Finding That Elusive TDC . . .

Sources of parts, and other hints to get your restration projects on the road.
Leo Bolter
Posts: 22
Joined: Tue Oct 07, 2008 10:43 pm
Location: R. D. 2, Palmerston North, 4472, New Zealand
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Finding That Elusive TDC . . .

Postby Leo Bolter » Thu Mar 05, 2009 6:39 pm

For quite some time, nothing seemed to be happening on this forum . . so I thought I'd break the silence! :wink:

NOTE: As it happens this post is pertaining to a tool made specifically for a LE Velocette motorcycle and was originally posted (by myself) on the LE Velo Forum. But of course with a slight mod to the size of the "spark plug" bit, it could be made to fit your machinery . . .
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To locate TDC exactly when you have a fully assembled engine, you could, with very little effort, make a tool similar to this one I made for my LE Velocette very recently. It is modeled on a larger version that fits into the more common 14 mm plug holes that all manner of engines use. I made and used that one extensively aeons ago when I was an apprentice Motorcycle Mechanic! . . and I still have it too!

To Make:
1 Remove the porcelain and the earth electrode from a LE plug.
2 Find (or make) a long 1/4" or 6 mm bolt with lots of thread. This bolt should have it's end rounded off to prevent damage to the top of the piston.
EDITED: 22 May 2009
3 Either drill a hole up the centre of the bolt or alternatively mill a groove (or file a flat) on the side to allow the compression air to pass.
4 Tap the hole with a thread to suit the bolt and the tool's made . . .

(A logical suggestion that was made by a LE Velo Motorcycle Forum member was to machine a knurled locking ring to thread onto the top end of the bolt’s thread. This disk could be run down the thread to abut onto the hex end of the old spark plug body at the appropriate moment, and so lock up any potential movement of the bolt.
1 This lock nut would hold the bolt more firmly once the necessary adjustments of the bolt had been made, whilst setting up to find TDC.
2 Also the bolt can be locked in position after it has been screwed down to set a point where the ignition time should be, ie. 5mm before TDC. That timing point will then be quickly reproducible).


NOTE: Don't be tempted to use a larger diameter bolt as it could run foul of the valves as they open and close. Using the recommended diameter, it's fairly close to doing that on a LE, as it is!

Image

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To Use:
1 Replace one of the spark plugs with the X spark plug part of the tool.
2 Start with the bolt screwed in so it just protrudes into the the combustion chamber.
3 Gently turn the engine over while carefully screwing the long bolt inwards.
4 Continue until the piston only just "bumps" past the rounded end of the bolt as it continues to be turned. Take care during this stage! If it feels like more that a gentle effort is required, unscrew the bolt a little!
5 Rotate the engine to the compression stroke. Turn the engine a few more degrees. Then stop rotating the engine.
6 Now screw the long bolt inwards a couple of more turns.
7 Carefully rotate the engine forwards until it's rotation is prevented by the piston contacting the bolt end.
8 Make a mark across the moving part (the flywheel?) and a stationary component nearby (the crankcase?).
9 Reverse the rotation of the engine until the piston makes contact with the bolt again.
10 Make a further mark on the crankcase opposite the mark made previously on the flywheel.
11 TDC is exactly when the single flywheel mark is halfway between the two crankcase marks!

Oh . . and . . . don't forget to take the tool out . . and put the spark plug in before endeavoring to start the beast!

Cheers.
Leo
Last edited by Leo Bolter on Wed Jan 13, 2010 2:19 pm, edited 3 times in total.
R. Leo Bolter,
Palmerston North, New Zealand.
Skype name = jupiter1951

(VCC of NZ - Member No 09/16128
JCC of NZ - Member No 0741.
JOAC - Member No 0161)

Leo Bolter
Posts: 22
Joined: Tue Oct 07, 2008 10:43 pm
Location: R. D. 2, Palmerston North, 4472, New Zealand
Contact:

Postby Leo Bolter » Fri May 22, 2009 9:20 pm

Please note:

My post above: Thu Mar 05, 2009 6:39 pm Post subject: Finding That Elusive TDC . . . Has been edited in blue

Cheers.
Leo
R. Leo Bolter,

Palmerston North, New Zealand.

Skype name = jupiter1951



(VCC of NZ - Member No 09/16128

JCC of NZ - Member No 0741.

JOAC - Member No 0161)

goneps
Posts: 1
Joined: Wed Nov 19, 2014 6:45 pm

Re: Finding That Elusive TDC . . .

Postby goneps » Wed Dec 17, 2014 10:27 am

With every respect, this seems a very complex and potentially dangerous method. A member of the Morris Register in Britain came up with the following idea:

Drill through the centre of an old spark plug and seal a small tube into the hole. Bring piston close to TDC on compression stroke, screw plug into head and place a small blob of washing-up liquid on the end of the tube. Gently easing the crankshaft back and forth, the bubble of liquid clearly indicates true TDC.


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