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TDC Tool

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er Jerry, at the risk of sounding S.A.'d, why not just take the V/C off + pull plug #1 + rotate motor until you see #6 valves rocking + stick a bar straw in #1 and watch it as you SLOWLY finish rotating to point of both #1 valves closed??? /ubbthreads/images/graemlins/angel.gif
/ubbthreads/images/graemlins/cheers.gif
Ed
 
1/8 graduation marks should really get you there. Thumb actually sounds about as accurate. Alsways used a dial indicator here, so I am a bit subjective. hmmmmm
 
When you need something accurate, I always us a dial indicator too. I have one that screws into the plug hole and has a rounded end that touches the piston for accuracy.
 
Heck, a small screwdriver does the same thing - & you've probably already got one in your box!
 
I was thinking a screwdriver too... but only if your careful - don't let it get lodged in there.
 
Hello all,

as Tony says,

"Heck, a small screwdriver does the same thing - & you've probably already got one in your box!"

Which will be as accurate as the tool Jerry is asking about, which is not very. For most things that is near enough but if you want true TDC then a dial indicator and a degree wheel is necessary to do a really accurate job, and if the plug port is not vertical to the piston top, also remove the head.

Alec
 
I not familar with this tool but I've seen tools like it, one I like is sold by CV Products, it has the same threads as the spark plug then a bolt that is threaded thru the center of it so you can adjust it up or down as piston stop. A piston stop is a very acurate way of determining TDC maybe the only fool proof way including using a dial indicator when the head is off. Here's the deal when you reach TDC there will be a few degrees of dwell, to truely to find TDC you must divide the dwell in half, that will be truely TDC. So with a tool that screws into the spark plug to act as a piston stop you would rotate the engine by hand in one direction until it hit the piston stop take a reading form the degree wheel, then rotate in the other direction until you hit piston stop, take a reading, split the diference and you have found TDC, you can achieve the same thing by marking the bottom pulley in each direction and then center of the two marks would be TDC rather than using a degree wheel, but the degree wheel is the most acurate. Missing TDC by just a degree or two when degreeing a cam can put you off several degrees on the cam as it is mutiplied by two against the crankshaft.
 
Hello Hap,

I would be very wary of the accuracy with any engine that has an inclined spark plug port. I don't believe that you will get a consistent 'stop point' with such a device. As you say a couple of crank degrees is a few degrees of cam inaccuracy.

Alec
 
The angle doesent matter providing you dont change the setting of the post in the spark plug hole.

Follow Haps methodology and you can get accurate to seconds when you mark the dampener.

If your really cheap and industrious you can make a tool with a spark plug body and some all-thread. Ready made pieces see Mr. Gasket, Moroso, local performance shop etc.
 
[ QUOTE ]
Hello Hap,
I would be very wary of the accuracy with any engine that has an inclined spark plug port. I don't believe that you will get a consistent 'stop point' with such a device. As you say a couple of crank degrees is a few degrees of cam inaccuracy.
Alec

[/ QUOTE ]
Hi Alec,
Hap is saying that using a positive stop in a plug hole, while bringing the piston up to the stop from both directions of rotation, will give two degree wheel readings that are split equally on each side of TDC. Splitting the two readings equally will give a very accurate indication of actual TDC.

For this purpose, it makes no difference whether the plug hole/stop is angled. The stop is still the same distance from the cylinder head. The whole purpose is to eliminate the rather indefinite indications of actual TDC caused by the several degrees of piston dwell at TDC. To my knowledge, all precision engine tuners use the positive stop method to determine TDC in order to check cam timing. Most times the cylinder head would not be installed & a bridged stop on top of the cylinder would be used. If necessary, for some reason, a positive stop in a plug hole will accomplish the same thing.

For the purpose of precisely measuring ACTUAL piston distance from TDC, the inclined plug measurment would indeed introduce an error. For example, an inclination of 30 degrees would result in the "measured" distance that the piston moves, being only 87% of the actual distance.
D
 
Hello Dave,

my reservations are that the tool has a sliding centre and that the perceived stop is unlikely to be the same every time.
A solid stop would be better (i.e. an extension welded to an old spark plug body), but it is still down to how much effort is applied at the stop position. This is unlikely to be the same to within a few thou which can be quite a few degrees.

If true accuracy is required I would remove the head and use a clock gauge.

Alec
 
I tried to use a screwdriver and frightened myself by getting it jammed. The answer was/is a length of wooden dowel of a diameter that only just fits down the spark plug holes, so it cannot fall sideways or get crooked.

For some cars, including my Rover, beware. The fan-belt pulley is not rigidly attached to the end of the crank-shaft because there is a damper (I forget the name, ah, Lanchester) which operates by friction. I carefully marked TDC on the pulley, and next time I needed it, it was way off!

Ken G, 1925 Rover 16/50 (San Francisco)
 
Most times a dial indicator here as well, and the dwell can be seen/measured with degree wheel attached. If it's a race engine or an exotic I'll use a stop block and index TDC well before the head(s) are on. Dial indicator works well enough in most applications tho, IMO.

...though it's difficult to get the magnetic base to work on those Alfa blocks! /ubbthreads/images/graemlins/devilgrin.gif
 
hehehehehe
 
Alec, just go to the CV products website, it will make sense to you then. As long as the reference point stays excatly the same in both directions of rotation it can not be be flawed. I made my own tool with a old spark plug. I'm not sure how just sticking a screwdriver in the hole would tell you anything, you must have the the same refererence point.
A positve piston stop in my opinion is the most acurate way to determine TDC, even more so than a dial indicator, which in a assemble engine is not a choice anyway. Feel free to call me at the shop on weekdays and I'll be glad to explain it you in more detail. (864) 370-3000
 
Hello Hap,

I fully understand what it is intended to do but accurate, I'm afraid not. I am refering to the original tool as per the link. As you know, to get accuracy of measurement rigidity is essential; that tool is flimsy, hence my reference to angled plug ports which will exacerbate the problem. The blurb mentions calibration of 1\8", a whole 0.125", hardly a precision tool. Add to that possible carbon deposits on the piston crown I do not believe that it will be any more accurate than feeling with a screwdriver.
When building an engine it is usual practice to set TDC anyway, so I don't think this tool will be an asset to any serious mechanic.
I still incline to the use of a test dial indicator as the reference points can be reached without any variation due to force of rotating the engine, unlike when hitting a stop. Even with a cheap instrument an accuracy of 0.0005" can be achieved and repeated consistently.

Alec
 
Not to add further confusion to this but....

The only way a dial indicator(or anything else that uses the top of the piston to find TDC)can be correct is to have a sharp point on it AND have symmetrical pistons. If the piston is unsymmetrical, fly cut for valve clearance or if the indicator point is too large, you will not find the exact TDC because approaching it from two different directions will not land you on the same(opposite) spot.
 
Hello Craig,

I'm afraid that you have managed to confuse me.

The piston does not move laterally to the block so as it rises and falls it will come back to the same spot. As we generally are swinging an arc about TDC the pointer does not leave the piston anyway.

Alec
 
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