Find the TDC

MexicoMike said:

Depends on how accurate you want/need to be. There are all sorts of methods for determining TDC but the most accurate method involves using a degree wheel and piston stop. If you just need an approximate TDC - the eyeball method with a pencil (or whatever) in the spark plug hole with the eraser end sitting on the top of the piston will do just fine. Just watch the pencil move upward until it quits moving upward!

Heck, for a very rough TDC you can put your thumb on the spark plug hole, turn the engine over manually and feel when the compression stops! I have used that many times to install a new ignition when you just need the timing to be "right enough" to start!

SO...it really depends on what you are working on as far as the necessary accuracy!

Click to expand...

But i have the head apart!

marshg246 said:

Agreed with the head on, but if I have the head off, I setup a dial indicator and choose a reading just before TDC and set the degree wheel to 0. Then turn the engine in the forward direction until I have that dial indictor reading again. I can then read the number of degrees exactly, divide by two and remember. Then turn the engine forward until I once again have my dial indicator reading and set the degree wheel to the the remembered degrees before zero. Finally, turn the engine forward to whatever I want before TDC and verify/correct the timing marks.

Click to expand...

Yes I agree with your method but I would still use a positive stop
I'd bolt a piece of steel stock across the barrel with a steel packer against the piston as a stop

MexicoMike said:

If you are looking for maximum accuracy, whatever you put across the top of they cylinder barrel needs to STOP the piston before it reaches TDC - same process as with the head on. IOW, clamp something there that wont move when the piston meets it.

Click to expand...

Pierodn this is all you need to do for maximum accuracy

baz said:

Yes I agree with your method but I would still use a positive stop
I'd bolt a piece of steel stock across the barrel with a steel packer against the piston as a stop

Click to expand...

Before I had a dial indicator, that's how I did it and it's way more accurate than needed. However, my way removes any backlash since you're always turning the engine in the normal direction and it takes less time to setup. On bikes with alloy cylinders, the dial indicator method is harder to setup so on those, I do it your way.

acotrel said:

I always use the soap bubble method with an old spark plug with the electrode removed. Bore clearance introduces an error if you use positive stop or a dial micrometer. As you rotate the crank, the pistons usually move slightly backwards and forwards in the bores while the rings do the sealing. It is the compression peak which is important. The soap bubble method is very sensitive. I usually try to rotate the crank forward, when finding TDC, so that any error ends up creating more advance. If I ever disturb the ignition timing, I always check my jetting. It does not matter if you get it slightly wrong as long as you jet to suit it.

Click to expand...

Another vote for the soap bubble method for determination of TDC, I use a spark plug with electode removed and a clear plastic tube attached, this method is very accurate.
Of course this only works in compression stroke, and is irrelevant for the O. P since his cylinder head is off .

Perhaps he could stretch a condom across it instead.... Less chance of f***ing it entirely....

EstuaryBoy said:

Same guy asking the same question!!

Click to expand...

By golly and seven plus years ago, time does fly.

I use a machined slider block positive stop on a headless Norton for minimal crank rotation, I also have a spark plug version that has a double threaded brass insert so the stop end can be changed to suit different pistons on bikes with the same plug thread OD.
Of course on some bikes you need to be wary of valve movement within the area (clash) of the stop probe.

This is the Moto Guzzi set up after the using the positive stop to machine the permanent degree wheel, the removable index finger shows both cylinders.
The Norton has a removable version but there seems little real interest in these sort of things.



The reality is a across the bore positive stop with the head off is the most accurate and also the cheapest to make, you could even print a degree wheel.
These would be for accurate settings in general, bubbles and straws would be fine for setting valve lash.

Out of interest Pierodn what are you setting tdc for?
Ignition timing etc

How is the soap bubble method going to work with the head off as Pierodn has said, with the head off the dial indicator is the most accurte way, they are a cheap investment for any workshop.

Ashley

You can find TDC, or extremely close to it using a straight edge or even visually or by feel if you don't/can't or have time for coming up with a degree wheel or a dial indicator.
It's not an F1 power plant.
If your skills are in order it will be fine.

As my attorney friend often reminded me "Don't let the lawyers kill the deal".

Peter R said:

Another vote for the soap bubble method for determination of TDC, I use a spark plug with electode removed and a clear plastic tube attached, this method is very accurate.
Of course this only works in compression stroke, and is irrelevant for the O. P since his cylinder head is off .

Click to expand...

Why does he want to find TDC while the head is off ? You usually only do the cam and ignition timing while the head is on the motor. I sometimes use a dial mike to map the cam lift. I don't use it to find TDC - too easy to introduce errors if the piston rocks a few thou.

acotrel said:

Why does he want to find TDC while the head is off ? You usually only do the cam and ignition timing while the head is on the motor. I sometimes use a dial mike to map the cam lift. I don't use it to find TDC - too easy to introduce errors if the piston rocks a few thou.

Click to expand...

I do it with the head on , spark plugs out and using a pencil down the hole , bike in 3rd or 4th gear and on a stand to lift the rear wheel up for the small back and forth movements to find the TDC sweet spot. Snug down the E.I. plate central if possible , new plugs in , find neutral , fire her up. Any back fires or kick backs to be avoided , just move the plate a tad until running warmest best . It's not a Maserati engine.

"with the head off the dial indicator is the most accurte way, they are a cheap investment for any workshop."

No, it isn't. The piston stop/degree wheel is the most accurate way in any situation. There will be several degrees of crankshaft movement that the dial indicator will not pick up. Same is true of any method OTHER than piston stop/degree wheel. OK...you can do the same thing without the degree wheel but the piston stop is mandatory. You could simply mark the damper/flywheel/whatever at each point where the piston stops when rotated in clockwise/counterclockwise and measure the distance between those marks. TDC is precisely 1/2 way between the marks. But a degree wheel is easier and you can actually read degrees!

HOWEVER, I do agree that, as I said earlier, on this 1940's engine, this sort of precision won't matter! So, if we are arguing what is the "best method" for a Norton, I wouldn't argue against soap bubbles, dial indicators, pencils moving up/down, feeling the piston with your hand (head off), looking into the spark plug hole with a flashlight, etc.. But if we are discussing the most precise method for finding TDC on an engine it's the piston stop and degree wheel - the largest diameter degree wheel that will mount on the crank.

Just tak

ashman said:

How is the soap bubble method going to work with the head off as Pierodn has said, with the head off the dial indicator is the most accurte way, they are a cheap investment for any workshop.

Ashley

Click to expand...

it just takes a lot more soap! LOL

The piston stop/degree wheel method is in accurate when you cannot divide half degrees by two in your head and remember where you were. The only reason I would ever try to find TDC with the head off, would be if I was going to machine the top of the cylinder barrels to get more compression. When you do that you are probably fooling yourself. Combustion conditions are affected by a balance between comp. ratio, jetting and and ignition timing . If your motor is jetted rich, raising the comp. ratio can make it run a bit leaner and faster.
I usually build the motor, then do the cam and ignition timing, then adjust the jetting to suit the changes. Getting speed out of the motor is all about cam and ignition timing, the exhaust system, but mainly the jetting. Even if all the others are a bit wrong, the jetting usually corrects the situation. When you use petrol as fuel, you might have a problem getting fine enough adjustment of your carburation. The step between a 0.106 and a 0.107 needle jet is huge. Mikunis are probably better.

acotrel said:

The piston stop/degree wheel method is in accurate when you cannot divide half degrees by two in your head and remember where you were. The only reason I would ever try to find TDC with the head off, would be if I was going to machine the top of the cylinder barrels to get more compression. When you do that you are probably fooling yourself. Combustion conditions are affected by a balance between comp. ratio, jetting and and ignition timing . If your motor is jetted rich, raising the comp. ratio can make it run a bit leaner and faster.
I usually build the motor, then do the cam and ignition timing, then adjust the jetting to suit the changes. Getting speed out of the motor is all about cam and ignition timing, the exhaust system, but mainly the jetting. Even if all the others are a bit wrong, the jetting usually corrects the situation. When you use petrol as fuel, you might have a problem getting fine enough adjustment of your carburation. The step between a 0.106 and a 0.107 needle jet is huge. Mikunis are probably better.

Click to expand...

The only reason I check with or without the head is to verify/correct (where possible) the ignition timing marks. If I were building race bikes it would be different but I rebuild to stock only.

acotrel said:

The piston stop/degree wheel method is in accurate when you cannot divide half degrees by two in your head and remember where you were. The only reason I would ever try to find TDC with the head off, would be if I was going to machine the top of the cylinder barrels to get more compression. When you do that you are probably fooling yourself. Combustion conditions are affected by a balance between comp. ratio, jetting and and ignition timing . If your motor is jetted rich, raising the comp. ratio can make it run a bit leaner and faster.
I usually build the motor, then do the cam and ignition timing, then adjust the jetting to suit the changes. Getting speed out of the motor is all about cam and ignition timing, the exhaust system, but mainly the jetting. Even if all the others are a bit wrong, the jetting usually corrects the situation. When you use petrol as fuel, you might have a problem getting fine enough adjustment of your carburation. The step between a 0.106 and a 0.107 needle jet is huge. Mikunis are probably better.

Click to expand...

Al
the bloke is asking how to find tdc with the cylinder head removed! That's all he's asking! Nothing more nothing less why not just answer what the bloke wants?
A positive stop on the piston and divide by 2 on a dial gauge is the best and simplest way to get this spot on

EstuaryBoy said:

Perhaps he could stretch a condom across it instead.... Less chance of f***ing it entirely....

Click to expand...

Or getting "knocked up" !

Given these times and time to spare in the mask free garage.

Soap bubble blowers look away. (which would be a sizable bubble with the cylinder head removed)
In the transition from needle stop to one μm (0.001 mm / 0.000039") after that point there is around a 1.5 degrees of no man land. (Connecting rod bearing and piston pin clearance might come into it when chasing fairy's )
I will never get this time back but to some degree proves a solid stop up to and back up to TDC with a degree wheel is hard to beat although some two strokes with overhead spark plugs do use a mm before TDC ignition setting.

1.5 degrees ish is probably not the end of the world but would not normally put a magnetic base anywhere near engine internals (hence the printer paper)



With the head on I have this spark plug tapped M10 internally with a threaded brass insert.
The alloy end threads in and can be changed to suit other engines with the same plug thread.
Movement of the valves need to be taken into account for possible clash.



I look forward to getting out more.

I have the same piston stop tool. Don't try to get out more. Hazardous to your health.