964 or 993 models (like the head studs being 993).

I am not really concerned about the noise itself, just that something may wear in the engine unnecessarily, precipitating expensive repairs again or at worst damage the engine severely. I am not quite certain how bad piston slap is. I only heard the term the first time today and did some quick checks online.

Any noise is worth investigating, so do your due diligence. The answer cannot be found on this forum. Its in the engine where you are.

I’ve had worn rockers that sounded like piston slap. I put in new Mahle sets and still had the slap.
Changed the rockers and it went away.
Bruce

Just an update after fetching the car and driving it for about 40 miles.
The mechanic said there is a possibly play on a rod (I forgot now which exactly it was). HE also explained how it could be the piston too but it is not something to be concerned about.
After warming up the car it is indeed much quieter and smooth with great power delivery and sound. And bringing a smile as only a well tuned 911 engine can.
He said the sound is definitely due to the old parts that were not replaced, but that it is not a problem at all. Just, as always, remember to not race the engine hard while cold. Which is normal behaviour.
So it looks like I am just a bit too paranoid for now.

Thanks for the pointers though. I will still monitor and have a healthier knowledge to go with it.

Have you thought about a second shop for an evaluation of the noise. Rob

see any soot near the exhaust ports that might indicate an exhaust leak? Easy to look for.

I've also been told that rockers can cause ticks or knock noises like mentioned earlier.
I would also have another shop eval the noise. And maybe a 3rd if you are loosing sleep over it.

It's unfair to challenge an opinion as there are always another side to every explanation.

However, there should never be any rod noises coming from an engine. The rods in these engines are bottom guided so the clearances between the crank and the rods BE is around 0.005" max. The bearing clearances should be around 0.0008"- 0.0025" or so, and the difference is taken up with oil. The pin to pin bushing clearances are approx. 0.0008" to 0.002' also, with an oil film. So no noise should be heard. Clearances of these sizes will not make any noise that can be heard.

The use of existing old parts is Ok if the sizes are in spec, their condition is good, and the expected use after rebuilding is within a time period those parts can last. If you have any piston noise there is a chance the clearances are beyond those that will not make a noise.

Best to ask the builder the clearances measured on assembly, and the piston manufacturer. Some pistons made from 2618 require more clearance than those pistons made from 4032 aluminum.

Any noise has to be investigated. It could be the pistons hitting the head, valves just touching pistons, all sort of reasons some also suggested in other posts. The devil is always in the details and the builders build sheets with all measured dimensions will tell. Or remove any doubts.

If it is quieter after the motor is at full temp, I doubt it is a rod. AHIK.

A stethoscope is your friend.

Just curious...

Do rod bearing and crank bearing clearances tighten up as a motor heats up?

I didn't think so...?

Re: rod clearance, certainly you shouldn’t be having any bearing clearance to hear a noise, but I’ve heard of some engines having side clearance on rods and causing an audible noise. Not sure if such a noise would tend to go away as the engine warms up.

Scott

I sent my old 95MM cylinders/pistons to Olly's for inspection/measurements, I was warned by Olly's if I chose to use my close to out of spec worn cylinder/pistons, that I may get piston slap/slop. Well, that triggered me to go 3.4 with CP Carrillo 98MM pistons and EBS bored/restored 98MM cylinders. Olly's had no teeth in the game.

How close were they to warrant that disclaimer from Ollies ? Followup is whether Porsche's spec is on the optimistic or conservative side.

Warrant? Well they gave me the numbers off their gauges, got them back and measured them as well. It was right at or near the maximum wear limits.

I think Porsche was conservative but why take a chance after all the work and cost. Plus a good excuse to upgrade, at least that is what I told my wife. :-)

A stethoscope will let you know where the sound is coming from. Most mechanics will have one.

A rod knock will get worse as the motor gets warm and piston slap will or should be less.