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Equating degrees before TDC to mm before TDC


Having very carefully set my Boyer MK1 electronic ignition to 31 degrees before TDC according to the inbuilt scale attached to the inside of the left crankcase cover visible through the plugged hole, I suffered a vicious kick back needing a visit to A&E, nothing broken but one very sore, swollen and bruised ankle.

I'm keen not to repeat this experience and have concluded that the inbuilt scale isn't to be trusted. I therefore want to verify the accuracy (or rather lack of accuracy) of the scale in a way I'm familiar with on two strokes I've owned, that being by measuring the piston position in mm before TDC measured by a clock gauge (DTI) through the spark plug hole. This figure of mm before TDC isn't quoted in the manuals I have for the Commando.  Now it's a long time since I was at school and my triganometry is more than a little rusty. However, knowing the stroke is 89mm and the con rod length between centres is 129.275mm I've calculated that 31 degrees before TDC equates to 8.6mm before TDC. 

Can anybody confirm my calculation or show that as I thought my triganometry is rusty. 

Has anybody else done this calculation?




From past experience the following stuff may cause a nasty kick back with a Boyer, so perhaps first check your battery has at least 12v charge, if not the Boyer will default to full advance when starting.

Then check you have the pick up wires connected the right way around. The white/ black and yellow /black cables tend to fade when old and can easily be confused, especially under the tank where they connect to the main harness.

The chain case marker on my Commando seems accurate enough.

Hope this helps.  


Found this Ignition Timing Chart hidden away inside Chapter 30 of the Dominator Service Notes. Not too sure how accurate it is but has got to be a reasonably good starting point.


Hi Neil,

Attached drawing of your arrangement. It draws out at 8.4040 mm but remember that this is a true vertical dimension and if your dial indicator isn't also a true vertical you have a further calculation.




Thanks everybody for your help. It's easy to see how just a couple of degrees error can put you in trouble, particularly if you err on the side of over advancement. Looks like my schoolboy triganometry wasn't too bad, a little tarnished but not entirely siezed up. 

I intend replacing the MK1 Boyer with something newer and not susceptable to having less than 12v coming in to it. As advised by Gordon, I don't want full advancement every time the battery voltage drops a bit, that seems to be a major flaw in the design which I've read the newer designs have eliminated.

All Boyer's up to MK3 are voltage sensitive, MK4 are fixed. But all these boxes are made from electrical components with tolerances so until you strobe them the timing can still be over advanced even if statically correct. As part of a fault finding exercise I changed the Boyer box alone, this changed the timing by 8 degrees at the crank and had to restrobe.


Having set my engine at 8.4mm before TDC (31 degrees)  with an allowance for the DTI being inclined in the spark plug hole relative to the movement of the piston (as quite rightly pointed out by Richard)  I find that the internal scale inside the left hand crankcase is showing 34 degrees before TDC, a 3 degree error.

Next stop is a new electronic ignition system which can tolerate a lower voltage supply without failure or unreliability.

Thanks everybody.


Just a comment on low voltage problems on the Boyer. It really should not be a problem on any machine other than a MkIII when using the electric starter.

It will work down to 9V and if your battery is anything other than fairly flat, you should be OK. Below this it can vary between +/- about 30 degrees from one bang to the next.

So if you see 12V at the battery with the ignition on, the Boyer should be as good as they ever are.



According to Boyer it should work down to 9v but my experience and nasty kicks made me check the battery voltage was above 12v with a multimeter if the bike had been parked up for a couple of weeks or more to avoid a broken ankle.

A Pazon Altair which needs only 3v has replaced the Boyer, it also has a similar advance curve as the original mechanical AR unit and reaches full advance around 3000rpm.

Much easier to strobe when setting up and a vast difference in performance, starting and idle speed.

But not cheap as I had to source it direct from New Zealand.    


The advice varies on what voltage the Boyer needs to operate reliably. To be perfectly honest, I am no longer interested in any ignition system where I need to concern myself whether the voltage is adequate or whether it might default to full advance, kick back and break my ankle. This is particularly important to me as my Commando is wired such that the front and rear "side lights" are on whenever the ignition is on and thus exert a slight drain on the battery. I'm sorry Mr Boyer, it's a different system for me.


The old Lucas RITA can't do that.

I did once put together a spreadsheet to calculate the piston drop.  The outstanding problem is - I've never seen the spark plug angle defined anywhere, and it's not easy to measure.  Last time I had the head off I painted the marks on the alternator (Dommie).  But RITA almost never needs any attention.

One reason piston drop probably fell out of favour might be because engine strokes became shorter and multi cylinder engines have smaller pots, so the same error in piston drop makes a worse error in timing.



If one really wants to perform a proper theoretical calculation of this figure, with an outcome correct down to the ten thousands of a milimeter, than one should also take in account the cilinder axis versus crankshaft axis offset.

Whether there is a factory design value for this offset, I do not know, but the offset has been found to be there indeed, on Norton twin engines that is.

Guess it will not effect occurrence or prevention of kickbacks though..


I'm sure somebody will shoot me down in flames if this is wrong. I have in my mind that the two axes on the 745cc motor were congruent. I was also told that to avoid boring through the cylinder wall into thin air on the 828cc motor, the crankshaft axis was displaced by 0.1" from the piston axis. In practical terms it probably doesn't make any difference but it's a nice nuance to the maths if you do allow for it.

Having set quite recently my Commando timing to the chaincase reference, then read one of the posts above, I'm now going to look at it again. I was once told to keep advancing it until it nearly blew my ankle apart when kickstarting, then take it back a couple of degrees. Simple but risky and not advisable.


I once put the electronic ignition pickup on a bit quickly after some repair or other to the Commando. Kick started it, seemed a bit brisk running, someone was on the throttle while I bent down to peer at the primary chaincase timing mark with the strobe gun, and had a major shock when it showed something like 20-30  degrees advance at 1,500 rpm and went clean off the scale before it reached 3,000 rpm. I shouted "stop" to my throttle person and adjusted the pickup plate, with a mental note to take more care next time.

It is therefore possible to start them with full advance. Also, a 500 single can be kickstarted at 38 deg advance. It just needs a decent swing and the TOE POINTING DOWN on the kickstart.

The pickups can only be positioned +/- 5 degrees static before you start it and see where it is on the strobe, unless you have scribe marked one that has been timed previously.

In reply to by chris_grimmett


Trust me, I'm sitting here with my leg up, my ankle surrounded by frozen peas, a serious kick back from a commando will break your ankle. It's not to be advised as a method of testing your timing. Best to do it in a technical manner. I will be setting my newly acquired Wassell,/Vape system at no more than 28 degrees BTDC using a timing disc and a DTI to determine TDC. 


Richard Hudson's sheet is fine...except it seems to have the wrong mm conversion for con rod length. His 129...should be 149. I believe the length is 5.875 inches, or 149.225.  I think this conversion appears somewhere on this site.

With 5.875" rod length, 31 degrees is only 7.07mm. And 8.4mm is nearly 33 degrees. But the 8.4 is roughly correct if measured at the plug angle.  Does anyone know the plug angle off hand? I'll measure it tomorrow maybe.


Back in the 60's, early model Tiger Cubs with upright distributor: set points opening roughly just before TDC. When (if!) it fired up, twiddle the dizzy back and forth a little bit until cracking open the throttle got a nice sharp response.

Tighten up the dizzy clamp & ride off.

If the front of the exhaust pipe glows cherry red, its not quite right!

It was all a lot easier then.

PS when the exhaust pipe stub pops out of the head and your exhaust clatters down the road, dont pick it up with your bare hands......


The degree plate has slotted holes but if the timing mark is visible within the window and therefore around 28° / 32° BTDC at full advance after locating a Boyer pick-up by eye then it will be close enough to start without any problems. As Gordon mentioned, the particular problem with the Boyer is low battery volts. If the battery is not holding charge then it will not retard when powered up. Reversed pick-up leads will cause the same effect.

Personally, I've had a sore foot from it but can't imagine how it could break anything if the kick is followed through.

If you want to check the degree plate then a stop bolt in conjunction with a degree disc is the best way to do it.

In fairness to Richard I think it was me who quoted the con rod length as 129.275mm. this figure was taken from the Norton Workshop Manual of 1973, part number 065416. It says 5.875" or 129.275mm. But of course 5.875" x25.4 is actually 149.225mm as rightly stated by David. So what is actually right and is this a typo in the Norton Manual? Anybody got a con rod to measure?


Just for those that are interested, according to the drawings I have, the plug angle is 29 degrees.

So take your measurement and multiply by 1.143 and this is what you should be looking for, as long as you are square down the plughole.

1.143 is 1/COS 29





I have just done a quick and dirty measurement on a con rod and got 149.5mm, so the accurate figure of 149.225mm looks good.

Many thanks Neil for accepting responsibility for my incorrect drawing, I am a Dominator man so didn't spot the error.

A handy dimension for the records Tony, now recorded in my little black book.

I modified an old plug body by removing the insulator and earth point, then machined it out and fitted a downward sprung probe. When it is fitted into the plug hole I only have to use the dial indicator on the outer end knowing that there is no sideways movement.

However this method is still only a good guess unless the top of the piston is new or has been cleaned during a rebuild, as variations in the thickness of carbon can produce a false reading.

If dome topped pistons are fitted it isn't just a matter of multiplying by the reciprocal of the Cosine, the curvature adds another dimensional calculation. I wouldn't even go there, just use the probe to accurately find T.D.C. for setting the timing disc.

Best Regards



The Atlas is fitted with an early Boyer and is often left for months ,but has never kicked back. I have not seen the Boyer since installed 25 years ago. Not bad. A Rudge Special or Ducati 250 with Borgo HC pistons need the head off  to find the piston positions ,so I don't time them at all. I just go by the sound and feel of the motor. 


For those playing with the trigonometry - yesterday I measured the spark plug angle (with a handy little electronic device).  No surprise:

The spark plugs on the heavy twins are placed at 30 degrees to the vertical.


Now we all know something the books don't tell us about!  (...we know more and more about less and less until we know everything about nothing at all...)



As threatened earlier in this thread, I have had a closer look at the accuracy of the timing indicator plate in the outer primary.

This required the sacrifice of an old N9YC that had seen better days and its repurposing to hold a dial gauge. There was a further delay while I had to machine a short extension to the gauge as it did not reach the piston crown. If you don't have access to a lathe, you can buy a ready-made screw-in dial gauge t.d.c. indicator (the ones I found were £25 + £5 shipping).

After that, it was easy. A quick setting of the motor to t.d.c., then a 31º offset using the timing disc and back on with the outer primary to check the reading. To my great surprise and delight, the mark on the rotor was at exactly 31º.

In my opinion this is going to be more accurate a method than trying to set t.d.c. with the linear type indicator where you are looking for a minimal change in length as t.d.c. is approached. In addition, it does not require a qualification in maths.

[Tip:- If you mount the timing disc into a short length of JG Speedfit 15mm plastic plumbing pipe, it is an exact fit into the alternator rotor retaining nut]


Thanks Tony! Maybe my bike stand is lop sided! Seriously though..I've never seen it in print. Perhaps their machines were worn out. Who in their right mind would choose a prime number for the angle? And who in their right mind would construct a drilling machine to make such an angle? Never's close enough. If we consider the variation in timing accepted by those who run Norton Singles with manual advance, it's all a bit academic. And we like being pedantic.


I must be missing something here! Surely if the big end moves down say 6mm, then the small end will  move down 6mm, so the length of the con rod is irrelevant? On that basis I make the BTDC figure 6.4mm at 31deg. That's 31deg at the big end BTDC.



Hi George,


The length of the conrod is very relevant. Consider if you had an infinitely long conrod, then the variation in angle between the big end being at the top and say 90 degrees will be zero. therefore if the big end moves down 6mm then the piston will move down 6mm

Now consider you have a conrod that is the same length as the throw. And again if we consider 90 degrees. if the big end moves down 6mm it is pulling the piston down at a steep angle so it will put the piston down more than 6mm.


If you look at suggested earlier by Michael and play around with conrod length, you will see it does make a difference.


The shorter the conrod the greater the difference.


Having said all that, using Piston BTDC for getting the timing right is ok to get it roughly correct, but a degree disk is the proper way.






George, as you correctly say, the total piston stroke will always be the same as that driven by the big end between TDC and BDC, but the rate of movement at any point in between, and hence piston position in relation to crank degrees, will differ with length of conrod. The shorter the conrod, the faster the piston is moving at around 90 degrees, but the slower it moves either side of TDC and BDC.

(edit - Tony got there first, but we have given different illustrations of the same answer!)

Hi George,

another way of looking at it is that when the crank is rotating at constant speed the piston travels up and down the cylinder first accelerating to maximum speed for 90 degrees of crank movement then decelerating for the next 90 degrees. It’s speed/distance graph traces a sine wave as not a straight line.

This is what started the old conundrum that the piston in a reciprocating engine never stops at the end of each stroke. As it is fastened to the crank and it doesn’t stop then neither can the piston. If it did stop then it should be able to calculate for how long which cannot be done.

I just threw that in as the thread was starting to go stale, hope you don’t mind.





I think a chap named Zeno would like a word with Richard Hudson …

Also, unless one has the infinitely-long con-rod mentioned in a previous post, the piston's speed/distance graph will not be an exact sine wave. The rate of acceleration away from TDC will be greater than the rate of deceleration approaching BDC.


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