Head on, head off what do you use to get TDC

What do you use as piston stop or do you use a TDC gauge with the head on to establish TDC. Which is best or it doesn't matter if you get there in the end?

What are you going to do with TDC ??

There are timing marks on the alt rotor.
Strobe timing works pretty good...

Its also quite possible to use a pencil down the plughole.
Just don't let go of the pencil...

Rohan said:

What are you going to do with TDC ??

There are timing marks on the alt rotor.
Strobe timing works pretty good...

Its also quite possible to use a pencil down the plughole.
Just don't let go of the pencil...

Click to expand...

The last time I set my boyer I did it with the timing marks and then a strobe, but on this occasion it has no head and is on an engine stand.

J

auldblue said:

What do you use as piston stop or do you use a TDC gauge with the head on to establish TDC. Which is best or it doesn't matter if you get there in the end?

Click to expand...

I would use a piston stop, either through the plug hole or a bent steel strip under a head bolt if the head is off.
Make a mark with the piston against the stop both before and after TDC and then split the difference.
Be careful when the head is in place. A piston stop in the spark plug hole will bend a valve when you rotate the engine. Jim

Rohan said:

What are you going to do with TDC ??

There are timing marks on the alt rotor.
Strobe timing works pretty good...

Its also quite possible to use a pencil down the plughole.
Just don't let go of the pencil...

Click to expand...

the timing marks on the alternator are not alway accurate ,,,,,,,,,,,,,,baz

comnoz said:

auldblue said:

What do you use as piston stop or do you use a TDC gauge with the head on to establish TDC. Which is best or it doesn't matter if you get there in the end?

Click to expand...

I would use a piston stop, either through the plug hole or a bent steel strip under a head bolt if the head is off.
Make a mark with the piston against the stop both before and after TDC and then split the difference.
Be careful when the head is in place. A piston stop in the spark plug hole will bend a valve when you rotate the engine. Jim

Click to expand...

Cheers jim
Did it like that the last time also, with the piston stop running single mik. Bike started pretty easy but the headers were glowing red after 30 sec or so. Gradually advanced it round and eventually it was normal. What did I balls up? It wasn't drawing air.

J

I agree with you there baz.

Maybe you set the timing at TDC last time. I don't know how far back down the bore is the equivalent of 28 degrees, but if you know that measurement, then turn the engine back to that position and then check your timing marks. That will be 28 degrees and you can then prove your timing marks are correct.
I think you also need to know what the basic timing at idle will be for your Boyer because that will be the timing you should set it at on the bench. 28 degrees is for timing at [ is that 3000 rpm, check your book ]. Then it will be a matter of running the engine to set the timing accurately according to the Boyer manual.

with no head on, use a dial indicator to find the TDC. [ rotate engine back and forward to prove the position as Jim has mentioned. ]
Dereck

auldblue said:

comnoz said:

auldblue said:

What do you use as piston stop or do you use a TDC gauge with the head on to establish TDC. Which is best or it doesn't matter if you get there in the end?

Click to expand...

I would use a piston stop, either through the plug hole or a bent steel strip under a head bolt if the head is off.
Make a mark with the piston against the stop both before and after TDC and then split the difference.
Be careful when the head is in place. A piston stop in the spark plug hole will bend a valve when you rotate the engine. Jim

Click to expand...

Cheers jim
Did it like that the last time also, with the piston stop running single mik. Bike started pretty easy but the headers were glowing red after 30 sec or so. Gradually advanced it round and eventually it was normal. What did I balls up? It wasn't drawing air.

J

I agree with you there baz.

Click to expand...

That method is going to give you TDC. Correct ignition timing is going to be 28 degrees before TDC at full advance. You will need to use a degree wheel to get back to 28 degrees and make a mark to set the timing with a strobe. Jim

Determing TDC without a piston stop and a degree wheel is not accurate. That's the only way to do it. Any other method can easily be several degrees (or more) off. You can make a piston stop with an old spark plug/ bolt.

FWIW, when I checked my Commando, the timing marks on the rotor were about 1/2 degree off. I have heard stories of all sorts of "factory errors" of 4-5 degrees + but I have to admit that I wonder if there was, in fact, a factory error or whether it was just poor measuring by whoever was checking them and then reporting the "factory error."...

Carefully sandwich a degree wheel between two washers on the crank, (outboard of the alternator rotor); snug up the alternator nut just about where the wheel stops spinning freely, then back it off ever so slightly.

Take a section of coat-hangar wire with a sharpened tip, bend it as required to just hang over the edge of the timing wheel pointing at the scale, and make a small loop on the other end that will align with one of the nuts fore or aft of the cylinder base case studs. Mount it up.

Get a hold of a dial guage and set it up to where you can accurately measure the piston arriving at TDC, working both ways and settling in the middle between where the needle rises and falls. Rotate the refernce dial to the "0". Double-check it with a bit more crank rotation back and forth. It should stop moving right at "0" in both directions.

Without disturbing the pistons or crank, gently rotate the timing wheel to the "0" mark, and carefully hold it in place as you nip up the alternator rotor nut.

Check your indicator needle alignment (both the dial guage and the timing wheel). If all is well, back the crank off 28 degrees and install your electronic ignition with the proper manufacturer recommended marks in alignment, etc. MAKE A NOTE OF YOUR DIAL GUAGE INDICATION, write it down.

Using whatever method you find easiest, taking care not to disturb crank or pistons, slip the timing wheel off and verify the timing mark on the alternator rotor. IF NEED BE, etch a new mark on it after re-verifying the correct position of the dial guage.

Should fire right up if you have a good battery, properly timed cam, new spark plugs, properly adjusted valves, and properly set up carbs being fed an adequate flow of gasoline. Strobe timing will likely need VERY LITTLE tweaking (if any)

Who installed the timing chain??? Are you sure all the dots are lined up and correct... If not it will make timing marks (strobeing side) dysfunctional.. Maybe off by one tooth and need to verify all is correct..

auldblue said:

What do you use as piston stop or do you use a TDC gauge with the head on to establish TDC. Which is best or it doesn't matter if you get there in the end?

Click to expand...

As someone who was brought up and demonstrated to by a motorcyclist of the old school, with head on I use a long thin 15 inch (or more) screwdriver through the plug hole, In conjunction with a timing disc bolted onto the crankshaft.
With the head off I would use a dial gauge, in both cases above, ALWAYS bear in mind you turn the engine one way and one way only (the correct direction) to take up the timing gears/other slack before arriving at the TDC.
It will take several attempts in both cases to arrive at TDC.

If you try the stop/degree wheel and any other method on the same engine, you will find that the stop/degree wheel will produce exactly the same results every time where the other methods will "shotgun" around the true TDC position. Sure, it's a lot easier to stick a pencil in the spark plug hole and watch it and then, when it stops moving, call that "TDC" but it's not. If you were sharp-eyed and sensitive-of-hand enough to stop at EXACTLY the point where it quit moving up, you are prior to TDC. So the whole TDC thing using any method but the stop/wheel is a rough guess - and possibly no better (if as good) than whatever that beat up, old, out of tolerance Norton factory machining came up with.

The same is true of any of the other methods - dial indicator, tube of water, whatever. The slop cannot be factored out as the piston goes over the top. So trying to "correct" for that is just guessing.

With the head off, TDC would be when the piston is at the top of it's stroke....I think you should be able to just look to find this..

No, you can't because the piston stays at the "top" while the crank/cam continue to rotate. Eyeballing it is probably the least accurate method...well, other than sticking your finger over the spark plug hole and "feeling" when the pressure stops.

I know I'm probably sounding "over the top" on this subject and it probably doesn't matter much on these old engines - I doubt if a few degrees of timing (cam or ignition) will make any practical difference on these very mild stock engines. But no engine building shop or cam/crank builder will use or recommend any method other than piston stop/degree wheel to ensure TDC positioning.

I guess I can sum up my view as: If you are going to check a TDC mark, check it with the stop/wheel. Otherwise, why bother? Any other method is not accurate enough to prove the factory mark is right or wrong. You could end up making the TDC mark LESS accurate than it already is.

mike996 said:

...I have heard stories of all sorts of "factory errors" of 4-5 degrees...

Click to expand...

Yep, using a piston stop and degree wheel, my factory marks are 4 degrees "optimistic". I set my full advance to the 32' mark to get a "true" 28'. It now makes sense why it seemed so doggy when set to 28'.

Nathan

It's common to find the factory mark off by a couple degrees.
Eyeball, screwdriver or even a dial indicator may get you close. A piston stop will get you dead nuts every time.

Ironically, I have to admit that despite my piston stop/degree wheel rant, I have yet to inscribe a timing mark on my Alton E-start rotor. I did the TDC/degree wheel check of my oem Lucas rotor some years ago and it was about 1/2 degree off. But I never did a check so as to accurately mark the Alton rotor which does not have a timing mark. So at the moment I have NO timing mark at all! Hopefully the Trispark, installed and timed with the oem rotor will keep working!

olChris said:

Who installed the timing chain??? Are you sure all the dots are lined up and correct... If not it will make timing marks (strobeing side) dysfunctional.. Maybe off by one tooth and need to verify all is correct..

Click to expand...

Sorry to say chris ,but the set up was by me . Have a deko and see if you think it is correct.


J

Rohan said:

What are you going to do with TDC ??

There are timing marks on the alt rotor.
Strobe timing works pretty good...

Its also quite possible to use a pencil down the plughole.
Just don't let go of the pencil...

Click to expand...

TDC sets the zero on the degree wheel you fit to the end of the crank when setting the cam and ignition timing. I usually fit the broken spark plug and use a soap bubble to find TDC. Pull the centre electrode out and move the crank backwards and forwards over TDC on the compression stroke until you get maximum expansion of a bubble of detergent. I also have two more keys broached at random into my camshaft sprocket that allows me to get within about 2 degrees of the desired timing.
Anyone who uses the factory marks in a motorcycle engine doesn't care how it performs. With older Triumphs in particular the idler gear 'hunts', and that can make life a bit more difficult. From memory, you need to turn the motor in one direction about 48 times before the marks line up again.