Absolute TDC

[Mike V]

Has anyone ever determined absolute TDC with the use of a degree wheel and piston-stop tool?  Curious. Long story but involves view angle through engine case flywheel window. I've discovered (dubiously) it "does" make a difference. I won't bore you with the details, just want to hear from anyone that may have performed this task.

[bruce_launceston]

Hi Mike
I use the 'screwdriver down the spark plug hole' method and turn the engine back and forth with the rear wheel until I can feel it at TDC.
I don't know how 'absolute' you need to be?

[Rob Valdez 79 R65]

Hi Mike
I use the 'screwdriver down the spark plug hole' method and turn the engine back and forth with the rear wheel until I can feel it at TDC.
I don't know how 'absolute' you need to be?

I do like Bruce, except I turn my engine (clockwise - looking at the engine from the front) with a ratchet handle/allen socket in the rotor bolt.

I've heard it's not good to turn the engine too far in reverse?  Myth?

[Barry]

use the 'screwdriver down the spark plug hole' method and turn the engine back and forth with the rear wheel until I can feel it at TDC.

You might want to take care with this method. With youthful incompetence I did this on my first bike a ducati single and promptly broke off the end of the screw driver in the combustion chamber. You could hear the gumption hissing out of me as Pirsig would have it. Anyway I learnt to remove and replace a cylinder head pretty quick and still remember how easy it was to get the timing right by aligning the dots on the camshaft bevel drive.

It's very very difficult to do it accurately this way without a dial gauge. I tried once calculating vertical movement of the piston vs crankshaft rotation in degrees and concluded you need to be able to measure the piston movement to within a few thou for a decent result. I think Mike V is suggesting a much better method where you fit a physical stop through the plug hole so that the piston stops short of  TDC by an equal amount when the crankshaft is rotated from both directions. TDC is then half way between these marks on your degree wheel. The reason this works better is because near TDC a large rotation of the crankshaft results in very little vertical movement of the piston.  This relationship changes in a sinusoidal manner from max at 90 Deg to zero at TDC and BDC.

Have I done it this way? No I wasn't smart enough to figure this out when I had the Ducati.

[Mike V]

Barry & all,
I'm in the process of identifying and accurately archiving absolute TDC. Why go to all this trouble? I'll try to keep this short...I'm currently doing some Beta testing for John Rayski at EME for the Enduralast and Electronic Ignition system being developed. I discovered my past interpretation of view angle through the case window to the flywheel for static and dynamic timing may have been in error. I've discovered it's quite easy to accumulate a 3° to 5° error depending on what the individual may interpret as radial or perpendicular to the case notch and flywheel marking. Moving on; I am going to go through the procedure of determining absolute TDC by use of a degree wheel and piston stop tool. The procedure is rotating the engine cw to stop record degree wheel reading, then ccw to stop and record this degree wheel reading. Then by finding the mean or mid-point of these readings equals absolute TDC. At this point I will identify the case notch and the flywheel OT index mark. The rub is; there is only 2 points of reference, the case notch and flywheel mark. To be accurate another or 3rd reference mark needs to be established. Remember 2 points define a line which is a dynamic view based on your eye position and 3 points define a plane which is my target result. I have a few ideas of including a pointer or additional case mark to fulfill the 3 point reference idea. I have a special piston stop tool ordered from Summit Racing I will utilize. Anal? Maybe BUT, once absolute TDC can be accurately referenced I will have a strong baseline and initial point of reference to correctly adjust valves, static timing and full advance timing. Sorry, that was the short version. I'll keep you guys advised of the procedure and results unless you find this an exercise in minutia.

My reason for asking the question originally is to see if anyone else has experienced the frustration of view angle for timing and valve adjustment and your resolutions.

[Barry]

My reason for asking the question originally is to see if anyone else has experienced the frustration of view angle for timing  

I've certainly experienced frustration in not knowing if I have eliminated parallax error in lining up the notch with the timing marks. I've tried using a pointer with the centre of the flywheel tooth as a vertical reference with respect to the centre line of the crankshaft. It's far from perfect though. You might have seen a photo of the S mark I've posted to help show others what the mark looks like. That was originally taken as record when I set the timing to study afterwords.

You also have to wonder how carefully the man with the chisel was when that notch was made at the factory so I do understand why you are doing this and look forward to hearing your results.

[Mike V]

Barry,

I'm happy to see we're on the same page here. I have a few ideas for an adequate (additional) reference once obtaining the critical TDC information. I'm informed by the Guru's the OT dots or hash marks placed on our flywheels are very accurate. I'm not challenging this fact and am confident of the accuracy. The question that arises is just as you've stated...regarding the placement of the case notch from the factory.
A couple of things I've been tossing around is maybe an additional parallel notch in the case to use as a sighting ladder to center or straddle the OT mark with.  Maybe painted white or coated with a dab of white-out. The ideal instrument would be a pointer placed in very close proximity to the flywheel marking but how to accomplish this at the risk of a Rube Goldberg affair is the question.  I don't like the idea of drilling holes in my case with nuts and screws or relying on epoxy to secure the device and chancing failure to throw parts inside the clutch housing. Any ideas are welcomed. I'll keep you informed.

[Andrew]

What I learned on airplanes, and use on car engine flywheels, gets you pretty darn close without a degree wheel. Put a stop down in the sparkplug hole a specific distance. Turn the engine over til you GENTLY hit it (don't break it off). Mark the flywheel/alternator/crank gear/whatever against some reference point. Then turn the engine over fully the other way, again hitting the stop, and marking the same circular reference. Now measure halfway in between and that's TDC. Close enough in my experience for road engines without super-close valve/piston clearance and tall-dome pistons. Having the plug centered vertically over the piston is best for this, but I think the airhead position will work. There is a factory Chevy tool that screws into 14mm plug holes for this purpose, or you can use a bolt. The Chevy tool is hollow for a pointer.

Andrew

[Barry]

Pity the factory didn't drill a radial hole at the flywheel mark into which you could fit a pointer. It would be a flywheel off job now though.

By the way "Rube Goldberg" flew clean over my head until I looked him up and discovered he was an American cousin of our very own "Heath Robinson" who had a remarkably similar career and entered our language for the same reason.

[Mike V]

Drew,
The biggest challenge isn't the TDC determination per say, but the reference indicators at the case window once the TDC determination has been made. Here's the tool I ordered:

http://www.summitracing.com/parts/CCA-4795

After thinking about my last post and the mention of a parellel case mark on the window...that won't accomplish what I'm after. A little brain fart there. It's going to have to be a 3rd point of reference in addition to the actual OT mark and case window notch. Or a pointer at very close proximity to the flywheel. Appreciate the correspondence.

[montmil]

Mike, Your Summit 'Piston Stop' reminds me of a tool I still have from back in the early 70's. It's called a Micro Tune or something similar. Sorta like a vernier caliper, when screwed into the spark plug hole, and after consulting the product's manual for your particular bike, a certain number of turns would lower a hefty rod and stop the piston at the the spark plug's firing point. I used it quite a bit on 2-stroke MXers popular during that time period. It even self-corrected for the angular positioned spark plugs.

Are you working on timing or degreeing a new cam?

Monte

[Mike V]

Are you working on timing or degreeing a new cam?

Hi Monte,
No it's a timing issue - of sorts. Actually a timing "reference" exercise trying to dial in the correct view angle through the case window. And I sheepishly admit it started on the /7 and not the 650. Although with the 650 engine on the bench STILL waiting for my heads and jugs to be returned from Ted I will be doing the process on the 650 when I get the top end assembled. And it will be much more pleasant and accesible with the transmission removed and on the bench.

The tool is actually a simple piston stop. I don't have any knowledge of the tool you're referring to but it sounds very clever and interesting. This is a simple tool I'm using and is nothing more than a piston stop threaded through the spark plug hole. You can make one by hogging out an old spark plug and tapping some inside threads then using a bolt through the center threads. I didn't want to go through the trouble and these are very affordable. But, to be clear; this all has to do with the way we view what we assume is the correct angle through the case timing window down to the flywheel OT mark and the alignment of the hatch in the case. I will first determine absolute TDC then figure out a way to correctly refernce the position of the OT mark on the flywheel by adding a 3rd point of alignment with the case mark and flywheel mark.

I'm attaching a drawing I created with an exaggerated condition to better describe what I'm doing and use as a design tool as I go.

[montmil]

PARALLAX:  The effect whereby the position or direction of an object appears to differ when viewed from different positions, e.g., through the viewfinder and the lens of a camera...

...Or the timing hole on a farm implement. See also Anal Retentive. OK, OK... I added the last part. [smiley=vrolijk_26.gif]

All in jest,  Monte

[Mike V]

See also Anal Retentive. OK, OK... I added the last part

I can't get out of my own way at times...

[MrRiden]

I'm out of my league here but as someone who has trouble just finding the timing mark in the window I throw in my 2¢. I also work on Triumphs and they have a neat little plug at the base of the cylinders. I simply remove that plug. Screw in my bolt that has been drilled hollow. insert a hardened pin in the hollow bolt and spin the crank. The crank has a notch that the pin drops into. Viola TDC! Although how absolute is open to speculation. It does however eliminate parallax or the need for a dial indicator when finding a "good enough" TDC reference for static timing. Perhaps a mechanical "finder" could be cobbled for the R65?