Evening all.
Moving this post from another thread as it was not on topic.

I did an exercise this evening to check piston 1 TDC relative to Harmonic balancer marks.
A rod in through the plug hole with some markings to easily see/feel moment. Not a clock gauge for sure but pretty accurate.
I tried to locate the start and end of the dwell at TDC and record them relative to the HB markings.
I did this in both CW and CCW directions.
Results from 3 tests below - You can see a large difference in CW and CCW results. I assume caused by slack or slap or play in the whole system.
If I take the average of the overlap of CW and CCW results then exact TDC is actually 1.7deg after the OT mark on the HB.
If I only consider the CW results the exact TDC could be as much as 5deg before the OT mark.

What do you guys think of these results?
There will be a little human error in this but its pretty clear CW and CCW differ greatly - maybe 15degress difference.
As the engine actually runs CW maybe only the CW results should be considered and if so is the 5deg discrepancy acceptable?
If both CW and CCW results are considered then this would give the same result as using a piston stop - but if there is play in the system as seen here, then the calculated TDC would differ from the effective TDC when the engine is running .

Anyway - would love to hear some opinions on this one!

 

Never rotate the crankshaft CCW on an interference engine, you may bend valves. Do you have the WSM? Contact Roger at 928'sRus and get the Jim Morehouse thumb drive, Lots of valuable info in there. What Model Year (MY) is your 928?

 

those few degrees won't hurt turning it CCW... but i assume you do need a clock gauge ! at TDC the piston doens't move much with cranckshaft rotation as the con-rods are "in line " .

 

Rotating the engine backwards is not a good idea- theoretically the belt should not skip a tooth with the idler gear fitted that prevents this but why do it?

There is nothing to cause an error as the HB should be a key fit and it would be most strange if there were an error.

Not at all sure what exactly you are doing but if I had no visual reference to tell me where TDC was I would take something like a wooden chop stick and mark the travel depth at points indicated by the HB. Obviously the depth must be the same for a given angular rotation- the bigger the angle the more accurate the assessment is likely to be. As I recall there are marks at 45 degrees before and after but...?

 

I had read cautions on that but made a call that doing it very carefully was ok as it is described as one method to finding TDC using a piston stop..
It is an 83S Eur0.

 

Fred - my explanation is not clear.
I inserted a rod to feel the position of the top of piston 1.
Then rotated the crank clockwise and marked on the HB where the piston stopped rising and then again where it started falling. I marked this as the dwell in green on my sketch.
After continuing CW for 1/4 turn I then reversed and did the same thing in a CCW direction and recorded the dwell in green on my sketch.
This was done three times.

The CW dwell is completely offset from the CCW dwell.
To me that indicates slack in the system.

I guess my question is, does these seem normal? (keeping in mind my ongoing search for missing HP!)

Robert.

 

As long as the timing belt is close to the correct tension and you have a stock tensioner (don't do this with a Porken tensioner) you can turn the engine backwards (by hand), until Jesus returns.
Won't hurt a thing.
The warning in the Workshop Manual is for the idiots that would turn the engine backwards with total disregard for the belt tension (or for people with a Porken tensioner.)
German engineer to guy writing workshop manual: "Let's just play it safe and tell the idiots to never turn the engine backwards!"

There's probably enough "play" in your rod (moving around on the top of the piston) to explain the discrepancy of your readings.
You really need a positive stop, right near TDC...or a spark plug hole dial indicator.

A quick and dirty tool is to take an old spark plug and "gut it".
Tap internal threads to match whatever all thread rod you can get your hands on.
Install spark plug and thread in the rod until it stops the piston from moving up (around 5-10 degrees BTDC)
Turn the engine backwards until it stops again.
Simple math to calculate true TDC.

This method is accurate to a degree or two...which is plenty close for whatever you are trying to figure out.

If you want to know closer than this, do not allow the threaded rod to actually stop the piston...but turn the engine over super slowly, until the threaded rod "moves" inside the slop of the threads in the spark plug. Then repeat that 350 (or wherever you set the threaded rod to touch the piston) degree going backwards.
Same math.

Super accurate.

 

Robert,

I rather suspect that what you are doing, although an interesting "find", probably has little practical value given the current state of play.

Trying to ascertain the position of the piston with the methodology you are using would likely be accurate only to within plus or minus around 10 degrees thus why it is better to ascertain depth and known reference points before and after TDC.

On the other hand bearing clearances in the mains, big ends and small ends will create some finite degree of "lash" and I would think you should not be able to pick such up in a healthy engine to give repeatable results in the way you have, I have never attempted what you are doing so will reserve opinion- suffice it to say excessive bearing clearances would induce and quite possibly explain what you are measuring but I would politely recommend not to let your mindset go down that avenue unless or until you have a clear reason to believe such may indeed be a problem- like for instance very low oil pressure.

 

Thanks Fred - A red herring maybe.

 

Why not with a Porken tensioner out of curiosity?

 

turning CCW puts the belt tension for turning cams over the tensioner. The oem one is almost no "spring" and can take that load.
I suspect the Porken tensioner is a "spring" which would be compressed in before the cams even begin to turn.
Don't know if it can take that "abuse" mechanicaly ??

 

I also thought the reason to not turn CCW was because the sale of the cam loves were designed to only ride up the cams in the proper direction and not the reverse direction, but maybe that's not the case.

 

The sides of the cams (ramps) are symmetrical on all 928 engines, so rotation direction doesn't matter.

 

Thanks.

 

Exactly.