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Old Dec 09, 2012, 06:38 AM
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setting the valve clearance on 4 stroke motors

Guys,
I`m sure this topic has been discussed many times so please forgive me for asking again. I have been out of fuel motors for a few years but lately been flying some of my warbirds with Saito and Ys 4 strokes. What is the best way to find TDC and what shim thickness should be used for setting gap clearance on 4 stroke motors? Thank you in advance.
Frank
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Old Dec 09, 2012, 06:45 AM
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Shim thickness should be in the manual, so can't help you there. Maybe you can find the manual for yours in the thread in this section dedicated to engine manuals.

Finding TDC is easy if you remove the plug and just look where the piston stops (or if you can't see the piston crown you can try and feel with something non-damaging like a toothpick).
But TDC is not critical, anywhere from 90 degrees before TDC to 90 degrees after TDC will do as long as the engine is on the compression or expansion stroke.

Brgds, Bert
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Old Dec 09, 2012, 07:34 AM
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What I do is rotate the engine until I see that both intake and exhaust valves are closed. Both valves are closed on the compression stroke of course and I am looking for that condition. It is easier to get to TDC with removing the glow plug, but you can leave it in too.

The recommended range is 0.002 to 0.004 inches (or 0.03 to 0.10 mm) and I typically try for setting the value to about 0.002 - 0.003 inches.
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Old Dec 09, 2012, 07:41 AM
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The most accurate point to set the valve-lash, is where the lifter/tappet is on the middle of the base-circle.

TDC is more difficult to determine and significantly less accurate, since neither tappet is on the middle of the base-circle; and it is possible for a cam not to concentric with the shaft.

Finding the adjustment point - the middle of the base-circle, is done for each of the valves separately.
Find the point of full lift for the valve its lash you want to adjust.
The rocker-cover is open.
Remove the glow-plug so compression will not interfere.
Hold your finger on one rocker, at the push-rod end and turn the prop slowly to feel for the point of max lift (one could use a dial-gauge, but most don't have one).
From that point turn the prop one complete turn (360).
There, adjust the lash.
Repeat for the other rocker.
You're done!
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Last edited by DarZeelon; Dec 09, 2012 at 08:24 AM. Reason: Corrections.
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Old Dec 09, 2012, 08:13 AM
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Nope, it's not....

Since the base circle is a circle, the valve clearance should be the same anywhere on that base circle. That is a mathematical fact

Camshafts can sag or be unround due various reasons, but the "calibration point" has been internationally adopted to be "TDC following the compression stroke" by ALL engine manufacturers all over the world, no matter if it is model engines, car engines, ships engines.... it is just THE definition of the point at which to set valveclearance.
It even does not matter if an engine has a single camshaft, double camshaft, or even a "strange working principle" camshaft like the Ducati Desmo's.

But it is NOT "the middle of the base circle" simply because the engine manufacturers if at all, use that TDC as the starting point for their calibrations. In fact I have even worked on engines that explicitly stated to set valve clearance in TDC because the roundness of the base circle could not be guaranteed but the clearance on the calibration point could....

Saying something like "the most accurate point is the middle of the base circle" is the same as saying "100 deg F is equal to the standard human body temperature, but it is most accurate if you calibrate your thermometer on a Monday".

As to ease of determination: I can probably come up with over 5 different ways of determining TDC without removing even the glowplug and with more than sufficient degree of accuracy.

Brgds, Bert
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Old Dec 09, 2012, 08:20 AM
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DarZeelon, that is most interesting. I have never tried that. But the next time I go to adjust one of my four stroke engines, I'll give that a try.
Thanks
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Old Dec 09, 2012, 09:02 AM
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Quote:
Originally Posted by Brutus1967 View Post
Nope, it's not....

Since the base circle is a circle, the valve clearance should be the same anywhere on that base circle. That is a mathematical fact

Camshafts can sag or be unround due various reasons, but the "calibration point" has been internationally adopted to be "TDC following the compression stroke" by ALL engine manufacturers all over the world, no matter if it is model engines, car engines, ships engines.... it is just THE definition of the point at which to set valveclearance.
It even does not matter if an engine has a single camshaft, double camshaft, or even a "strange working principle" camshaft like the Ducati Desmo's.

But it is NOT "the middle of the base circle" simply because the engine manufacturers if at all, use that TDC as the starting point for their calibrations. In fact I have even worked on engines that explicitly stated to set valve clearance in TDC because the roundness of the base circle could not be guaranteed but the clearance on the calibration point could....

Saying something like "the most accurate point is the middle of the base circle" is the same as saying "100 deg F is equal to the standard human body temperature, but it is most accurate if you calibrate your thermometer on a Monday".

As to ease of determination: I can probably come up with over 5 different ways of determining TDC without removing even the glowplug and with more than sufficient degree of accuracy.

Brgds, Bert

Yes, it is, Bert!

It has been well documented that due to production inaccuracies, it is possible for the base-circle of each cam, not to be concentric with the camshaft.

It is even more likely with the very small production batches hobby engine manufacturers produce.
Grinding accuracy of cams isn't the best either in hobby engines.

TDC on the firing stage would place both lifters/followers of that cylinder, at either side of their respective cam-lobes, on the base circle, but not on its middle.
The middle of the base-circle is the point furthest from the cam's peak, so it is the best choice for selecting the point to adjust at.

The instructions for adjusting valve-lash for some American V8 engines, state to place one valve at full lift; and then to adjust other valves which are nearest to the middle of the base-circle.


...And I believe the correct human body temperature is 98.2F...
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Old Dec 09, 2012, 09:11 AM
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Originally Posted by earlwb View Post
What I do is rotate the engine until I see that both intake and exhaust valves are closed. Both valves are closed on the compression stroke of course and I am looking for that condition. It is easier to get to TDC with removing the glow plug, but you can leave it in too.

The recommended range is 0.002 to 0.004 inches (or 0.03 to 0.10 mm) and I typically try for setting the value to about 0.002 - 0.003 inches.
When you say both are closed, you mean the part oppisite of what you are adjusting?When they are closed they are down?
Frank
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Old Dec 09, 2012, 09:35 AM
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I've always considered that, center of the base circle, a pure load of bulloney. Who's to say that with the equipment that we have that the inaccuracy if any is somewhere other than the center of the base circle?.
The most accurate way would be to measure the base circle for high and low spots, mark them, then set the lash at a point equal to the average. We just can't measure that accurately so TDC is close enough. I've had fourstrokes since 1990 and have never had a valve problem caused by doing it at TDC.
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Old Dec 09, 2012, 09:42 AM
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Originally Posted by skrez View Post
When you say both are closed, you mean the part oppisite of what you are adjusting?When they are closed they are down?
Frank
Yeah my wording wasn't great, but the idea was when the valve itself is closed, not when the rocker arm is pushing down on it. There is a place during the compression stroke where the two valves are closed so the engine can build up compression for the spark to fire it off, that both valves are closed. There would be no pressure from the rocker arm to push against the spring to open the valve.

But then DarZeelon brought up a good point about that other method that was something I hadn't thought about before or at least for a very long time, I forget now.
The technique he describes would work well too.
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Last edited by earlwb; Dec 09, 2012 at 09:48 AM. Reason: correct misspelling
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Old Dec 09, 2012, 09:45 AM
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The fun never stops with Dar.

Dar,
What tolerance (runout) have you measured on model engine camshaft base circles?
How many engines have you found to be damaged or non functional while following the manufacturers gapping method at TDC?
Lastly, how many model four stroke engine have you owned?

We can get into all sorts of reasons why being accurate to the micron on valve lash is irrelevant, but that won't suit your style. Within the means of Average Joe Modeler any method is suitable even those that adjust the gap simply by feel.

Greg
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Old Dec 09, 2012, 10:35 AM
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Quote:
Originally Posted by earlwb View Post
What I do is rotate the engine until I see that both intake and exhaust valves are closed. Both valves are closed on the compression stroke of course and I am looking for that condition. It is easier to get to TDC with removing the glow plug, but you can leave it in too.

The recommended range is 0.002 to 0.004 inches (or 0.03 to 0.10 mm) and I typically try for setting the value to about 0.002 - 0.003 inches.
Maeby the unexperienced will have trouble as to determine at what precise moment of compression the valves must be checked. I always advise to simply turn the engine shaft around until the piston is at top dead center (the toothpick is one good idea but can also be a tiny screwdriver etc) Now turn the shaft slightly to the left and to the right (waggling) and, when by doing this, both rockers move about then this in NOT the correct position for adjusting. Now turn the shaft around again until the piston is at TDC and you will now notice that when you turn the crankshaft again a little to the left and to the right it will take a much further turning of the crank in both directions before each of the rocker starts moving about. THAT is the correct moment for adjustment. I do write this because I still see alot of engines from people that have adjusted the rockers at the wrong stroke.
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Old Dec 09, 2012, 11:26 AM
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Maeby the unexperienced will have trouble as to determine at what precise moment of compression the valves must be checked. I always advise to simply turn the engine shaft around until the piston is at top dead center (the toothpick is one good idea but can also be a tiny screwdriver etc) Now turn the shaft slightly to the left and to the right (waggling) and, when by doing this, both rockers move about then this in NOT the correct position for adjusting. Now turn the shaft around again until the piston is at TDC and you will now notice that when you turn the crankshaft again a little to the left and to the right it will take a much further turning of the crank in both directions before each of the rocker starts moving about. THAT is the correct moment for adjustment. I do write this because I still see alot of engines from people that have adjusted the rockers at the wrong stroke.
Okay that seems simple, when the piston is at the top and the rockers are waggling to left and right then would you turn the shaft preceisely 360 degrees from that point and then set the gap, correct?
Frank
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Old Dec 09, 2012, 01:26 PM
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Its easier than that, with the rocker covers off, watch the rocker arms as you rotate engine the proper direction, when the left, (viewing from the rear), rocker moves the valve downward and then back up watch for the piston to come into view through the glow plug hole. When both rockers are loose is the time to adjust.
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Old Dec 09, 2012, 03:52 PM
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Originally Posted by gkamysz View Post
The fun never stops with Dar.

Dar,
What tolerance (runout) have you measured on model engine camshaft base circles?
How many engines have you found to be damaged or non functional while following the manufacturers gapping method at TDC?
Lastly, how many model four stroke engine have you owned?

We can get into all sorts of reasons why being accurate to the micron on valve lash is irrelevant, but that won't suit your style. Within the means of Average Joe Modeler any method is suitable even those that adjust the gap simply by feel.

Greg
Greg,


I have never measured the tolerance on any model four-stroke.
I don't even own the instruments to do it.
My knowledge of inaccuracies in the grinding is based on rumors; that I know not to be baseless.

I have always helped people here adjust their valves and then I always did it the way I recommend here. I always double-check, remeasuring after spinning the engine a few time. They ALWAYS ran better after the adjustment.
It does not take longer than finding and relying on TDC; and to me it seems more trustworthy.

I still own only one four-stroke.

It's always fun when you're around, Greg. There's never a boring moment and I mean it in a positive way.
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