Piston stopper for TDC on an A series head

[rolesyboy]

When using a DTI to measure TDC with the cylinder head in place its always difficult to get an accurate measurement, as the spark plug hole is at an angle to the piston crown.
Is there a 'piston stopper' type tool or device that will allow me to use my DTI to get an accurate TDC. I've managed ok in the past but there has to be a better way
Cheer. Mark

[Polarsilver]

back in the day i would use a Moor & Wright Dial Indicator that came with extension rods of different lengths that screw into the dial plunger ..many years later i home made an extension rod for my DTI to find TDC .

[timmy201]

If you smash the porcelain out of a spark plug, you can tap it and then use a bolt as the piston stop. If you round the edge over and go slow it won’t damage the piston

There’s also plenty for sale online if you want to save the effort

https://lugnutz65chevystepside.weebly.c ... -tool.html

[imack]

I made and use a piston dead stop to find true tdc to set valve timing if the cylinder head is installed.

[Exminiman]

I must be missing something here, If you dont use a dial gauge or a telescopic measuring tool how do you know you are not stopping the piston before TDC ?

[MiNiKiN]

I must be missing something here, If you dont use a dial gauge or a telescopic measuring tool how do you know you are not stopping the piston before TDC ?

You actually stop the piston before TDC --> mark the crank angle. Then turn the other way round until you "hit" the piston-stop --> mark the crank angle.
Then half the distance between the two markers and you have TDC. Bob's your auntie.

[Exminiman]

I must be missing something here, If you dont use a dial gauge or a telescopic measuring tool how do you know you are not stopping the piston before TDC ?

You actually stop the piston before TDC --> mark the crank angle. Then turn the other way round until you "hit" the piston-stop --> mark the crank angle.
Then half the distance between the two markers and you have TDC. Bob's your auntie.

This is what I am getting at, not sure how that works with a fixed stop

[Peter Laidler]

PolarSilver Norman has hit the nail on the head. Same here. I just use a DTIndicator into the plug hole (or firmly mounted above it) with a probe with a rounded end and the other end screwed into the DTI plunger (Moore and Wright have this facility). It really doesn't matter what angle the probe is at, TDC will always be TDC

If you were really clever you could mount the DTI in an old broken off spark plug. BUT WHATEVER YOU DO, DON'T USE A LOOSE PROBE THAT COULD DROP INTO THE CYLINDER. Been there and done that. Removed with the aid of a magnetic probe!

[MiNiKiN]

I must be missing something here, If you dont use a dial gauge or a telescopic measuring tool how do you know you are not stopping the piston before TDC ?

You actually stop the piston before TDC --> mark the crank angle. Then turn the other way round until you "hit" the piston-stop --> mark the crank angle.
Then half the distance between the two markers and you have TDC. Bob's your auntie.

This is what I am getting at, not sure how that works with a fixed stop

Following sketch should help to explain. Basically this is the more precise method to find the TDC, compared to a DTI.
Because the closer you get to the TDC, the smaller the stroke per degree of crank rotation. This means the last +-5 deg. crank rotation you have hardly any to no measurable stroke at all.
I.e. ideally you set your piston stop so it it stops the piston, say 20 deg short of the TDC. Then you rotate the other way until stopped by the stop.
When you now measure distance between the two marks and half it, you get the exact position of the TDC.

Watch the video demo, shown on a Vespa flywheel and check out my illustration and you'll grasp it.

Finding_TDC.jpg

https://www.youtube.com/watch?v=WMn5GyP5Sgk

[rolesyboy]

Great info gents. I didn't know you could buy such a thing ready made. Loads of Scooter boys use them. I bought one off Ebay for £6.
Not even worth the bother of making one from a Spark plug
Cheers. Mark

[kit of bits]

Great info gents. I didn't know you could buy such a thing ready made. Loads of Scooter boys use them. I bought one off Ebay for £6.
Not even worth the bother of making one from a Spark plug
Cheers. Mark

Have you got a link Marc?

[rolesyboy]

There's loads on there now I know

https://www.ebay.co.uk/itm/184394323735 ... R_D3qt6IYw

This looks even better . Its got a PTFE end on it
https://www.ebay.co.uk/itm/123470469281

[Oneball]

We were talking about this and I said that a dial indicator in the centre of the piston was more accurate than an offset stop.

I didn’t argue the point as I’m not sure I’m right. Any thoughts?

[rolesyboy]

I think you're right and there's no question that if the heads off the car that it's so much more straightforward. My enquiry was to find the best available solution when the head is on.
Interested to hear other people's thoughts too

[Peter Laidler]

Yep, Oneball is dead right. You can see Top Dead Centre (TDC) on the dial EXACTLY when the piston is at the top of its stroke. Some are fooled by a bit of fuzzy logic, that the DTI pointed might not be in the centre of the piston. It doesn't need to be. Regardless at which point the probe is sat on the piston, TDC as shown on the dial will always be TDC.

As for the piston stop idea.......... Not so easy when the engine is in place

And using a bit of school boy trigonometry and a couple of known datums/angles you can then work out EXACTLY when the piston is at and angle BEFORE TDC

[timmy201]

The problem with using a dial indicator to check the TDC point is that the piston will dwell at TDC for a few crankshaft degrees and you may not get a true reading. The standard method is to check it on the way up (say 1/16” before), note the mark on the crank marker, then check it on the way down at the same point (1/16”). Average the two readings to get true TDC

The same process is used for valve timing as it’s difficult to get a decent reading at full lift

When you are trying to measure TDC when the head is on, the piston stop is one of the best ways when you can’t get an accurate dial indicator reading.

[BAD942B]

The problem with using a dial indicator to check the TDC point is that the piston will dwell at TDC for a few crankshaft degrees and you may not get a true reading. The standard method is to check it on the way up (say 1/16” before), note the mark on the crank marker, then check it on the way down at the same point (1/16”). Average the two readings to get true TDC

The same process is used for valve timing as it’s difficult to get a decent reading at full lift

When you are trying to measure TDC when the head is on, the piston stop is one of the best ways when you can’t get an accurate dial indicator reading.

You don't use a DTI like that, fit 360 protractor to crank with pointer bolted to block, turn engine over past TDC to get an idea of where the dial tops out, turn back & bring slowly up to TDC, make a note of point reading on the protractor at say 0.025" BTDC then slowly turn on to 0.025" ATDC, & note the point reading, divide by 2 & you have TDC

[Peter Laidler]

I can see this 'how do I find TDC thread being another of those subjects that gentlemen - and forumers - never discuss in the Common Room, their Lodges, the mess or their clubs. Alongside religion, politics, silicon brake fluid and waterless coolant! Add to than TDC..........

[Exminiman]

…..did somone say silicone brake fluid ?

[Oneball]

You don't use a DTI like that, fit 360 protractor to crank with pointer bolted to block, turn engine over past TDC to get an idea of where the dial tops out, turn back & bring slowly up to TDC, make a note of point reading on the protractor at say 0.025" BTDC then slowly turn on to 0.025" ATDC, & note the point reading, divide by 2 & you have TDC

And seeing as how you use a stop the same way is it less accurate?

My initial thinking was because it’s at an angle, (on an a series) it will be. But does the before and after bit remove that inaccuracy?