Sounds like a knock
 

I just rebuilt a 1975 911s engine. It was dismantled and sent out to be totally machined and refurbished. I put in 92mm JE pistons, bigger Mahle cylinders and had cam ground to SC specs and a new SSI exhaust There is a change in sound in area of cylinder #6 when warmed up to 210. It sounds like a knock and increases with RPM. Valves checked and rechecked and are adjusted correctly. Cylinders sound perfect through stethoscope. Timing chains sound smooth as well. Don't want to drive it until I can figure it out.

What about sending the oil out for analisys? If it's a rod or a bushing You should get a clear answer. Cheaper then tearing it apart.
luca

Thanks Luca for the suggestion. Rods have been refurbished and bushings are new and meticulously reassembled. Under load the sound does not go away. If it was a rod the sound would stop for a moment. I have changed the oil and filter 5x to see if I could see anything, nothing in oil or on magnet. Dismantled oil filter and spread out paper and wicked oil off paper to see if there were any metal in filter, nothing, very clean. Sounds from cylinders are consistent and very smooth sounding. I don't thing it's a rod or a bushing, but I admit I am at a loss for an answer. Some think I am imagining it. I guess that's possible.

Sounds like a knock
 

Does it go away when it warms up or get worse? Piston slap which is pretty common can sound like a rod

And a rocker worn past needing refurbishing sounds like piston slap
Bruce

The engine was totally refurbished by Ollie's and meticulous care was taken when put back together. I put in new pistons(92mm JE's) bigger cylinders, reground cams to SC spec and a new SSI exhaust, basically everything is new. I have changed the oil + filter 5x to check for any metal specks, nothing. I took the oil filter apart and unwrapped the paper and wicked the oil from the paper to see if I could see or feel any particles, nothing. There was also nothing on the magnets on the drain plugs.
Valves checked and rechecked and adjusted properly. Timing chains perfect. I also listened to the cylinders through a stethoscope and they all sounded smooth and all sounded the same. Maybe that is the way it should sound. It sounds perfect when warming up, not perfect at 210 on temp gage.

Do they show the cylinder and piston measurements on the spec sheet?

Do not have the specs handy. If you are referring to deck height, we set it at 1mm and the cam overlap was set at 1.5mm according to the recommendation in the book Rebuilding Your 911. It takes into consideration the piston size etc.

Piston to cylinder, back in the early 2000's mine were .0015 thousands but understand J&E changed the alloy used for pistons. I don't know if this change required a different clearance or not?

Hey Bubba, does the sound change as you load and unload the engine? Some bearings like piston pins when they have too much clearance increase and decrease their rapping noise as you speed up and back off on the throttle. Otherwise, I'm having a difficult time thinking how piston slap would increase as the engine warmed up, so I'm agreeing with flat6pac that it's likely to be a rocker arm or something related.

The sound does not increase or decrease under load, it's consistent once it warms up. It could be a rocker arm, but they were totally refurbished at Ollie's. There is a possibility that if there was a rough spot on the bushing when put it on the shaft that when expanding it could swell to the point that it locks on the shaft. I will loosen the rocker all the way and rotate it on the shaft to see if I can feel any catching. I am told there should be a little slop when the rocker is moved side to side as well (very Little). Does this make sense?

When you are in there, make sure that a rocker shaft hasn't "walked" sideways.

Good morning RedCoupe,
Only had time to check intake rockers and all were fine, none had walked. All were positioned where just a little of the shaft was extending out of the thin side. All had a little movement side to side and none felt like there was any binding or sticking. Cam lobes looked and felt fine as best I could tell. Will have time tomorrow to do exhaust.
Thanks to all for your suggestions.
Bubba

loose head"
or nut?
Did you replace the studs?

What afterburn just posted made me think about the sound of an exhaust or compression leak on one cylinder. Either one of these can make a noise that can sound like a metallic tapping. More possibilities....

Real real common for mag cases with out new and improved stud holding pwr.
The studs will not stay put. (they pull out of the magnisum case .
try ReTorq your head nuts to 30 lbs.
Do this by backing nut off just a little then then go all the way till you hit 30 lbs torq.
DONT do this with out some sort of case savors in place....

Case savers put in when machined. Head studs are torqued properly. I will be looking at the exhaust rockers tomorrow and resetting the valves after looking at the cam lobes. I have to admit I am baffled as to what is left to cause the sound. Everything I have checked thus far is set properly and looks great. I will however solve this problem.
Thanks all for your suggestions as to what it could be.
Bubba

Was piston to head clearance measured piston by piston when you reassembled motor? One of my JE's was just touching the heads (done by Ollies). It ran well for thousand miles then began to show combustion leaks around the top of that cylinder. There was no obvious noise but we weren't looking for one either. I think we took a couple of thousandths off the pistons at the edge.

I listened to each cylinder with a stethoscope and they all sounded the same and no audible difference. I would think that if the piston was hitting then head I would hear it. We checked each one and made sure we had 1mm of clearance.

Well I have readjusted all the valves and took a good look at the rockers and what I could see of the cam lobes and nothing abnormal. One point of interest: the lateral measurement of the rocker on the shaft as it relates to the cam tower, not sure it's relevant, varied from .0012 to .0016. I talked to the machinist and it is not something they ever work with and they said that difference is acceptable and shouldn't cause a knock.??? We only measured it because we noticed the shiny shaft beside one of the rockers and therefore measured it. The next step is to analyze the oil. Now beginning to think it could be a piston/head issue.
Thanks all for your interest and suggestions
Bubba