Ducati Owners Club of Victoria

True TDC at Last or Old Tips for New Players

One of the more common jobs confronting Ducati owners is that of finding TDC (Top Dead Centre) prior to ignition or valve timing checks.

I have seen various methods employed, from the proverbial bit of wood with biro marks to the DTI (Dial Test Indicator). The latter obviously being the most accurate however due to running clearances (smallend, bigend, main bearings) even the DTI can't achieve much better accuracy than 1 deg. measured on the crankshaft. So what do you do if you want your TDC 'spot on'. Well here is a nice, simple and cheap tool which most people will be able to knock up in about half an hour

THE TOOL
tdc

First get an old spark plug, cut off the external electrode and with a hammer and punch destroy the porcelain core so you are left with the bare metal body. Alternatively a detachable type plug is easier cos all you have to do is unscrew the nut. OK now tap a thread down the centre of the plug body and die a length of rod about 3 ½ long (90mm) any thread system you like to suit then round off one end of rod (see diagram). Screw the rod into the plug body and run a nut down the rod so that you can lock the lot up. File flat across the plug body thread (to allow an air passage when screwed into cylinder head). Now you have a TDC tool second to none.

THE METHOD
Rotate engine to just before top dead centre - the exact distance is unimportant, then screw your tool into the cylinder head, unlock the locknut and thread the rod down until it touches the piston crown, relock the nut. The important part is to stop the piston going over TDC, then place your degree disc on the crankshaft and fix up a pointer in the normal manner. Next turn the disc to where you think it should be, again accuracy is not all that important at this stage, next turn engine so that the piston is hard up against the rod of your TDC tool. Now look at your degree disc, say it reads 42 deg. BTDC (before top dead centre) make a note of that figure, now turn engine backwards until piston rises and contacts the TDC tool once again. Holding in hard contact look at degree disc, this time say it reads 18 deg. ATDC (after top dead centre). Now comes the interesting part, add your first reading (42 deg. BTDC) to the new figure (18 deg. ATDC), resulting in 60. Now divide 60 by 2 and you have 3O deg so still holding engine against the stop move your degree disc to read 30 deg. ATDC, to check if you are correct, rotate the engine forward again until the piston contacts the stop and check the disc - it should read 30 deg. BTDC.

Next remove TDC tool and turn engine till disc reads 0 deg. this will be true TDC. Easy isn't it? But just in case, another example: Engine contacts stop at 61 deg. BTDC, turning the engine back the other way stop contacted at 33 deg. ATDC. 61 plus 33 = 47 degrees. So set the disc to 47 deg. ATDC to check, rotate the engine backwards and the reading should be 47 deg, BTDC. Remove tool and set engine to 0 deg. as indicated on the disc. As will be seen by the above, the action of holding the piston hard against the stop from both directions removes any inaccuracies due to the various engine clearances. This results in a true TDC impossible to reproduce by other methods.

Note: For beginners, the Ducati Bevel drive twin runs backwards, so don't get your BTDC and ATDC confused. Pantahs of course run fowards.

Original article by John Withers, March 1986