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

grandpaul · Nov 4, 2014

Let's examine...

You have a dial indicator set up above a piston.

You slowly and carefully rotate the crank in the forward direction.

As the piston approaches TDC, the dial indicator makes contact, and the needle begins to rotate.

At the exact point that the needle stops rotating, THAT is "true" TDC, NOT an average of fore/aft rotation-influenced error where the pistons are essentially standing still and the crank turns "over center".

Logically, this is the same result as with a piston stop.

Dances with Shrapnel · Nov 4, 2014

Grandpaul,

Unfortunately there is no "exact point that the needle stops rotating" as there is piston rock and translation of the piston in the bore. It will get you in the field but not close enough.

Much more accurate to do a dead stop of piston some distance down the bore where there is higher gain in the reading (or even an identical dial indicator read - ex distance in thousandths of an inch BTDC and ATDC) then split the difference on the degree wheel.

Dances with Shrapnel · Nov 4, 2014

What I mean by higher (or greater) gain is that once the piston is down the bore a bit you gain greater movement per degree of crank angle which should give you greater sensitivity on angular position.

bluemax · Nov 4, 2014

Can you just use a dial gauge with a longer tip and rotate the engine until it just touches or note a specific reading and do the BTDC, ATDC ,degree wheel thing to workout TDC.
Or have I missed something?

comnoz · Nov 4, 2014

bluemax said:
Can you just use a dial gauge with a longer tip and rotate the engine until it just touches or note a specific reading and do the BTDC, ATDC ,degree wheel thing to workout TDC.
Or have I missed something?

That will get you closer but there are still variables such as piston rock which may be different moving forward than backward.
A piston stop is the only sure way as that takes up any clearances that may affect the reading.

Deets55 · Nov 4, 2014

Jim,
I guess you want to get the stop right over the wrist pin for best results ?
Pete

mschmitz57 · Nov 4, 2014

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?

I always use a dial gauge with a long probe down the plug hole.

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

rx7171 · Nov 4, 2014

When I'm adjusting the valves and finding the exact TDC is less critical I use a long plastic straw down the plug hole.
No chance of scratching the piston top and will bend without breaking.

Brooking 850 · Nov 4, 2014

Jim can you post a pic of your piston stop?
Regards Mike

comnoz · Nov 4, 2014

Here is my timing tool kit

14mm and 10mm piston stops
pointer
undersize rocker shafts for test assembly
washer to hold degree wheel
various size wires for checking valve to valve clearance through spark plug hole

comnoz · Nov 4, 2014

Deets55 said:
Jim,
I guess you want to get the stop right over the wrist pin for best results ?
Pete

It will not make a difference when using a piston stop. All clearances will be taken up when the piston is moved up firmly against the stop -either before or after TDC. Jim

Brooking 850 · Nov 4, 2014

Thanks for the pic
regards Mike

aceaceca · Nov 4, 2014

Does your pictured piston stop clear the valves???? Were you referring to someone using a full spark plug o.d . rig in the hole bending the valves?

comnoz · Nov 4, 2014

aceaceca said:
Does your pictured piston stop clear the valves???? Were you referring to someone using a full spark plug o.d . rig in the hole bending the valves?

It will just clear with a stock size valve and stock cam but I always remove it before I turn the engine to the other side of TDC.
IE, I never rotate the engine all the way around with the stop installed to get to the other side of TDC -just to be safe. I bent one once. Jim

gortnipper · Nov 5, 2014

mike996 said:
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."...

My rotor's timing marks are way off, 6-7 degrees retarded. Set it up static, and it wont start at all. So, the timing gun is also useless, and I have always set it up by feel.

mike996 · Nov 5, 2014

My rotor's timing marks are way off, 6-7 degrees retarded. Set it up static, and it wont start at all. So, the timing gun is also useless, and I have always set it up by feel.[/quote]

Heck, back in the old days, for performance use we almost always set timing by "feel" - how the engine reacted to the throttle. Timing marks/specs were just a "guide." Typically you would advance the timing until you got a slight bit of detonation on WOT acceleration and then back it off about 3 degrees. Actually we did the same thing later when I got into serious engine building except we did it on a dyno - advance timing to produce max power. It was always interesting to find that two examples of exactly the same engine could "want" a different timing setting for best power - sometimes as much as 7-8 degrees different.

auldblue · Nov 5, 2014

gortnipper said:
mike996 said:

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."...

Click to expand...

My rotor's timing marks are way off, 6-7 degrees retarded. Set it up static, and it wont start at all. So, the timing gun is also useless, and I have always set it up by feel.

When I timed my last engine for the first time using an engine stop and degree disc on a boyer and a single mikuni it started easy but the headers were red hot in 30 sec , would it have started at 31 deg after TDC?
I gradually advanced it till it was ok and then strobed it.

After about two hand red miles or so it started missing and then cut out, the bolt holding the boyer rotor had slackened off and I retimed it using the scale on the primary and rotor marks and it fired up first kick, but move,d it advanced a couple of deg on the strobe and it has been ok since.

The new engine has been timed pazon hopefully it will start and not roast the headers this time.

mike996 · Nov 5, 2014

"When I timed my last engine for the first time using an engine stop and degree disc on a boyer and a single mikuni it started easy but the headers were red hot in 30 sec , would it have started at 31 deg after TDC?"

Pe the manual, the timing is supposed to be set at 31 degrees BEFORE TDC, checked at full advance. I'm not sure I understand your post but if you were setting it to be at 31 AFTER TDC ( I don't know how you'd actually set that with the stock timing index/mark), I'm not sure it would run at all. However, if it did run, the timing is SEVERELY retarded and would easily cause the header pipes to turn red vey quickly.

auldblue · Nov 5, 2014

What I'm saying is ,have I cocked it up before and set after TDC. I know it should be 31 deg before TDC, but would the engine run that far retarded if set at 31 deg after TDC. it did get hot rather quickly after start up and I do not want a repeat if I can help it.

J

mike996 · Nov 5, 2014

The engine should run, even at that retarded setting, since the incoming fuel/air mix is still in the CYL and could be ignited. It wouldn't have any power at all and would run extremely hot.

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

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