Topic: Using a piston stop question

[Bailgang]

When using a piston stop why is it more accurate than say using a dial indicator to determine TDC? The reason I ask is because if a piston is at absolute TDC then isn't the piston at a dead stop already and at that point have no where to go but down? The way I'm seeing it is that if I'm using a piston stop to prevent me from turning the crank any further once at TDC then that's telling me I'm not at absolute TDC then because the piston is still wanting to come up a bit more therefore it can't be at absolute TDC at least not just yet because if it were at absolute TDC the that's as far up the cyl the piston is going to travel anyhow and have no where else to go but down. Is using a piston stop more accurate because it's more consistent? No arguments or debates here, I'm just willing to learn and need an explanation.

[bwaller]

With a positive stop, I usually install at about 20d before TDC, but doesn't matter. The idea then is with a degree wheel installed, turn the engine in one direction until it contacts the stop, then in the opposite direction and average both numbers and adjust you degree wheel pointer to the exact number. Finding exact TDC is very important, and takes a little time going in both directions until the degree from TDC is exactly the same.

With these SOHC4's there are a couple issues working against our using a dial indicator. Plug hole angle and the inability of installing the guage close enough to the plug hole to get a good reading. Certainly it is an accurate way in a combustion chamber with a center spark plug hole. That said some guys do use a dial indicator.

Simply one opinion Bailbang.

[Bailgang]

With a positive stop, I usually install at about 20d before TDC, but doesn't matter. The idea then is with a degree wheel installed, turn the engine in one direction until it contacts the stop, then in the opposite direction and average both numbers and adjust you degree wheel pointer to the exact number. Finding exact TDC is very important, and takes a little time going in both directions until the degree from TDC is exactly the same.

With these SOHC4's there are a couple issues working against our using a dial indicator. Plug hole angle and the inability of installing the guage close enough to the plug hole to get a good reading. Certainly it is an accurate way in a combustion chamber with a center spark plug hole. That said some guys do use a dial indicator.

Simply one opinion Bailbang.

Ahha, now that makes sense. I had never seen how a person would set up a piston stop therefore never understood it's logic but now it makes sense. I knew it had to be a simple explanation. Thanks.

[martin99]

A TDC finder and piston stop are two different things.

A TDC finder is used in conjunction with a degree wheel mounted on the crank. As you turn the engine and the piston rises, you select one of the markers on the TDC finder, before it reaches TDC. Then set your pointer at 'zero' on the degree wheel. Next, revolve the engine in the other direction until the piston position corresponds with the same mark on the finder, and note where the pointer is on the degree wheel. Remove the finder, continue to revolve to exactly half that reading you have just taken from the degree wheel, and you are at TDC.

A piston stop is a tool, often home-made using a bolt and spark plug body, to easily and quickly ascertain when you are at a specific number of degrees before or after TDC. For example, my old Norton is timed at 32' BTDC. Having found TDC as described above, I turned the engine until the pointer on the degree wheel indicated I was at 32' BTDC. I then screwed my bolt through the sparkplug body until it touched the piston, then unscrewed the sparkplug body, complete with bolt, from the head. Peen or tack the bolt so it can't rotate in the sparkplug body, and I have a piston stop which will ensure I am at 32' BTDC whenever I need to revisit my timing, without going through all the rigmorole of finding TDC each time.

[Raul CB750K1]

For example, my old Norton is timed at 32' BTDC. Having found TDC as described above, I turned the engine until the pointer on the degree wheel indicated I was at 32' BTDC. I then screwed my bolt through the sparkplug body until it touched the piston, then unscrewed the sparkplug body, complete with bolt, from the head. Peen or tack the bolt so it can't rotate in the sparkplug body, and I have a piston stop which will ensure I am at 32' BTDC whenever I need to revisit my timing, without going through all the rigmorole of finding TDC each time.

But in such a case you have to make sure the sparkplug body is always inserted at the very same position, isn't it? That is, not overtightened or undertightened in respect with the time the adjustment was made...

[martin99]

For example, my old Norton is timed at 32' BTDC. Having found TDC as described above, I turned the engine until the pointer on the degree wheel indicated I was at 32' BTDC. I then screwed my bolt through the sparkplug body until it touched the piston, then unscrewed the sparkplug body, complete with bolt, from the head. Peen or tack the bolt so it can't rotate in the sparkplug body, and I have a piston stop which will ensure I am at 32' BTDC whenever I need to revisit my timing, without going through all the rigmorole of finding TDC each time.

But in such a case you have to make sure the sparkplug body is always inserted at the very same position, isn't it? That is, not overtightened or undertightened in respect with the time the adjustment was made...

Absolutely. Just remove the crush washer and bottom it in the head each time.

[Bailgang]

A TDC finder and piston stop are two different things.

A TDC finder is used in conjunction with a degree wheel mounted on the crank. As you turn the engine and the piston rises, you select one of the markers on the TDC finder, before it reaches TDC. Then set your pointer at 'zero' on the degree wheel. Next, revolve the engine in the other direction until the piston position corresponds with the same mark on the finder, and note where the pointer is on the degree wheel. Remove the finder, continue to revolve to exactly half that reading you have just taken from the degree wheel, and you are at TDC.

A piston stop is a tool, often home-made using a bolt and spark plug body, to easily and quickly ascertain when you are at a specific number of degrees before or after TDC. For example, my old Norton is timed at 32' BTDC. Having found TDC as described above, I turned the engine until the pointer on the degree wheel indicated I was at 32' BTDC. I then screwed my bolt through the sparkplug body until it touched the piston, then unscrewed the sparkplug body, complete with bolt, from the head. Peen or tack the bolt so it can't rotate in the sparkplug body, and I have a piston stop which will ensure I am at 32' BTDC whenever I need to revisit my timing, without going through all the rigmorole of finding TDC each time.

Believe it or not in all my years playing with cars and all things mechanical that's the first time I've ever heard of a TDC finder so I had to look it up. I understand though how using a piston stop in conjunction with a degree wheel will determine TDC as well.

[Don R]

The piston will remain at TDC for a few degrees also depending on rod length, pin height, clearances etc. I think that's why the piston stop/ degree wheel method is better.

[madmtnmotors]

The piston will remain at TDC for a few degrees also depending on rod length, pin height, clearances etc. I think that's why the piston stop/ degree wheel method is better.

+1

[martin99]

The piston will remain at TDC for a few degrees also depending on rod length, pin height, clearances etc. I think that's why the piston stop/ degree wheel method is better.

+1

Yep, I agree entirely. I was just making the differentiation between the two, and how I apply it to my old bike, where a degree or two either way is not too critical. Actually I think the most accurate method is with a dial gauge.

[bwaller]

Sorry, but if a degree or two isn't critical, then just go by the mark on the advance mechanism. The worst I ever saw was 1.5 degrees off, most much less than that.

[2wheels]

They say you learn something new each day, this is mine for today.
I would think the piston stop method would be more accurate for finding TDC.  I bet someone has experiments that would prove which way is the best.
It might depend on your tool budget though.

[martin99]

Sorry, but if a degree or two isn't critical, then just go by the mark on the advance mechanism. The worst I ever saw was 1.5 degrees off, most much less than that.

Again, I stress that I'm talking about a 56 year old motorcycle where the parts are unobtainium or out of financial reach. Twin carbs so worn they think they are on permanent choke, compensated for by using plugs two heat ranges up from normal; dizzy shaft so slack that if you set the points gap on opposite cams you get a .0010" variance...

Just to get it to run reasonably well is good. To actually manage to arrive anywhere is a bloody achievement

[Bailgang]

Sorry, but if a degree or two isn't critical, then just go by the mark on the advance mechanism. The worst I ever saw was 1.5 degrees off, most much less than that.

That's how I degreed the 650 cam in my 550, I was using a slotted cam gear/sprocket and wanted the valve timing 6 deg advanced but only had 1 dial indicator which was on the cam and even then it wouldn't have been able to reach all the way to the top of the piston so I used the TDC mark on the advance. Not as accurate as it could've been but the engine ran great none the less. I guess it would be personal preference on using an indicator or piston stop but I now understand the logic behind using a piston stop and it would seem to be a pretty accurate method.

[bwaller]

oldskoolnuts, I understand what you're saying, but we should never settle! 

[dave500]

ktm use a built in positive stop bolt method,it locks into the crank though,no marks on the crank.