linear piston travel and advance degrees

motorson said:

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.

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Had I known it that the offset was that small, I wouldn't have posted my BLATHER....

o0norton0o said:

motorson said:

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.

Click to expand...

Had I known it that the offset was that small, I wouldn't have posted my BLATHER....

Click to expand...

Must have missed motorson's reference unless it is from another thread. If the offset is on the order of 1.0mm then we are talking about 1/3 of a "fart in a windstorm". And yes, it is good to check the timing marks.


Using the spreadsheet I referenced above and Rohan's early remarks about piston positions for ignition timing of 1/4" piston movement from TDC:

Degrees/ piston depression (inches)

27 0.245
28 0.263
29 0.281
30 0.300
31 0.320
32 0.340
33 0.360
34 0.381

Looks to be in the ballpark at a hair under 28 degrees.

The Excel program posted gave the same numbers as LAB's TorqSoft page. When I put in an offset of .039" (1.0mm) it changed the piston position at 31 degrees by .006" to .314" instead of .320. Don't forget my little Hypotenuse vs adjacent side ratio of .8829 to compensate for the spark plug hole angle.

motorson said:

The Excel program posted gave the same numbers as LAB's TorqSoft page. When I put in an offset of .039" (1.0mm) it changed the piston position at 31 degrees by .006" to .314" instead of .320. Don't forget my little Hypotenuse vs adjacent side ratio of .8829 to compensate for the spark plug hole angle.

Click to expand...

Same numbers as the TorqSoft reference as I would expect. Fortunately the spreadsheet allows for offset evaluations whereas the TorqSoft requires that you buy or register for something in order to have that capability.

I did a mental extrapolation and find that the 0.039" wrist pin offset scenario equates to approximately 1/3 of a degree. Although I like the thought process of calculating the angle of the spark plug hole, using a dial indicator like that is impractical as it will tend to bend and drag over a less than frictionless surface (top of piston) and give inaccurate measurements.

What you can do (I have done this many times before with a Commando) is set up your dial indicator with a magnetic "V" chuck and adjustable rods set to somewhere on the bike near the spark plug hole. See URL below:

https://www.summitracing.com/parts/...Kw60f0pRCZadwEFvSF_D6x4P7WxZcOpMRkaAtoX8P8HAQ

You will find that the dial indicator stylus can be situated through the spark plug hole that it is damn near vertical (perpendicular to the piston travel) and gives a direct reading.

Another thrifty approach is to use a common machinist steel rule and a stiff steel wire, rod, or screwdriver with a reference scribe mark. A common machinist scale graduation is 1/64" (0.0156"). If you are steady and can read accurately you will get within a degree of where you want to be. With the plug out, place and hold the rod and measure from a benchmark to the scribe mark. Move the piston while holding the rod and measure again - might be better with two people; one taking measurements while one is bumping motor rotation. It would be a quick and dirty check for anyone wondering if their timing marks in the chaincase are correctly indexed.

I had a small victory this afternoon. I used my data adjusted for the 62° angle of the spark plug hole and my eBay acquired TDC finder dial indicator. I rounded the end of the brass rod. I found Top Dead Center (TDC) to be quite repeatable. Then lowered the piston 9.20mm Before TDC and the pointer was at exactly 31°! I cranked the engine back to TDC and my "0" mark lined up. I then checked The 3.92mm distance from TDC and got exactly 20° on my pointer.

My clutch cover pointer happens to be right on the money. The main little victory here is that the 20 and 31 degree marks were BOTH right indicating that my data accurately predicted that they would be 11° apart.

I got full repeatability with my gauge and engine. I can say that the engine is tight on both the big end and gudgeon pin end of the rods. As Les said, all I have really done is confirm that my pointer and rotor are accurate. If I had found that to be off I would have simply recorded the amount of the error so I could correct for it when I time the ignition.
Cheers, Dan.

If the ignition timing is off by one degree or possibly even slightly more, tuning the carbs will probably compensate for it.