Jherriott

Hey guys,

I have searched and researched on the topic of engine break-in procedure ad nauseum and unfortunately hit that cliff of sifting through elaborate theories that may or may not be based on "empirical evidence"... Also, my engine was not a total rebuild, so I don't have to break in reground cams etc. So a lot of the information does not apply to me.

So here's the deal; the motor was dropped due to worn valve guides. Excessive oil consumption and accompanying blue smoke when driven with "normal' deceleration. This is the first rebuild since new at a mileage of 137,000 miles. The heads were sent out to get rebuild with all factory upgraded materials. The case was not split, cylinders looked very good with crosshatching still visible from factory. Rings were replaced and everything buttoned back up.

Now the thing that has got me worried is the mechanic told me to of course, get the car up to temperature every time I drive it, but that I keep the RPMs below 4,000 for... 5,000 kilometers. Then come in for a valve adjustment and oil change.

To me, that seems like a loooooong time before checking the oil for anything worrisome even with a light rebuild. And it seems like a loooong time before running the RPMs any higher in the spectrum. Even seems a long time to check valve clearances?

What do you guys think? I greatly appreciate any input as I am in a bad spot where my gut feels I should do something other than the mechanics advice, but my head tells me to do it because he built it.

Many thanks!

Jase.

77tony

air cooled ? I'd post here: https://rennlist.com/forums/993-forum-58/

Jherriott

I tried over at Pelican... no answer. Ill try a rennlist post too. But like I said, it's the minds here that I want to reach. Thx.

Jase.

Jherriott

...

r911

don't worry - it's why you have an oil filter

jamesjedi

I asked a similar question on the bird for my engine. You need to seat the rings. The non detergent oil is mentioned as being important. Brad Penn may be not be as effective in seating the rings. Also, you should do a leak down test before, then later to see if your rings have seated.

Here is my thread from the bird: http://forums.pelicanparts.com/911-engine-rebuilding-forum/946673-not-rebuild-just-rings.html

n8kruger

It's a little bit of a what's important to you question. If you want the engine to last longer take the mechanics advice. If you don't mind spending money drive the hell out of it right now.

Jherriott

Thx James, to the others... not so much.

I don't know where I said I want to drive the hell out of it, I am curious why changing the oil before 5,000km seems like a recipe for disaster. I want to know the engine is coming along before 1 or 2 seasons of driving.

And before people harp on, of course I am in discussions with the builder.But, I would like other opinions. Opinions from people that build and know and have real world experience... not just people with keyboards.

Jase.

cairo94507

I have always done the first oil/filter change after the first drive where the engine gets hot and I have driven it for say 30 miles. I have then done the next oil/filter at 1K miles and then I do it every 5K or once a year, whichever comes first. But it has been almost 25 years since my last 911 motor rebuild and things may have changed.

My fresh 3.2 is still waiting to be installed in my Six. Once that car is together and ready to be fired, I will do exactly what you are doing and try to mine the braintrust to determine the best way to handle this.

Mark Salvetti

I'm certainly no expert, but staying below 4,000 rpm for 5000 kms - for many of us that could be an entire driving season, maybe more. Seems unnecessary.

Over on the Pelican Engine Rebuild forum, seems the rule of thumb before exceeding 4,000 rpm is just 500 miles.

The 911SC owners manuals say not to exceed 5,000 rpm during the first 1,000 miles.

Mark

m42racer

Here goes my opinion. Sorry for the long winded post but I think its worth knowing what you are doing when making a decision of how long.

First off, you need to understand what “running in” an engine achieves. The builder or mechanic if they did their job correctly is only concerned with just a few engine issues. In this case, it should be, did the original fault get put right. Leaks, unwanted noises etc., should all be part of rebuilding it correctly. The Valve adjustment is the only real issue he needs to address. That can only be done by re checking. When, that depends.

As the owner, you are concerned with leaks, unwanted noises, the whole gambit. So, does it leak any oil and does it make any unwanted noises. If the answer is no, then you are almost there. Remember, engines usually only leak oil when they are hot.

Bearing clearances may change by 1/10’s when the engine is hot and running, but if the engine is assembled correctly, this is all taken into consideration. No amount of engine speed or time changes anything here. If any bearing clearance is not correct it will fail regardless of engine speed or time. Rod bolts can be considered the same. If one was not stretched or tightened correctly, these will fail immediately as well. The Oil pump is simple as well. It should have been disassembled and checked for wear and clearances. It either builds pressure or it doesn’t. If the oil temperature is good, pressure remains stable after running for a short period of time, you can assume the scavenge side of the pump is working as it should.

This is the engine bottom end covered. No leaks and no noises. Jump past the obvious, Cylinders and Pistons, and look at the heads and valve train. If the heads were rebuilt correctly, the Valve sealing and guide clearances will not change with any amount of engine speed or time. Springs can be considered the same as well. Rocker arms and their clearances are something that will and often change.
If the Ignition timing and fueling was checked and set correctly, then these also have no bearing on the running in process.

The only parts left are the Piston rings and valve lash. The clearances for the Pistons to the cylinder walls should have been measured and these are typically set to manufacturers specifications. If the RA number was known when the cylinder went back in the engine, this can be a huge determinator in the bedding in time. This should be known when rebuilding the engine. The cylinder supplier will have this number. Typically, in street engines this was in the 8- 12RA region and often is seen between 10 -20RA. The higher the number the rougher the surface, the more time is required to bed the rings in. Today we see mirror finishes to lower friction, so bedding rings in is something of an ”old fashioned thing”.
Taking the rings first, what you are doing when running in, is to allow the rings to bed themselves with the cylinder walls. New cylinders will have some amount of cross hatch honed into them to retain oil. This cross hatching is quite rough when magnified. The ring faces look the same under the same magnification. What you are doing is micro polishing these two components with one another to help increase the sealing. This is why using synthetic oils when performing this is often a no, no. They have modifiers built into them that make them very slick and inhibit the fiction required to bed the ring against the cylinder wall. Understanding what you are trying to achieve should give some idea why high Piston speed and high cylinder pressures are often said to be detrimental to good sealing. High piston speed can cause the rings to skip across these high spots on the cylinder walls ending with uneven polishing of the walls. High cylinder pressure pushes the rings harder into the walls causing the rings to wear faster against these high spots. Slower piston speeds and less pressure gives the rings a better chance to marry with one another and a better chance of an even wear pattern over the length of the cylinder. Under cylinder pressure the rings tip in their grooves to produce the contact points on the ring faces. This wants to happen but at the same time the cross-hatching needs to be introduced to the rings in a controlled fashion.

The other part to consider is the valve lash. Under running conditions the valves are slammed against the seats. At speed, the forces here are huge. You always get some movement here. The valve faces can deflect and the seat angle can readjust itself against the valve faces. The angle that seals is the 45° on each. If the valve seat work was done correctly this sealing should not change. But the valve position can change very slightly. This can change the lash. But the real change happens when everything heats up. Valve lash numbers are given as a cold-numbers typically and these clearances have the heat expansion built in. The real number should always be the hot lash as this the oil clearance at running temperature and the designed ratio between the cam profile and the valve control. The only way to check the lash in a car is cold. On an engine dyno, it is checked hot as it’s so much easier to remove the valve covers. In the car, it should be done cold and the lash numbers are cold numbers, so the only change in the cold number will be any settlement of the valves onto their seats. Engine speed and running time have really no affect here. Any change will happen within a very short period of time. The higher the engine speed in the same amount of running time, the more times the valves will hit the seats, the quicker this will happen.

Changing oil after the running in period removes the debris from the inside of the engine from these parts bedding against one another. Hopefully nothing else.

This is what “running in” a Porsche engine does. Long winded but it covers the basics I think.
In your case, you changed only the rings and guides. Guides are covered above. As for the Piston rings, these will try to bed themselves against cylinder walls that have had their high spots already removed. It may take a little longer or it may not happen at all. I don’t think any amount of time or running -speed will help here. In fact, I would lean on the engine a little more than if the Cylinder walls were fresh. I would not suggest running the engine at low speed for 3000 – 5000 miles. I would aim for 500 – 1000 miles maximum. If they don’t seal within this time, I would guess they won’t in 3000. In reality, there is nothing the ring faces are bedding against. You need to hope that the cylinders were round as this too will affect the sealing. The only issue will be the valve lash. I think after 500 miles the lash should be checked in case a valve is held off the seat. This will cause the valve to over-heat and hurt itself. A loose lash will be noisy and lower the engine performance but not really hurt anything. Zero lash will.

So, what the magic number of RPM’s and time? There isn’t one. What’s important here is to understand what you are doing. Once you fully understand what you are doing you can then use good common sense and judgement in deciding when to push the engine. Good inspection, measuring, component repair work and assembly all play a huge part in assuming everything is good. If there are any doubts here, all of this can be put in the trash bin and means nothing. There are few things in life you have to go with and up to this point one of them is the way your engine was repaired.

Jherriott

Awesome reply Racer, my sincere thanks for your time.

My main worry is what you described. The difficulty new rings might have seating to old smooth cylinder walls. I think I will take her in at 500 miles and see how things look in that regard and check the valve lash at the same time. I just couldn't see myself driving possibly 2 seasons with those questions nagging at me. I certainly wouldn't be enjoying the cruising like I should.

Once again, many thanks for your well tailored input. A concise and logical read compared to all the flotsam that I have come across out there.

Jase.

P.S. Thanks Mark and Cairo!

m42racer

You're welcome.

The only thing I can suggest is to fit a vacuum/pressure gauge to the crankcase. You could tap into the breather cover. You could then monitor the crankcase pressure and if it doesn't change from this start, then you can assume the rings are not bedding in. It may read a positive pressure as the pumps in those engines are not good at pulling a vacuum and I think the system is not closed.

Jherriott

Okay, I will check with the builder to see if he took any prior readings to compare to. Thx!

Jase.

m42racer

That will not mean anything. You changed the rings as they were worn out.

Just establish what the crankcase pressure is now and hopefully it will go down as the rings bed in. If it stays the same that could tell you they haven't of that they have and no change can be measured.

Then push the engine and see if the crankcase pressure rises. If it does and a lot, then you can probably assume along with any oil use they haven't. .

The builder should have either used rings that are compatible with low cross hatch, or had the cylinders honed slightly to start you off again like new. Changing the ring tension and or the RA number would have been the way to go here. At the very least, measured the RA to see if you were to have an issue or not.

Piston Rings and the sealing are pretty important in an engine. All too often they are taken for granted.
Books and manuals tell you the specs when the parts are new or need to be replaced. Don't give an idea when you are not doing either. Comes down to understanding what you are doing.