TDC tool

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Oct 25, 2008
was reading a old post about setting up timing someone mentioned degree wheel and tdc tool .my question is do they make a top dead center tool .checked service manual tool section and no mention of such a tool ? i'm familiar with triumphs tdc tool does norton make one? thanks mike
The TDC tool is just a piston stop, take an old sparkplug and break the ceramic bit and remove with the center elctrode, then take off the side electrode. Now glue a 1 1/2" piece of wooden dowel into the hollow metal bit.

To use first get the pistons to Bottom dead center and screw in the piston stop, slowly turn the engine forward until you feel resistance, note the degree reading on your crank mounted degree wheel, turn the engine back until you again feel resistance and note the degree wheel reading. You can now calc the movement you need to adjust the degree wheel so both resistance points are equidistant about TDC. Now move the degree wheel the required amount and double check you get say 45 degrees before TDC and after at the 2 resistant points and then remove the piston stop. Set the bike to your fully advanced figure, remove the degreewheel without moving the crank, replace the primary cover and check the reading to see how far off it is.

You mentioned in a previous thread that you have a "75 850 Commando"?

So I'm guessing it's an electric start MkIII - but I haven't been able to find any clue from your previous posts whether it actually is a MkIII model or not?

If yours is a MkIII (and not a late MkII) - then it will have a timing hole (3/4" hexagon plug) on the lower right hand side of the crankcase immediately below the timing cover, a slot in the crankshaft lines up with the hole at the 28 degree BTDC position.
There is a tool available, but it isn't completely necessary, as the position of the crank slot can be viewed through the hole quite easily, (unlike Triumphs). There's no additional TDC slot on the MkIII Commando crank, as you would expect to find on Triumphs.
If you make one for your Commando from a spark plug body some thoughts...
Once you break out the ceramic core the steel hole remaining will need to be a 5/16 tap. Now you need an inch and one quarter sticking out the threaded end of the spark plug threads. But a 5/16 bolt shoulder will be way too thick it needs to be turned down to 3/16 diameter and have a bullet shaped tip for piston contact. The 3/16 is to miss the moving valves and the 1 1/4 lengh gives about 30 degrees on each side of TDC. The 30 degrees keeps the math about right less lengh would be less accurate more and it would weaken too much. Triumph's and BSA's don't have such a problem with the diameter so beware of advice from these guys they just don't know about Norton valves. Best to have a lathe to do the job. A drop of red Loctite for the 5/16 bolt install into the spark plug body well previous of the lathe work.
Isn't the Norton plug at an angle? These things are dangerous in such an engine if the center pin extends down and then gets caught on the piston which may try to bend it sideways rather than push it up. If at an angle it is only useful for determining TDC not vertical distance anyway... unless you know trig.

So just use a plastic drinking straw with some markings made with a Sharpie marker to find TDC.

Was not thinking of a piston stop tool... sorry. I have a TDC tool with a sliding rod in it with a scale on it. So oops.

The tool description is very misleading: "Accurately displays piston movement"

Except this is only true if the plug is exactly parallel to the bore axis, and if the piston pin location in the piston is centered (symmetrical between the thrust faces).

Trig will recover the 1st error - if you know the exact plug angle.
Moving the piston down as far as possible reduces the pin offset error almost to zero.
batrider said:
Was not thinking of a piston stop tool... sorry. I have a TDC tool with a sliding rod in it with a scale on it. So oops.


I have the TDC tool but have also seen a Norton timing tool which looks different.
thanks guys for the great answers. I like the timing inspection plug answer similar to the triumph lines right up to the drilling on the crank.If you can visually see the slot i guess no tool will be needed.
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