74 2.7 engine questions

Ohio911 · 11-12-2006, 01:32 PM

A little bit to read here
I recently purchased a 74 911 for parts. For the price, I assumed the engine was shot, but I needed many of the other parts for my current project. Rather than nit picking away a part at a time this allowed me to get all the parts I need at once plus many other parts I could use in the future or sell to re-coupe some of my monies paid.

Well for ****s and giggles I decided to start the engine. After a lot of ground cleaning and re-wiring etc she fired up. Not to my surprise, there is a low knocking noise which appears to be coming from the passenger side of the engine right around cyl #4. I suspected chain tensioner and a few other things so I dropped the engine. This engine has solid tensioners and the chain felt loose.

At first I thought" hey this is it" but upon further reading on here I found that with solid tensioners the chain has to be lose to compensate for expansion when the engine heats up. This is especially true with the magnesium case. So I then pulled the valve covers and checked the head bolts. Weirdly enough the bolts up toward cylinder #4 where a tad loose. All the rest where fine. Would this have caused any noise at cylinder #4? Now that I have tightened up the head-bolts does anyone think its worth the time to put the engine back in the car and see if that corrected it? The chain ramps that are visible are all black and seem in great shape so i do not suspect the loose chain. Should I bag this project and sell for parts?
Thanks All

tab tanner · 11-12-2006, 02:54 PM

Piston slap will sound like a low knocking noise

Ohio911 · 11-12-2006, 03:16 PM

I was told by one person whom was able to have a listen that it sounded like piston slap. What is the usual cause of this in a 911 engine?

If we assumed that it was indeed Piston Slap, and I was fairly ceratin this engine had been rebuilt not too long ago could this be fixed with a top end overhaul or does it get more involved than that? I have two 2.7 parts engines in various stages of tear down that I may be able to borrow parts from.

Basically looking for direction. I would be happy if I would get another 30,000-50,000 out of it as a recreational driver for my brother.

Thanks much for all input.

tt_targa · 11-12-2006, 03:26 PM

Bump

I'd like to here about suggestions for piston slap.

I think I read that slap is a start up thing. Anothersuggestion was that it took quite a while for an engine to kill itself with an audible slap.

afterburn 549 · 11-12-2006, 03:49 PM

Hmm....... tie the thing dwn and fire it on the bench?????????

911pcars · 11-12-2006, 04:29 PM

Piston slap is due to excessive clearance space between piston skirt and cylinder. Often heard during warmup before the piston warms up. Not a usual sound in a 911 as long as the factory piston-to-cylinder clearance is to spec. More commonly heard with aftermarket pistons because their greater expansion rate when warm requires more clearance when cold.

Excessive clearance not only results in piston slap but allows the piston to rock slightly causing the rings to wear faster. Replacing pistons/cylinders is usually not part of a top end overhaul; usually related to cylinder heads and stuff contained within it, but they're easily replaced with the heads removed. If the rebuilder managed the same piston clearance on all pistons, you'd hear slap from both sides.

However, the noise might be coming from the loose chain flopping around in the chain box. Good thing you pulled the cover off to inspect it. You can adjust the tension using the screw adjuster and the noise might go away which would confirm the loose chain theory. The real fix is to replace them with the factory tensioners that self-adjust for chain wear and temperature. The mechanical adjusters are not recommended just because no single tension setting will be adequate for all engine conditions.

But then again, all rebuilds are not equal. Who knows what the rebuilder did; how careful was he in selecting, measuring and assembling? There are a hundred places to screw it up if given the chance to neglect it.

Perform a close visual inspection, check for metal bits in the oil, mate the gearbox to the engine and perform a compression test. If it looks okay, I'd reinstall it, but with the factory tensioners. You can borrow a couple from the extra engines you have.

Sherwood

Ohio911 · 11-12-2006, 04:30 PM

I would probably be easier to just throw it back in the car. Are you suggesting that I bench test it so that I could better pinpoint the noise? No bumper etc in the way?

afterburn 549 · 11-12-2006, 04:32 PM

yup/ either is ....work

Ohio911 · 11-12-2006, 04:38 PM

Can anyone who has experienced piston slap on startup answer this one question?

If you where to rate the loudness of the knock on a scale of 1-10 with 5 being the noise made by a bone stock engine at idle, what would you rate it. I am going to say that the knock on this engine is at least a 6. How loud can piston slap be?

911pcars · 11-12-2006, 04:53 PM

Reinstalling the engine is 1-2 hours of work. If the engine needs work, why reinstall it? You can perform a compression test on the ground as long as the gearbox and starter motor is attached and you have a battery with jumper cables.

Not sure about the loudness from piston slap. That would depend on the piston skirt clearance. More clearance, more noise. Piston slap gradually decreases during warmup. If the noise doesn't subside or go away, it's something else.... like a spun rod or main bearing or chain slap. You could have done this with the engine in the car, but that's history now. Do the most expedient thing, but be thorough. Don't jump to conclusions.

Hope this helps,
Sherwood

Ohio911 · 11-12-2006, 04:57 PM

sherwood,

How would you reccomend I plug off the oil hoses if i where to do this out of the car as i assume you would have to fill it with oil in order to get proper compression tests and to not roach the engine?
thanks

Ohio911 · 11-12-2006, 05:07 PM

I forgot to add that oil pressure was perfect when I started the car. I only ran it for around twenty seconds for fear that I was doing damage. I have a compression tester on hand!! would this in any way help to prove for or against the piston slap theory? Would a spun bearing cause the car to report low oil pressure while running at startup? I think someone told me that

911pcars · 11-12-2006, 06:46 PM

"How would you reccomend I plug off the oil hoses if i where to do this out of the car as i assume you would have to fill it with oil in order to get proper compression tests and to not roach the engine?
thanks"

You don't need an oil tank connected to perform a compression test. However, you do need a bucket to hold the oil that will come out of the hose via the oil pump.

"I forgot to add that oil pressure was perfect when I started the car. I only ran it for around twenty seconds for fear that I was doing damage."

That's a good sign.

"I have a compression tester on hand!! would this in any way help to prove for or against the piston slap theory?"

No. It only shows a particular cylinder will seal adequately during the combustion process. You should have 6 good ones. If one or more cylinders exhibit no/weak seal (aka low or not consistent compression numbers), the engine will be down on power.

"Would a spun bearing cause the car to report low oil pressure while running at startup? I think someone told me that."

Possibly. Depends on how thrashed the bearing is.

As you might have gathered, at some point, you'll probably have to start the engine to diagnose the noise, if any. Once running, there are ways to eliminate and/or pinpoint abnormal sounds and sources.

Sherwood

Ohio911 · 11-12-2006, 07:29 PM

update.. I decided to take some of the chain ramps from one of my other parts engines to have a comparison with the ones in the engine in question. It turns out that the ramp on the left side of the alternator/fan is warn big time allowing for excess movement from front to back. There is evidence that the chain has dug into the case a bit. This could very well be the extra wear and tear that solid tensioners cause because of the necessity to leave the chain looser at startup.

Could this make a knocking noise?

tab tanner · 11-13-2006, 03:15 AM

Yes, I think you may have found one of the problem. This may not be all that wrong but a good place to start. I would replace the solid tentioners with OEM ones as was metioned earlier