I do not have a chart but you can work it out from geometry. The rod angle µ is related to the crank angle ß by: R*sin(µ) = C*sin(ß) R is the rod length, C is half the crank stroke. So µ = asin(C/R*sin(ß)) or ß = asin(R/C*sin(µ)) The drop of the piston is the rod and 1/2 crank stroke minus each length times the cosine of their respective angles. Piston drop = R + C - [R*cos(µ) + C*cos(ß)] Substitute the equation for µ from the above equation: Piston drop = R + C - [R*cos(asin(C/R*sin(ß))) + C*cos(ß)] Solve for ß given the rod length, 1/2 crank stroke and piston drop from TDC. You can drop that equation into a spread sheet and plot piston drop versus ß then put a trend line through it if you do not want to do the inversion.

Last edited by DMadigan; 03/21/133:02 am. Reason: cannot display Greek correctly

Re: Chart to convert inches to degrees
[Re: DMadigan]
#482229 03/21/133:44 am03/21/133:44 am

I do not have a chart but you can work it out from geometry. The rod angle µ is related to the crank angle ß by: R*sin(µ) = C*sin(ß) R is the rod length, C is half the crank stroke. So µ = asin(C/R*sin(ß)) or ß = asin(R/C*sin(µ)) The drop of the piston is the rod and 1/2 crank stroke minus each length times the cosine of their respective angles. Piston drop = R + C - [R*cos(µ) + C*cos(ß)] Substitute the equation for µ from the above equation: Piston drop = R + C - [R*cos(asin(C/R*sin(ß))) + C*cos(ß)] Solve for ß given the rod length, 1/2 crank stroke and piston drop from TDC. You can drop that equation into a spread sheet and plot piston drop versus ß then put a trend line through it if you do not want to do the inversion.

The complication is that you know the angle of the crankpin from TDC (or from cylinder axis),but you would need to calculate the rod angle. I do it a little differently to simplify arithmetic ,but the result is the same.

If R = rod length, and C = crank radius (which is 1/2 stroke), and B = crank angle (from TDC) and H = height of gudgeon pin from crank centre

H = C(cos.B) + sq. rt.( R squared - [C(sin.B)]squared )

Height of gudgeon pin at TDC = R+C. Distance from TDC = R+C-H

Re: Chart to convert inches to degrees
[Re: Red Lapierre]
#482249 03/21/139:43 am03/21/139:43 am

Wow! May be I should of asked what degree would I set timing at for a BSA A10 SR 3/8" BTDC is what I need. I will drop a pin in spark plug and measure it. Hope that will be close enough.

Red, I don't understand why you're starting with inches. I would think that the timing spec for your bike, in DEGREES, would be a well known and published bit of information.

This might explain it better but the equation is the same:

You want H, the distance the piston drops from TDC at the timing angle. The distance A is the same for the crank throw and the rod. From trigonometry, R^2 = RT^2 + A^2 and S^2 = ST^2 + A^2 A can be calculated from the timing angle by A = S*sin(timing angle) From this you can calculate RT and ST, the projected vertical length of the rod and stroke. The distance the piston is down from TDC at the timing angle is H = R + S - RT - ST

Re: Chart to convert inches to degrees
[Re: DMadigan]
#482425 03/22/131:08 pm03/22/131:08 pm

This means that there if some 4° difference in the timing between a BSA M20 and M21. M20 stroke 94mm, M21 112mm both with 8.25 inch rods (209.5mm.) Using the calculator on line and 7/16 BTDC at full advance for both engines I get approx 32.8° BTDC for the M20 and 36.8° BTDC for M21. Is there really this much difference? Does it matter?

Cheers Pete

Cheers Pete

Re: Chart to convert inches to degrees
[Re: Red Lapierre]
#482430 03/22/131:49 pm03/22/131:49 pm

Similar question: On my 70 A50R there is a raised portion on the alternator spinny-thingy onto which is inscribed a timing mark. The timing line is to indicate full advance and it lives in the middle of that raised pad. Anybody know how many degrees wide that raised pad is? Knowing that would make it easy to interpolate minor plus or minus timing variations. No, I don't have a degree wheel or a degree in trig for that matter. Thanks, Leon

October of 69-A50R

Re: Chart to convert inches to degrees
[Re: Red Lapierre]
#482433 03/22/132:09 pm03/22/132:09 pm

Mark Z Red, I don't understand why you're starting with inches. I would think that the timing spec for your bike, in DEGREES, would be a well known and published bit of information. My Haynes Workshop manual goes by inches BTDC not degrees. I google it but found nothing that help.

Mark Z Red, I don't understand why you're starting with inches. I would think that the timing spec for your bike, in DEGREES, would be a well known and published bit of information. My Haynes Workshop manual goes by inches BTDC not degrees. I google it but found nothing that help.

The factory manuals for most preunit bikes gave timing in inches.

A smattering: '53 Gold Flash '67 Royal Star '71 Rickman Metisse '40 Silver Star '37 Rudge Special sixtyseventy Lightboltrocket road racer...and many more.

Re: Chart to convert inches to degrees
[Re: Alex]
#482532 03/23/1312:57 am03/23/1312:57 am

I guess because there's no place to put a degree wheel on it?

Along with a lot of other things in my toolkit, I carry a 1/2"-wide machinist's ruler with the timing specs of ~20 models of all the major British bikes in tiny print taped to the back of it. The ruler has a 'T'-bar/pocket clip that slides up/down when locating TDC. Because it's a lot easier to add mm than 1/32nds, that's how I have all the timing specs tabulated. I can get the timing to within 1/2mm with this ruler, which is within a degree. I can (and have) field strip someone's faulty magneto on the side of the road, fix it, reinstall, and reset the timing with this "calibrated" ruler. This would be essentially impossible to do any other way.

Re: Chart to convert inches to degrees
[Re: Red Lapierre]
#482548 03/23/132:40 am03/23/132:40 am

Not to take up any more of this thread for my education on pre-unit bikes, but, if this is the prescribed method for timing them, I'm wondering now why Red wants to convert the spec to degrees.

Have you ever tried to measure how far a piston is from TDC,while the head is on?

It would be easy enough if the spark plug hole was vertical and the piston top was flat. It would still be easy enough with a domed piston,if the vertical spark plug hole was in the centre of the head.

With an angled spark plug hole and a domed piston,it becomes awkward to get an exact measurement.

I don't doubt that Magnetoman can get it right within one degree.Many others would find that difficult.

With an angled spark plug hole and a domed piston,it becomes awkward to get an exact measurement.

I should have mentioned that the quality control of these rulers isn't great, so it was easy for me to find a few at a used machinery store that were a tiny bit narrower than 1/2" (maybe 0.01" narrower?). These insert vertically in the spark plug holes of everything I tried them on the the garage and that I faced on the road, i.e. BSA A10 and Gold Star, Matchless G12 and G80CS, Triumph T110, and a Velocette (maybe others, but I don't remember).

The following photo shows the ruler (Snap On brand) in a Matchless G12 engine that's on its side (sorry about the quality, but I just made a quick hand-held snapshot and there wasn't sufficient light to get a good depth of field):

As can be seen the ruler is vertical, so even with a high compression domed piston there wouldn't have been an issue (the steepest parts of the tops of most pistons are cutouts for the valves, which aren't directly under the spark plug hole).

The engine is rotated back and forth near TDC and the sliding clip is used to "lock in" the arbitrary height when the piston is at the top. The ruler is removed and the required height BTDC (say, 9.5 mm) is added to that height and the clip moved to that position. The ruler is replaced and the engine then rotated backwards and the ruler slides down until the clip stops from moving it any further. Again the engine is rocked back and forth to determine where it is just touching the ruler. That's where the points need to open. I can determine this position to ~1/2mm, which is equivalent to ~1 degree. Or, I've convinced myself I can determine it to that accuracy. In any case, trying to get the points to open precisely there is a feat in itself. But, all the engines I've done this with have run fine afterwards. Maybe not at max. hp, because I try to err on the side of having the timing too retarded if anything. Certainly they ran a lot better than had I not had my calibrated ruler with me to get the guys' bikes back on the road.

Also, I'm not recommending this for doing a proper job of timing, but it's great for getting things pretty close in the garage, as well as for checking/repairing things on the road. Even if it isn't as serious as a failed magneto condenser that needs repair on the road, if a bike won't start, is it because the timing slipped, or because of one of other possible problems? If a quick check with the ruler confirms the timing is OK, it removes that from the list of possible problems. For this purpose it's essential to have the timing specs on the ruler (or somewhere readily found), because in the stress of the moment not many riders remember how many inches BTDC their timing should be set at on their bike.

Last edited by Magnetoman; 03/23/135:42 am. Reason: added photo and some text

Re: Chart to convert inches to degrees
[Re: Red Lapierre]
#482581 03/23/131:00 pm03/23/131:00 pm

Mark Z I drilled and tapped the end of crank on the engine sproket side to put degree wheel on. I have made several measuring devices to do the timing but have not try the ruler Magnetoman is showing. Timing sometimes takes awhile so I figure a degree wheel might make it easier with head on.

Ok I get it, and probably what I would try as well if I had one of those. Well you have the formulae now, but I'm still having a hard time believing that with all the A10s in our cyberspace, no one knows the timing spec in degrees. Does your service manual show different specs (in inches) for different A10 models?