Magneto timing and camshaft

coloradobluesky

coloradobluesky

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coloradobluesky
Engine is Lycoming 0-360.
Is this possible?
The cam gear is one cog off from where it is supposed to be.
There is still enough adjustment for proper timing.
Would this be ok? Or would there be balance or other problems?
 
How would the cam gear be one cog off? Do you mean the cam to crank timing is one cog off? If so, the engine isn't assembled correctly.

If your mags are one cog off, then then need to come off and get retimed... Takes about an hour, maybe less.
 
Yeah right, the engine got assembled wrong. Cam gear one cog off. But its working. Just doesnt start quite right. And seems to vibrate different than others. Its just a suspicion on an engine I had some experience with. Just wonderin if its possible.
 
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Hell, just about anything is possible... I once reassembled my ignition harness with two cylinders swapped in firing order. Ran smooth, didn't backfire. Of course, it didn't pass the preflight mag check either ;-)
 
Sure you can get it to run and the cam being off will not affect your ability to set the mag timing but is it "OK"? No, it's not.
 
If that really is the problem, I think a guy could fix it under two days.
 
Yes, it is very possible to have the cam one tooth off and have it still run. Mag timing should be independent of the cam, though, since that should be referencing the piston TDC.
 
Is there any way to tell for sure without a lot of work (like pulling the engine and taking it apart?). Pardon my ignorance, but I really don't know this one. We don't really know if it is off a cog.
 
I've read of this before and correctly diagnosed it in the AOPA forum archives about 7 years ago. You can check it by removing a spark plug & feeling the exhaust-to-intake overlap vs TDC & comparing the crank position to another O-360. Obviously be very careful putting your finger in any spark plug hole while rotating the crank.
 
or I suppose you could just remove a rocker box cover and observe the valve overlap points vs crank TDC & compare that instead.
 
coloradobluesky said:
Engine is Lycoming 0-360.
Is this possible?
The cam gear is one cog off from where it is supposed to be.
Click to expand...
There is still enough adjustment for proper timing.[/quote]
No. There may be enough adjustment to get it to run
Would this be ok? Or would there be balance or other problems?
Click to expand...
No It would not be OK, and Yes there will be problems.

Find out when the valve starts to open in relationship to the position of the crankshaft. Overlap is governed by the cam it's self, not the timing of the cam, and cannot be changed. Overlap is determined by the cam lobes. But if a valve, just for discussion I'll use intake valve, is supposed to begin to open at X* before TDC, and the event starts prior to, or after, then we have a cam timing problem. One tooth, it may well run, but you'll have trouble with it, either now or later. There is more to Timing than just ignition timing (mags).
 
Easy to get one tooth off, there are three gears involved, and no there is no way to see the marks with removing the accessory case.
 

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If whomever assembled the engine managed to get the cam/crank timing off, I'd be concerned about what else they got wrong during assembly.
 
Craig said:
If whomever assembled the engine managed to get the cam/crank timing off, I'd be concerned about what else they got wrong during assembly.
Click to expand...
How do we know they did? Assembly is not that difficult, the machining, and the new parts are seldom out of tolerance. the only thing you really need to pay particular attention to is the torque, and gear timing.
 
Long story. And someone is sure to call bs or me a moron so I'll decline. Just take it as a hypothetical. If you can figure out this one, you're a pretty good mechanic IMO.
 
Pull upper plugs, remove #2 rocker cover. Turn prop until #1 is at TDC on the compression stroke. If cam is timed correctly the rockers on #2 will be even. Move prop to the left of TDC, one rocker will move in, the other out. Move prop back to the right of TDC, the rocker that moved in will now be out and the one that moved out will be in. Back to TDC and the rockers will be even again. If at TDC one rocker is already moving in, the cam is not timed properly (installation error) and power will be reduced and it will generally run like crap.

It happens more often than you'd think. Especially on H2AD's.
 
I'm not sure exhaust/intake valve "overlap" or "swap" occurs at exactly TDC. That's why I suggested comparing it to a known good engine. Or do you by chance know that it is at TDC? Somewhere I recall seeing the valve timing for Lycomings.
 
Using Tom's photos, and depending on which gear is off, you can estimate around 11 degrees for the crank or 5.5 degrees for the cam. The only way you would know for sure is to determine which set of gears is out of time.
 
Looks to me like there are 16 teeth on the crank gear. If the camshaft timing is off by 1 tooth, there will be a discrepancy of about 22 degrees of crank rotation. It should be very obvious. There is no need for any particular precision in the measurement!
 
I'm wondering if one were to pull both mags and insert a bore scope with a 90 degree head on it and take a peek. I don't know if you could see the marks or not, they may be crusted over,or there may not be room the get a scope in there.
 
Idler and cam are both 26 tooth gears, so about 13 degrees between them . Crank gear is 13 tooth. Idler to crank is going to be about 27 degrees. Gear ratio for the entire 3 gear train is 1:2. I simply estimated from the pictures originally and I did flop the cam speed . BTW... The graphics in the manual are general parts representation and not the actual part design.
 
Craig said:
Idler and cam are both 26 tooth gears, so about 13 degrees between them . Crank gear is 13 tooth. Idler to crank is going to be about 27 degrees. Gear ratio for the entire 3 gear train is 1:2. I simply estimated from the pictures originally and I did flop the cam speed . BTW... The graphics in the manual are general parts representation and not the actual part design.
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Well,,, someone finally realized that the cam always runs at half the speed of the crank. :)
 
Long night at work with a badly broke jet is my excuse :p
 
It seems to me that it is the number of teeth on the cam gear, divided into 360 is the degrees per cam cog. Once around on the cam gear isn't going to be once around on the crank gear so ignore the crank. Looking at it another way, if you fixed the crank so the piston is TDC then moved the cam gear one tooth, that would be the error. Move the cam gear all the way around, and that would be 360. Ergo, degrees per cog of cam = 360 / number of teeth in cam gear?? no?
 
nrpetersen said:
I'm not sure exhaust/intake valve "overlap" or "swap" occurs at exactly TDC. That's why I suggested comparing it to a known good engine. Or do you by chance know that it is at TDC? Somewhere I recall seeing the valve timing for Lycomings.
Click to expand...
It does. And watching the overlap is Lycoming's method of determining if cam timing is off. No need for borescopes. It's really obvious when it's out a tooth.
 
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