I'm trying to understand the piston stop bolt procedure. How do you know how far to screw the adjustable bolt down into the cylinder? How do i know that the piston will not hit the bolt? How do I know if i'm on the compression stroke or the power stroke? Still have not found a good explanation? Maybe someone can set my brain straight? AT LEAST ON THIS SUBJECT?
piston stop bolt confusion
- Thread starter artron9
- Start date
Mart
Well-known member
Whoa. Not on a flatty. The spark plug hole is not over the cylinder.
A piece of flat bar bolted to the deck face across the center of the bore is good, the crown on the piston is a reasonable distance for the piston to stop before top dead center.
You need a pointer on the crank pulley.
Bolt the stop over No.1 piston.
Rotate the crank clockwise until the piston contacts the stop. Mark the pulley (temporarily) at the pointer. Rotate the crank anti-clockwise until the piston contacts the stop. Mark the pulley (temporarily) at the pointer.
Put a permanent mark half way between the two temporary marks. When the pointer aligns with that mark, that is as close to TDC as you can get it by all practical means.
To find whether you're at the ignition TDC or at the end of the exhaust stroke, look at the firing order and split the first and second halfs.
1548
6372
No 6 is at top dead center at the same time as no.1.
Look at the valves. As no.1 piston rolls past tdc on it's ignition stroke (when it should be firing) the exhaust valve on n0.6 should just be closing and the inlet on no.6 should be just about to open. This is called "rocking".
When no.6 valves are rocking, no.1 should be firing.
As the exhaust valve is visible through the plug hole, this method can be used when troubleshooting the ignition on a built up engine.
Any questions just ask.
Mart.
A piece of flat bar bolted to the deck face across the center of the bore is good, the crown on the piston is a reasonable distance for the piston to stop before top dead center.
You need a pointer on the crank pulley.
Bolt the stop over No.1 piston.
Rotate the crank clockwise until the piston contacts the stop. Mark the pulley (temporarily) at the pointer. Rotate the crank anti-clockwise until the piston contacts the stop. Mark the pulley (temporarily) at the pointer.
Put a permanent mark half way between the two temporary marks. When the pointer aligns with that mark, that is as close to TDC as you can get it by all practical means.
To find whether you're at the ignition TDC or at the end of the exhaust stroke, look at the firing order and split the first and second halfs.
1548
6372
No 6 is at top dead center at the same time as no.1.
Look at the valves. As no.1 piston rolls past tdc on it's ignition stroke (when it should be firing) the exhaust valve on n0.6 should just be closing and the inlet on no.6 should be just about to open. This is called "rocking".
When no.6 valves are rocking, no.1 should be firing.
As the exhaust valve is visible through the plug hole, this method can be used when troubleshooting the ignition on a built up engine.
Any questions just ask.
Mart.
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glennpm
Well-known member
With the heads on the block you can use the zip tie method. There are also more elaborate ways that you can Google but this one works quite well and accurate enough for most people. I have used this method.
https://www.jalopyjournal.com/forum/threads/flathead-timing-trick.243565/#post-2590973
https://www.jalopyjournal.com/?p=10940
https://www.jalopyjournal.com/forum/threads/flathead-timing-trick.243565/#post-2590973
https://www.jalopyjournal.com/?p=10940
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Mart
Well-known member
The heads need to be off, or at least one head.
There is a method using a cable tie as a stop that people have used, but I've never tried it.
If your engine is the early type, (not 8BA) you can't really get the timing wrong, what exactly are you trying to achieve?
I'm not sure, but the bolt method you are referring to, is that normally used with overhead valve motors?
If you put anything hard down a flathead plug hole you will probably damage a valve. The plug holes are over the exhaust valves.
Mart.
There is a method using a cable tie as a stop that people have used, but I've never tried it.
If your engine is the early type, (not 8BA) you can't really get the timing wrong, what exactly are you trying to achieve?
I'm not sure, but the bolt method you are referring to, is that normally used with overhead valve motors?
If you put anything hard down a flathead plug hole you will probably damage a valve. The plug holes are over the exhaust valves.
Mart.
JSeery
Member Emeritus
Can not use a stop bolt with the heads on, but there are other methods that will work with the heads on. You need something you can slide through the sparkplug hole all the way over to the top of the piston. It needs to be something that is thicker than the clearance between the piston and the head and not damage anything but at the same time not be deformed or compressed. A thick zip-tie will work. The method is the same as Mart described. I would add that it is best to check it several times to assure you are getting consistent results. With the heads on it is easy to determine the compression strock by turning the engine over with your thumb over the #1 plug hole. You will feel the compressing.
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How about the compression/vacuum gauge method? Sounds good? Remove Schrader valve from rubber tube, screw on correct 14mm adapter, insert in plug hole. Hand crank motor clockwise, watch needle go to the right on vac scale and bring back counterclockwise into the compression scale beyond zero and then forward to zero. This is supposed to be TDC? But is this compression stroke or power stroke?
glennpm
Well-known member
Use just the hose from the compression gauge with the Schrader valve removed. When that piston is at the top most position, If you can blow into the hose, you are on the compression stroke. If you can't blow in, the piston is at the top, both valves are closed and you are on the power stroke.
Willit Stop
Well-known member
I think what you're referring to is compression or exhaust stroke.If you're on the power stroke,both valves will be closed.If you're on the exhaust stroke,the exhaust valve will be open. Pull all of the plugs,shift the transmission into third gear and place your thumb over #1 hole (Passenger side front). Have someone push the car forward. You will know it's on the compression stroke when pressure builds under your thumb. Good luck.
No, if you can blow in the hose, you are on either intake stroke or exhaust stroke. Both valves are closed on both the compression and power strokes.
Are you just trying to determine whether the piston is on the compression stroke or not? If so, this method will work.
If you're trying to determine exact top dead center, this method is about like trying to build a piano with an axe! It's nowhere near accurate enough! The zip tie method (using the head of the zip tie), as already explained, will work. The most accurate method would be using a positive piston stop with a degree wheel. This is, of course, done with the engine on a stand and a head off.
glennpm
Well-known member
Hi,
For four stroke the piston is at TDC either when the spark is lit and you have the power stroke. Both intake and exhaust valves are closed and you can't blow into the hose.
0r when the piston is returning to TDC again, the last of the exhaust is being pushed out and the exhaust valve is almost closed and for more performance cams, the intake valves are also just cracked open before TDC. When in this position you can blow into the hose.
For four stroke the piston is at TDC either when the spark is lit and you have the power stroke. Both intake and exhaust valves are closed and you can't blow into the hose.
0r when the piston is returning to TDC again, the last of the exhaust is being pushed out and the exhaust valve is almost closed and for more performance cams, the intake valves are also just cracked open before TDC. When in this position you can blow into the hose.
Ronnie
Well-known member
This method will only get you somewhat close but is not very accurate due to piston dwell at TDC.
Thanks: 40deluxe,Ronnie,Mart,JSeery,Glennpm,Willit Stop and Hoop. Its my 52 Vic. Its running fine . I just want to know these procedures. It seems everyone has their own methods. It's quite confusing and I am open to all suggestions. I still cannot figure out the zip tie method. It seems most of you advocate this option? Hard to believe that something as unstable as a zip tie could work when trying to discover a solution that requires a precise result?
Hoop
Well-known member
artron9, your engine should have a pointer and a "bump" or "pimple" on the crankshaft pulley ... if aligned they indicate 2 degrees BTDC for No.1. This is where you set your stock timing.
Don't know why you want to use bolts, cable ties, etc. to find TDC .... and I sure don't understand what the other guys are talking about.
Mass hysteria.
(.... blowing in hoses?)
Don't know why you want to use bolts, cable ties, etc. to find TDC .... and I sure don't understand what the other guys are talking about.
Mass hysteria.
(.... blowing in hoses?)
tubman
Well-known member
Nylon is very rigid. (Have you ever tried to "undo" a "zip-tie"?), They are flexible enough to wiggle theough the spark plug hole and it's not too hard to position them over the top of the piston. Most stock engines I have worked with have .080-.090" of clearance over the piston. Thus, any "Zip-Tie" with a head 1/8" or greater should work.
JSeery
Member Emeritus
One more time. You place anything in the combustion chamber for the piston to stop against, zip-ties work great. You rotate the engine until it stops against the zip-tie and mark the crankshaft pulley. You need a pointer near the pulley to use as an indicator (on an 8ba the original one on the timing cover will work). Now rotate the engine in the opposite direction until it once again contacts the zip-tie and again mark the pulley. TDC is half-way between these two marks. As posted before you can repeat this several times to assure a consistent reading. Simple and fast and accurate.