Hitchcocks Forum

Regarding the Top Dead Centre tool sold by our host has anybody done the maths (math for the Americans)to convert the distance when the piston is .8mm from TDC to the distance the tool moves on its millimeter calibrated scale because it is inserted in the head at quite an angle. I would estimate it could be more like 1.2mm but my trigonometry is too rusty to work it out exactly. I have done my timing when the head is off which makes it easy. I know the maximum advance timing of 32 degrees is the most important and that equates to 8.5 mm at the piston but what would it be on the tool? Nine or ten mm perhaps?? Any ideas?

Drive train back lash... can your eye discern the difference in movement between 8.4mm and 8.5mm... assuming the plug is at 45 deg for example, do you need a calculator to compute the cosine... is the engine a 350 or a 500... what is the actual combustion chamber capacity...
what altitude, and what percentage of ethanol in the the fuel...


Rather than relying on a predetermined vertical measurement, there are better indicators of suitability of ign timing for the Bullet; ease of starting, absence of kick-back, absence of 'rattle' under load, oil temp, performance, vibration, discolouration of header-pipe, and by no means least, use a strobe light.

Hi this is how i do my timing.get tdc mark a line across stator and rotor on a bjt of clear plastic mark a line across then 21mm from this line mark another line this is your 32 degree mark stick the plastic between rotor and stator align first mark with rotor and mark stator at 32degree mark. i use a timing light to match rotor mark with 32mark on stator. So you can set up with tdc tool which is pretty rough then adjust points plate to get it spot on marks line up at just over half throttle which i thinkk is full advance. I use this method to set boyer ign.
Calc used to find marks inside stator measurement 236.4mm
32 over 360 multiplied by 236.4 gives 21mm from tdc. It works for me.

Scotty, I have done other posts on this topic, maybe not on this forum but one problem I had was, with the chaincase cover off and turning the motor gently with a spanner on the crankshaft it is still hard to find exactly TDC so you can mark it. As you know there is a point when you can turn the crank a bit each way without the piston moving. I always use your method with my strobe if the cover is off as I can see it is advancing ok. My question was, how does .8mm relate to a reading on the measuring tool to use as a rough check. I still don't think a difference of 1mm in either direction makes much difference however.

A Positive Stop screwed into the cylinder via the spark plug hole, so that it prevents the piston from passing TDC.


Attach a tin pointer under one of the stator securing nuts. Bring piston up against the Stop. Mark rotor with a scribe line at pointer tip, rotate crank in opposite direction until it stops again, scribe second line. Halfway between the two lines is true TDC, now you can inscribe a permanent TDC line on rotor


1mm movement of the points backing plate is equal to approx 1.5 degrees ign timing adjustment.

I always used my patented coat hanger wire, cut off near the end so it can`t drop down the cylinder, and is perpendicular once in there. The top mark is Fully Advanced, the middle mark is 0.8mm, and the bottom mark is TDC. Quite suitable for the state of tune my engine was in.

May be splitting hairs, but further to Tims comment on 1mm of backing plate movement being 1.5 crank degrees- doing the maths on a 68.5mm Boyer stator plate give 1.19 crank degrees per 1mmm of circumferal movement. On a standard bike, not an issue, but on one making 45plus hp, its a different story. Horses for courses.

Get it somewhere near and just tweak it advance/retard til it runs right.
Measuring piston travel accurately so close to TDC is almost impossible with a TDC tool.

If you want to be super accurate you need a strobe but that's OTT on your average RE.

Tim, Can you explain in more detail please. What do you put in the plughole to stop the piston and could that cause damage. You mark that spot on the roter OK. If you then turn the motor backwards 360 to bring the piston to the top again and mark that spot, surely halfway between them would see the piston at the bottom (dead centre). I know all this is not that critical. I am just curious. And by the way my latest problem which got me into this discussion turned out to be an open circuit secondary in my coil. The bike ran ok most of the time but intermittently died only to run perfect again later.

I usually set my static timing with the points locked in the fully advanced position using our hosts small locking washer tool at 8.5mm BTDC. If it then cold starts, I call it a win. If it's latchy to start, I retard it a little.

The "eyeball" way, which is how it will be done on most bullets worldwide, is to have the stator plate only just nipped up and with the engine running, progressively advance it by tapping it round with a screwdriver handle then riding for a bit until it pinks on full throttle. Then retard it a touch until it stops pinking.