linear piston travel and advance degrees

motorson · Mar 21, 2017

I have heard and believe that the degree marker in the Commando clutch cover is not always accurate. I have a degree wheel I can use to find 31 degrees. Then I would have to carefully put the cover back on and see if the marker is correct and if not, then by how much is it off. (And, it just occurred to me, that I could generate the chart I am asking for myself if I go to that much trouble.)
So, I am wondering if any one has accurate data on the distance from TDC (Top Dead Center) the piston is at different crankshaft degrees. This is not going to be a linear formula since the piston moves down slowly at first and accelerates as the crank approaches 90 degrees. I think I remember 3/16ths being quoted as 31 degrees before TDC. I would like data that is in thousandths of an inch with more degree positions than just 31 degrees.
I have a Top Dead Center gauge with a dial indicator on it so I can get pretty accurate when I go to set the timing. Does anyone out there already have a chart of this information?
Dan.

L.A.B. · Mar 21, 2017

http://www.torqsoft.net/piston-position.html

xbacksideslider · Mar 21, 2017

I'm not understanding.
You can find 31* with the degree wheel so I'm curious about why you want to find the distance from TDC.

Rohan · Mar 21, 2017

He wants to see that the 31 degrees is accurately marked.

Some of them are reputedly inaccurate on the little scale.
How would you know that unless it was checked....

motorson · Mar 21, 2017

Thanks LAB! This is great. The reason I wanted this info was just out of laziness I guess. Now I can check my clutch cover marker for accuracy and then use it with a strobe light to check timing. My calculations from the Torqsoft page provided are these: 31(degrees) = 8.404mm (down from Top Dead Center, TDC) 30=7.891mm, 29=7.392mm, 28=6.908mm, 27=6.439mm, 26=5.984mm, 25=5.545mm, 24=5.121mm, 23=4.712mm, 22=4.320mm, 21=3.943mm, 20=3.583mm. All using an 89mm stroke and 129.275mm rods.

o0norton0o · Mar 21, 2017

I've often wondered if the easiest way to check the accuracy of the rotor mark and scale would be just to scribe a distinctive mark on the rotor somewhere between 25 - 30 degrees. (noting the exact degree angle from the TDC mark) Bring the pistons up to TDC by approaching TDC from both directions and averaging the position. Then look to see if the scribed mark on the rotor aligns with the same degree angle marked on the scale mounted on the primary cover.... IF they match then your strobe will give you an accurate reading. If there's a discrepency between the scribed angle mark and the scale then that is your adjustment to be factored into your strobe setting by adding or subtracting the difference.

Is that clear enough of an explanation? No degree wheel needed...

Deets55 · Mar 22, 2017

I think the most important thing is the method of getting true TDC of the piston. When I had my head off I used a stop block and degree wheel and then as Frank said I marked my stator at 0,28 &. 31 degrees. After the head was re-installed I checked for TDC with a dial indicator and threaded holder through the plug hole. I rotated the engine forward to TDC, then past the mark and then backward to TDC. There was at least a 5 degree error and no repeatability compared to my stator marks and degree wheel. I temporarily installed the primary cover and found there was a 3 degree error on the timing plate. Fortunately that was held on by small screws so I was able to adjust the plate into the correct location. The stop block takes all the free play out of the moving parts, something the dial indicator can't do. There are piston stops that thread into the spark plug hole but I worry about catching a valve so I haven't tried that.

Pete

toppy · Mar 22, 2017

If for some reason you have to remove the inner or outer primary covers the position of the scale may move slightly (mine did) i also found the outer cover could move relative to the inner as the dowel holes (again mine was yours may not be). When i installed my Alton Ekit i was left with no mark for the timing. So after using a degree disc to set the engine to 31º BTDC and improving the fit of the dowels to locate the cover acutely i left the engine set at 31º BTDC carefully refitted the outer then marked a new mark inline with 31º on the scale.

lcrken · Mar 22, 2017

You may have some trouble getting an accurate measurement through the spark plug hole. As I recall, you can't really get the dial indicator in line with the bore because of interference with the head. If the dial indicator is at an angle to the bore, your measurements will not be correct. It's really pretty easy to do it with the degree wheel as you originally described.

Ken

acadian · Mar 22, 2017

Take one of the standard TDC tools that threads into the plug hole, grind down so it doesn't interfere with the intake valve, tap a small bolt horizontally through the body of the tool to lock it and bingo, piston stop tool. Use with a degree wheel on the crank end (a 7/16 socket makes a good interference fit into the stator nut opening) and you can easily check your timing marks.

Deets55 · Mar 22, 2017

lcrken said:
You may have some trouble getting an accurate measurement through the spark plug hole. As I recall, you can't really get the dial indicator in line with the bore because of interference with the head. If the dial indicator is at an angle to the bore, your measurements will not be correct. It's really pretty easy to do it with the degree wheel as you originally described.

Ken

Ken,
Yea, I agree, the dial indicator was just a science experiment on my part. But the notch on the flywheel was right in the center of the access hole on the right side, found that to be interesting.

Pete

o0norton0o · Mar 22, 2017

lcrken said:
You may have some trouble getting an accurate measurement through the spark plug hole. As I recall, you can't really get the dial indicator in line with the bore because of interference with the head. If the dial indicator is at an angle to the bore, your measurements will not be correct. It's really pretty easy to do it with the degree wheel as you originally described.

Ken

BUT,..... isn't the essence of the timing mark on the rotor it's relationship with the piston?? so you still have to find some way to determin the piston position when you measure the timing mark.

When you mount a degree wheel, the piston postion has to be known or else how is it accurately mounted?

I don't see how any of this can be done without finding TDC since the rotor mark relys on TDC and the valve timing relys on TDC. Am I missing something here??

On a chainsaw, they make these piston stops that thread into the spark plug hole. It's primary purpose is to stop the piston so the crankshaft nut can be removed (since without it turning the nut would just spin the crankshaft) This piston stopper can stop the piston at the same point in both directions of crankshaft travel, so a piston stopper could be used to find TDC on any engine by making a mark on the rotor in both spin directions where the piston is stopped, then averaging the 2 locations. Isn't that how it's done on a commando? (whether they use a stopper or a dial indicator...)

I'm hoping to learn something here, that maybe I'm missing. thanks....

MikeG · Mar 22, 2017

L.A.B. said:
http://www.torqsoft.net/piston-position.html

About 15 years ago I was looking for just this kind of thing. Setting the timing on a BSA A10, the only figure given is pistons set BTDC, no degree figure. Works great with the head off, but impossible to do with it on due to the plug hole angle in relation to the piston. I tried simply dividing the stroke by 360 but it was pointed out that due to the pistons not being on the centerline of the crankshaft my calculation was wrong.
Thanks L.A.B.!

Nater_Potater · Mar 22, 2017

I used an adjustable piston stop through the spark plug hole (only on the compession and power stroke so as to avoid the valves) in conjunction with a timing wheel. Once 31' was found, the piston stop was left untouched while the primary cover was reassembled. Lo and behold; four degrees off!

An adjustable piston stop is easy enough to make. Knock the guts out of an old plug, then thread the now empty shell to allow a bolt to be threaded through. Add a jamb nut to the bolt before inserting in the plug shell. Once you snug the plug shell against the seat, you can adjust the bolt to set the piston part-way down the bore. You just have to be careful as you roll the engine back and forth that you don't open the valves into the bolt. I pull mine out while moving the crank around to the opposite side, then snug the stop back down as before. As long as you keep that torque similar, the stop will always go back to the same depth.

Nathan

Rohan · Mar 22, 2017

MikeG said:
I tried simply dividing the stroke by 360 but it was pointed out that due to the pistons not being on the centerline of the crankshaft my calculation was wrong.

Thats not the only difficulty !
Due to the varying angularity of the rod, the piston only moves slightly for each degree near tdc (and bdc),
but mid stroke the piston moves LOTS for each degree.
So taking an average of the piston movement is no help whatsoever...
hopethishelps.

lcrken · Mar 22, 2017

I think the use of mm down from tdc for timing is more common for 2-strokes, where that's how the factory used to specify it, at least back when I was racing the older 2-stroke Yamahas. It was easy on most 2-stokes back then. They were all air cooled and mostly had a simple head with spark plug in the center and perpendicular to the piston. I still have the old kit I used to use for that. Not so simple with most 4-strokes. As far as I know, almost everyone uses a degree wheel on the crank, and finds tdc with a piston stop, as several have described in this thread. If you have the head off, you can certainly also do it with a dial indicator, but your still doing it by finding the same piston height on both sides of tdc, and setting the dial indicator off that info. It's really not very accurate to try to find tdc by watching the dial at tdc. As Rohan pointed out, there's very little piston movement for a degree or two either side of tdc.

I use old spark plugs to make pistons stops. Just break all the porcelain out and weld in an appropriate length rounded plug. I have several now for the different plug threads.

Ken

Rohan · Mar 22, 2017

lcrken said:
I think the use of mm down from tdc for timing

Nortons quoted the timing figures in inches for most 1930s models (fractions of inches obviously)
and all earlier models - around a 1/4" to 3/8 and thereabouts was very popular.
Finding degrees of advance for early models, except for the racing models, could be quite difficult to find.
Degree wheels and their use prewar would have been scarce, for road bikes. ?

The 1st dommie twin Model 7 (1949) is quoted in the handbook as 31 degrees = 1/4".
The 16H, big 4 and ES2 are only quoted in inches in their handbooks.

Mr. Rick · Mar 23, 2017

Is the stock 850 rod length not 5.875?
This converts to 149.225 mm, so why is Motorson using the 129.275 number?

o0norton0o · Mar 23, 2017

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.

motorson · Mar 23, 2017

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.

linear piston travel and advance degrees

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