Finding TDC looking at piston through exhaust port?

B_Turner · Nov 5, 2008

I've been told that the best way to find TDC on a saw is to use a piston stop and a pointer on an degree wheel to find the center of where the piston stops in each direction.

How about looking through the exhaust port at the piston? Seems like one can tell pretty well where the piston is at the top.

What sort of accuracy are we looking for, like within a degree?

My motivation for all this is eventually I still intend to find out whether or not husky put a little more advance in the green coil/flywheel for the 3120. I have one of each now, and think I should compare them before I swap out the 9.6 coil/flywheel on the one saw.

timberwolf · Nov 5, 2008

You can eyeball it, but it is certainly going to be better to measure. A dial indicator down the plug hole can work too. Problem though with dial indicator or eyeball is near TDC there are a couple deg of crank rotation with nearly zero piston movement, you can guess middle of the pause at TDC but it won't be as accurate as it could be. A feeler gauge inserted in the exhaust port will give a more accurate stop for finding TDC by the half and half method.

Why do you need to find TDC to look at the coil advance, put a timing light on it, mark when the plug fires on one coil then check it with the other coil. when your done count your fingers to make sure they are all still in place.

There can be differences in coil timing despite the two looking identical from the physical mounting locations of the pols ect.

blsnelling · Nov 5, 2008

timberwolf said:
...when your done count your fingers to make sure they are all still in place.

LOL!!!

B_Turner · Nov 5, 2008

timberwolf said:
You can eyeball it, but it is certainly going to be better to measure. A dial indicator down the plug hole can work too. Problem though with dial indicator or eyeball is near TDC there are a couple deg of crank rotation with nearly zero piston movement, you can guess middle of the pause at TDC but it won't be as accurate as it could be. A feeler gauge inserted in the exhaust port will give a more accurate stop for finding TDC by the half and half method.

Why do you need to find TDC to look at the coil advance, put a timing light on it, mark when the plug fires on one coil then check it with the other coil. when your done count your fingers to make sure they are all still in place.

There can be differences in coil timing despite the two looking identical from the physical mounting locations of the pols ect.

Thanks for the reply. I am no saw tech, so any simpler way to do things is welcome.

The reason I ws thinking about finding TDC on each saw was so that I could get an aboslute reference with my exisiting setups. Since the flywheels are keyed so differently.

I want to figure out if the 9.7 k setup is more advanced or not before I spring for the extra parts. What I should have done is marked on my first saw (not flywheel) where full advance was with the 9.7k setup and then done the same on the second flywheel/coil.

Another thought I had was to use the keyway position itself on each saw as an absolute reference.

B_Turner · Nov 5, 2008

blsnelling said:
LOL!!!

With the marks in the right spot, you can see the flywheel with the top cover removed rather than the side cover. Much less scary.

timberwolf · Nov 5, 2008

With the marks in the right spot, you can see the flywheel with the top cover removed rather than the side cover. Much less scary.

That's no fun! Good idea though Easy enough to mark the outside of the flywheel about 25 deg BTDC somewhere where it will be visable from the top then with light on mark the case or take visual reference.

I would try to do the test on the same saw, just too many ways to get error using two saws. Even the plug gap and compression will affect the exact moment the plug fires.

Tzed250 · Nov 5, 2008

The dial indicator can be used like a piston stop to find exact TDC. Just split the difference on the degree wheel at say .040" or 1mm of travel in the indicator.

B_Turner · Nov 5, 2008

Tzed250 said:
The dial indicator can be used like a piston stop to find exact TDC. Just split the difference on the degree wheel at say .040" or 1mm of travel in the indicator.

I haven't come up with a way yet to hold the dial indicator solidly enough to try that.

timberwolf · Nov 5, 2008

Just toss the printed out degree wheel in the fire and make port maps then do the math, you won't look back.

Crofter · Nov 5, 2008

timberwolf said:
Just toss the printed out degree wheel in the fire and make port maps then do the math, you won't look back.

Is that with, or without the saw attached?

Just going by some point such as where the coil pole edges register with the magnets, will not determine exactly when the coil will trigger. electrical variables have the final say on when it fires. The key way should be close but the exact location to the magnets or their spacing can have some tolerance. A timing light is almost the only certain way of knowing exactly what the timing is. Not easy on a saw to get a visible mark for the light to work on the flywheel with the recoil housing on and not easy to take it off with the saw already running (thats where the finger counting come in. On some saws there is access via the air box with the recoil housing in place. It sure would be nice to have a full look at what you really are getting in timing on your saw because they do vary one to the next.

B_Turner · Nov 5, 2008

Crofter said:
It sure would be nice to have a full look at what you really are getting in timing on your saw because they do vary one to the next.

I have no idea of why, but one of the three brand new 3120s I have tried (owned) was noticeably stronger than the other two right out of the box. Absolute torquemonster, and you could hardly stall a 50 inch bar.

Next one not nearly as strong out of the box, but did pick up gains over maybe 10 tanks. Still not quite as strong as the first, though.

And the third one so far lags the second in grunt. Maybe some more tanks will tell. Wish I knew if it was compression, timing, or what....And wish I had that first saw back.

Taxmantoo · Nov 5, 2008

B_Turner said:
I haven't come up with a way yet to hold the dial indicator solidly enough to try that.

Pick a reading on the dial indicator that's about .040-.050 short of TDC. Roll the engine forward slowly until it reads your chosen number on the dial indicator, say .040" BTDC. Mark the flywheel position. Then roll it backwards to the same reading, like .040" ATDC. Mark that spot on the flywheel. TDC is exactly between the two marks.

timberwolf · Nov 5, 2008

That works, but better to do it further from TDC where the piston moves more in relation to how much the crank rotates. Just more acurate thats all same idea. But the same thing can be done with a fealer gauge in the exhaust pott, then no messing trying to secure dial indicator and read the numbers off it.

When you look at pistons you will see letters stambed on top "A"s and "B"s are the looser fits, "C"s and "D" are a little bigger, some piston fit just a little tighter than others, factory hones wear and get readjusted so no two bores are going to be exactly the same either, luck of the draw. Squish is in a range too seen identical saw models range nearly .010 and compression 20 psi on saws in good condition. one coild to the next seen 3 deg difference. If all these factors line up you get a above average stock saw, they don't line up and you get a dog.

Mad Professor · Nov 5, 2008

B_Turner said:
I haven't come up with a way yet to hold the dial indicator solidly enough to try that.

They made dial indicators to do just that, the base screws into the plug hole.

As a MC mechanic we useded them to time old school two strokes that had point ingitions. They were VERY sensitive (the bikes and indicators) to TDC/piston position, some were 3-4 cylinders.

You would set the gap on the points in the base plate, then time with the base plate. The other cyl/points timing/gap were set (e.g. only adjustable by gap) by adjusting the gaps only (e.g. all new points so gap was O.K. with proper timing on first cyl).

This was a time consuming process, even with all new points.

Anyway, to find TDC. Cycle the piston through a few cycles to break off any carbon residue with dial indicator installed. If you have a good indicator you will be within 0.5 degrees TDC at max height. If you need to be closer then a degree wheel (large) and measure 1.0 mm BTDC/ATDC, split the difference.

ShoerFast · Nov 5, 2008

There is a chance you might want to set up a spot to run a timing light on the saws at speed if there are two that have a lot different power. (knowing ovr-all spark advance)

Simple things like how crisp the Hall-effects tip the coil can have a lot to do with ing-timing and power output.

Tzed250 · Nov 6, 2008

timberwolf said:
That works, but better to do it further from TDC where the piston moves more in relation to how much the crank rotates. Just more acurate thats all same idea. But the same thing can be done with a fealer gauge in the exhaust pott, then no messing trying to secure dial indicator and read the numbers off it.

When you look at pistons you will see letters stambed on top "A"s and "B"s are the looser fits, "C"s and "D" are a little bigger, some piston fit just a little tighter than others, factory hones wear and get readjusted so no two bores are going to be exactly the same either, luck of the draw. Squish is in a range too seen identical saw models range nearly .010 and compression 20 psi on saws in good condition. one coild to the next seen 3 deg difference. If all these factors line up you get a above average stock saw, they don't line up and you get a dog.

Oh... to be able to pic and choose parts from the line....

Finding TDC looking at piston through exhaust port?

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