linear piston travel and advance degrees

I was not able to edit my previous list of figures that were generated using an incorrect metric con-rod length for the Commando. Here is the new list using an 89mm stroke, 5.875 inch (149.225mm) rod length.

31°= 0.320" (8.127mm) BTDC (Before Top Dead Center), 30°= .300" (7.63mm) BTDC, 29°= .281" (7.147mm) BTDC, 28°= .263" (6.678mm) BTDC, 27°= .245" (6.224mm) BTDC, 26°= .228" (5.784mm) BTDC, 25°= .211" (5.359mm) BTDC, 24°= .195" (4.949mm) BTDC, 23°= .179" (4.554mm) BTDC, 22°= .164" (4.174mm) BTDC, 21°= .150" (3.81mm) BTDC, 20°= .136" (3.462mm) BTDC.

My apologies for the previous mistake. The math error is still up on the Norton Owners Club web site at: http://www.nortonownersclub.org/support ... do/conrods

It seems to me that you could set up a dial indicator through the spark plug hole, then set the "zero" point up at the piston's high point, but how do you know if the dial indicator is traveling inline with the bore, because the spark plug hole is at an angle to the cylinder ???

Do you remove the head to set up your dial indicator inline with the bore??? do you just use the spark plug hole to probe the piston and index the dial indictor's body off of the head's fins?? How do you set that up??

o0norton0o said:

But also as Rohan said, if the crank isn't centered beneath the bore of the cylinder, then averaging any 2 stop points from either side of TDC does not yield the exact TDC at their midpoint.

Click to expand...

This is true but I venture to say in the case of the Norton big twins, if conducting the piston stop method properly, inaccuracy is so minor compared to what we are trying to reconcile and correct here that the minor difference (inaccuracy) amounts to a fart in a wind storm. Even with the specified degrees full advance the engine happiness will be different under different conditions where it may want a little more or little less full advance.

What is the bore centerline offset? It would be fun to see what the exact difference is at say 45 degrees before and after TDC.

I didn't say that about the desaxe engine design* - although I'd agree with those remarks.
Isn't the crank set 1/4" from the centrelines ?

*Although its more a case of the cylinders couldn't be expanded equally all around,
because of the cam and lifters, so all the cylinder expanding had to happen to the back of the engine.
Thus the crank is no longer under the centreline of the bores.
You can see this very clearly if you overlay the 1st 500cc dommie head gasket onto any of the 650 750 or 850's.
All the bolts cam gear pushrod tunnels etc at the front are the same, everything at the back has been moved back...

motorson said:

Mr Rick is correct. The mistake is on the Norton Owners Club web site on line 2 of the "Conrods" section where they list the imperial length correctly and the metric length incorrectly. (http://www.nortonownersclub.org/support ... do/conrods ) I didn't check their math! Sorry! I'll go back and re-do my little chart earlier in this thread.
Dan.

Click to expand...

Several years ago I pointed out this rod length discrepancy to NOC and yet it still exists.

Rohan said:

Isn't the crank set 1/4" from the centrelines ?

*Although its more a case of the cylinders couldn't be expanded equally all around,
because of the cam and lifters, so all the cylinder expanding had to happen to the back of the engine.
Thus the crank is no longer under the centreline of the bores.
You can see this very clearly if you overlay the 1st 500cc dommie head gasket onto any of the 650 750 or 850's.
All the bolts cam gear pushrod tunnels etc at the front are the same, everything at the back has been moved back...

Click to expand...

I've heard this before and when you look at the bores:
500 Dommie 66mm
750 Commando 73mm
Diff 7mm (say roughly 1/4")

The Dommie had a 72.6mm stroke, so did they run longer rods or short barrels? If shorter barrels, (8.2mm shorter?) this might have bought them a little more room between the bores and the pushrod tunnels.

Dances with Shrapnel said:

This is true but I venture to say in the case of the Norton big twins, if conducting the piston stop method properly, inaccuracy is so minor compared to what we are trying to reconcile and correct here that the minor difference (inaccuracy) amounts to a fart in a wind storm. Even with the specified degrees full advance the engine happiness will be different under different conditions where it may want a little more or little less full advance.

What is the bore centerline offset? It would be fun to see what the exact difference is at say 45 degrees before and after TDC.

Click to expand...

I guess I'm a little bit OCD about theoretical stuff. Until Rohan mentioned it, I had no clue that the crank was off center in the bore so I wondered if there was another proceedure to find TDC. As you said, the optimal spark timing across the range of RPM's also varies with mixture, altitude, and temperature anyway so marking the TDC location to within a single degree is probably close enough in reality.

Thanks for responding. I have a better perspective...

You guys are killing me! The calculations site that LAB posted has a formula for calculating the linear motion of a piston that is offset from the center line of the crank so I can get that data and go back through the calculations. I can also compensate with a little bit of trigonometry for the angle of the spark plug hole. The only problem then will be the accuracy of the dial indicator, the care of the mechanic and the tightness of the engine parts themselves. I once told Jim Comstock that we would one day solve all of the Commando's engine problems. He just laughed. Cheers, Dan.

How's it run?

Ha! Mr. o0norton0o", you certainly do not have a monopoly on OCD. All good stuff.

As for Mr. motorson. My OCD apparently let me down on what I had already seen from LAB's post. In my next dull moment I will attempt to run the numbers and answer my own question.

So, with respect to the title of this post, for a Norton Twin, it really depends. If I understand the evolution of the Norton twins, the degrees versus piston linear movement will be different for the 650, 750 and 850 because for each increase in bore diameter, the wrist pin is shifted further back.

Now, for some context, how fine a division is the timing placard (in degrees) inside a Norton primary and how many instances have we read where the card was not accurately indexed?

Good stuff.

But its dessaxe ,

Id thought it was 3/8 in. like a trumppy , at 31 Deg. :cry:

Dances with Shrapnel said:

Rohan said:

Isn't the crank set 1/4" from the centrelines ?

*Although its more a case of the cylinders couldn't be expanded equally all around,
because of the cam and lifters, so all the cylinder expanding had to happen to the back of the engine.
Thus the crank is no longer under the centreline of the bores.
You can see this very clearly if you overlay the 1st 500cc dommie head gasket onto any of the 650 750 or 850's.
All the bolts cam gear pushrod tunnels etc at the front are the same, everything at the back has been moved back...

Click to expand...

I've heard this before and when you look at the bores:
500 Dommie 66mm
750 Commando 73mm
Diff 7mm (say roughly 1/4")

The Dommie had a 72.6mm stroke, so did they run longer rods or short barrels? If shorter barrels, (8.2mm shorter?) this might have bought them a little more room between the bores and the pushrod tunnels.

Click to expand...

Well actually only the back of the bore would have moved back 7mm (about 1/4"); the wrist pin would have only moved back 3.5mm or a fraction more than 1/8". Really not a lot and in my opinion, in the wrong direction.

Sorry guys, but I can't help thinking that all of this concern to get your 31 degree marker set better than somewhere between 30.5 and 31.5 might make sense if you were generating your spark trigger from the crank, but you are not, it translates to the cam via two straight cut gears and a chain!, meaning it floats anyway, and you are strobing this, possibly with your head at an angle, possibly on your knees, possibly in bright sunshine!

Do a basic check of your chaincase marker with a degree wheel, well worthwhile, there are known errors. More than that?

I don't have a chaincase, so that part was no issue for me, I had to make marks on my pully and have a pointer on my belt cover. When I fitted a new pulley I used a piston stop to find TDC, and rotated to 31. I made a couple of other marks, and then Jim Schmitt says to me...you know...some guys are finding that reducing the advance with those looonng rods (I have a shorts stroke!) works well....so cautiously I backed off to around 29, not the 26 or so suggested.

So, since you guys have the time and inclination to do the maths.....what do I do with 77.5mm bore (cast 828 barrels on first overbore), 80.4 mm stroke and 6.57" (166.878mm) rods, having first decided what the appropriate timing figure in degrees is?

... but steve,... IF I verify that the mark on the rotor and the scale are both accurately indexed to the piston position, then when I strobe the bike and I adjust the timing plate to change the spark timing, I'm actually getting the advance numbers that my strobe light indicates, rather than wondering IF the timing numbers that I'm reading are right...

The whole point of this is to verify the location of the timing mark on the rotor and that the timing plate scale is accurately adjusted so the strobe number is accurate when it's read. When you strobe the bike, timing chain play is included in the equation because the strobe pulses are fired by the ignition wires which are triggered by the cam... so altogether verfifying the rotor mark and scale allow the numbers you read on your strobe to be verified numbers too.

edited to add: I've played with jet sizing and timing because I had experience racing dirt cars. Sure, you can retard your timing a little and your bike will run smoother and you'll probably notice no loss of power on the street. I think that if you're not "spot on" with your timing, you should be a bit retarded rather than be a bit over advanced...

If you draw a schematic where the wrist pin is halfway down the stroke, you can develop an algorithm to calculate the relationship between crank degrees and distance from TDC. I did it years ago when I was porting a T250 Suzuki two stroke. Back then I had access to a computer which would calculate a table from a formula in 0.2mm steps from TDC and give the relevant conversion. The relationship is dependent on rod length and stroke. So the table is not usually transferable to other makes of bike. To find TDC, I use the soap bubble method. - Pull the electrode out of an old sparkplug and put a bubble of detergent on the top of it. If you rotate the crank backwards and forwards on the compression stroke - when the bubble is max size, you have found TDC.
I don't usually measure from TDC to set the timing on a four stroke. However the exercise is necessary with two strokes when you are porting them. For a four stroke motor, the soap bubble and the degree disk are better.

If you decide to do the calculation, always remember that with some computer programmes it is necessary to convert degrees to radians. The conversion factor is 57.296, if my memory serves me well.

I must be mis-understanding something - if all that is needed is to verify the timing marks, then using a piston stop, rotate to stop in one direction and mark, then the opposite direction (making sure valves don't get tangled) and mark. You now have TDC at exactly half way between your marks. Mark the half way and delete the first two to avoid confusion in the future.

Use a degree wheel to scribe your desired timing marks.

A tried and trusted method of finding TDC and applying accurate timing marks.

nickguzzi said:

I must be mis-understanding something - if all that is needed is to verify the timing marks, then using a piston stop, rotate to stop in one direction and mark, then the opposite direction (making sure valves don't get tangled) and mark. You now have TDC at exactly half way between your marks. Mark the half way and delete the first two to avoid confusion in the future.

Use a degree wheel to scribe your desired timing marks.

A tried and trusted method of finding TDC and applying accurate timing marks.

Click to expand...

For all practical purposes, you are correct, and that's how everyone I know does it, but as already discussed above, because the crankshaft is offset from the bore, there is a tiny ('fart in a windstorm" as Dances puts it)difference in the number of crankshaft degrees from TDC to the piston stop in one direction, and the number of crankshaft degrees in the other. That means that when you divide the crankshaft angle (on the degree wheel) between the two stop positions in half, you're not quite at the real TDC. I haven't ground out the numbers, but the difference has to be really small. Way smaller than the accuracy to which we can set the timing with a strobe light. Also, the closer the two stop positions are, the less the error from this factor, not that it really matters in the real world. Just ignore all the blather and time as usual.

Ken

31°= .320" (8.127mm) BTDC (Before Top Dead Center), 0.362” (9.205mm)
30°= .300" (7.630mm) BTDC, .340” (8.642mm)
29°= .281" (7.147mm) BTDC, .319” (8.095mm)
28°= .263" (6.678mm) BTDC, .298” (7.564mm)
27°= .245" (6.224mm) BTDC, .278” (7.049mm)
26°= .228" (5.784mm) BTDC, .258” (6.551mm)
25°= .211" (5.359mm) BTDC, .239” (6.070mm)
24°= .195" (4.949mm) BTDC, .221” (5.605mm)
23°= .179" (4.554mm) BTDC, .203” (5.158mm)
22°= .164" (4.174mm) BTDC, .186” (4.728mm)
21°= .150" (3.810mm) BTDC, .170” (4.315mm)
20°= .136" (3.462mm) BTDC, .154” (3.921mm)

The figures in GREEN are corrected for the 62° angle of the spark plug hole. When the gauge measures 1 inch of travel the piston will have moved .8829" down so the gauge needs to read more than the actual piston travel to give you the actual piston travel. According to Jim Comstock the crank offset he has measured was only .5 to 1.0mm on 750 and 850 Commando engines which is not much. And, of course, if the piston is domed or dished the dial indicator will be hitting a different part of that dome or dish at the different distances from TDC as it moves across the piston because of the 62° angled spark plug hole.

lcrken said:

For all practical purposes, you are correct, and that's how everyone I know does it, but as already discussed above, because the crankshaft is offset from the bore, there is a tiny ('fart in a windstorm" as Dances puts it)difference in the number of crankshaft degrees from TDC to the piston stop in one direction, and the number of crankshaft degrees in the other. That means that when you divide the crankshaft angle (on the degree wheel) between the two stop positions in half, you're not quite at the real TDC. I haven't ground out the numbers, but the difference has to be really small. Way smaller than the accuracy to which we can set the timing with a strobe light. Also, the closer the two stop positions are, the less the error from this factor, not that it really matters in the real world. Just ignore all the blather and time as usual.

Ken

Click to expand...

So I obsessed about that "fart in a wind storm".

Found a neat spreadsheet piston position calculator available at the following URL:

https://will-miller-mwtb.squarespac...troke-dwell-rod-angle-piston-speed-calculator

It even does a side by side comparison with zero offset and you can plug in the Commando approximate 1/8" offset (yes it is pin offset and not crank to bore centerline offset but I say close enough).

Degrees/Piston Position/Piston Position w/1/8" offset
28 /0.263 /0.281
29 /0.281 /0.300


Net difference is about 1 degree. This 1 degree offset remains roughly true through 60 degrees BTDC and 60 degrees ATDC. So I believe the fart in a windstorm has been quantified. For those who really like to obsess or can't help themselves in obsessing, you can use this calculator to figure out which way you need to offset your degree wheel (by 1 degree) to get it right on the gnats ass!

Otherwise, the standard piston stop fore and aft is the most practical method.

SteveA said:

.............................I don't have a chaincase, so that part was no issue for me, I had to make marks on my pully and have a pointer on my belt cover. When I fitted a new pulley I used a piston stop to find TDC, and rotated to 31. I made a couple of other marks, and then Jim Schmitt says to me...you know...some guys are finding that reducing the advance with those looonng rods (I have a shorts stroke!) works well....so cautiously I backed off to around 29, not the 26 or so suggested.

So, since you guys have the time and inclination to do the maths.....what do I do with 77.5mm bore (cast 828 barrels on first overbore), 80.4 mm stroke and 6.57" (166.878mm) rods, having first decided what the appropriate timing figure in degrees is?

Click to expand...

So you have your answer above but in your case, you should really be determining your ignition timing on a dyno. You are probably in the ballpark with the 28 degrees (or maybe less) but it is a bit of a different motor from that of a standard Commando that you are now running with the shorter stroke and longer rods and god knows what other evil things you did to the inside of that motor.