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MG MGA - Finding TDC
I have looked at the archive for doing this and read about various methods. Have also looked at the John Twist video where they use a tube and oil down the plug hole (looks a bit messy). Nobody has mentioned a whistle. Has anyone any experience of using this method? If they are good where can I get one in the UK? On Ebay the only one for sale is in the US.
|I have a whistle but it is not very easy to use accurately because of the resistance when using the hand crank against all the gearbox input gearing which makes a smooth action and precise interpreting of the whistle not a particularly accurate method|
|Just found the following on the internet. Sounds like a lot better than pouring oil down the plug hole.|
1.Make an indicator from some clear plastic tubing, a jar of light oil, and an old sparkplug.
2.Break up an old sparkplug and attach a length of clear plastic tubing to it (make it airtight).
3.Remove all the spark plugs.
4.Stick your thumb OVER the #1 cylinder spark plug hole. Rotate the engine (see note below on tricks for this) until you feel pressure on your thumb. That's the compression stroke. TDC is at the top of this stroke.
5.Screw in the sparkplug with plastic tubing attached and insert the other end of the tube into a jar of light oil. Continue rotating the engine. Bubbles will appear until the piston reaches the top of its travel. When it starts down on the next stroke, the bubbles will stop and oil will begin traveling up the tube. Stop at a convenient point and mark the tube. Then mark the crank pulley and the engine body at a convenient spot.
6.Rotate the engine backwards and watch the oil recede into the jar. Continue rotating. As the piston continues past tdc and downward it will again suck oil into the tube. Rotate the engine till the oil again reaches the mark. STOP! Mark the crankshaft pulley where it lines up with the mark you made previously on the engine. You should now have two marks on the crankshaft pulley. The midpoint of these two marks lined up with the mark on the engine is tdc. Whoa! Almost like finding South with a wristwatch. Boy and girl scouts listen up.
1) Take out plug #1.
2) Use the crank to get it just before TDC.
3) Put the car in 4th gear
4) Look with a flashlight while slightly rocking the car.
There is another good step to use if you're going to be starting the car soon afterward, especially if you are starting it from under the hood; i.e.,
5) Take to car out of gear......(at least that's what I've heard)
|It is no use using any method that depends on actually BEING at TDC. This is because, when actually at TDC, the vertical piston movement variation with rotation is ZERO! So trying to dial in vertical motion as an indicator is no good. Any method which involves actual motion such as finger over the plughole or generating whistles is bound to be even worse.|
The best way is to make a piston stop from an old spark plug. This will stop the piston some distance, say 15 deg before TDC. Mark the pulley. Then rotate backwards and get it to stop the same amount after TDC. Mark pulley again. Then bisect the distance between the two marks and you have exact TDC.
As far as I can see this is the only method of determining TDC with accuracy, which is what is needed if you are timing valves or ignition.
|I just read your oil method properly! It should be as good as the piston stop as long as all the air is out.|
|Yes, I thought it was good - not my idea just found it while searching. It has the advantage of not having to insert anything down the plug hole potentially causing damage. I have read elsewhere that the only method for finding true TDC is to take the head off and use a dial gauge etc. I thought that this was maybe going a bitfar (unless of course the head is off already).|
Barney, if you read this then maybe this procedure is a candidate for your esteemed site.
|AP's first point seems good to me in that at TDC there will be a small rotation of the crank say 1/2 to 1 degree (a guess) for no discernible movement of the piston so if 'absolute' TDC is required piston movement will most of the time only give an approximate result.|
Its the rotation of the crank that important in timing and its fortunate that our engines are fairly forgiving for v.small differences in timing.
|J H Cole|
|Art gave the only correct accurate method above. The oil trick is clever but again not accurate, even if oil doesn't start running down past the rings. Doing it with the head off and an indicator, you are doing exactly what the solid stop does, measuring a fixed distance down in two directions and splitting the difference - you do not measure AT TDC. It is quite easy to lose ten or more degrees in trying to read anything at TDC.|
|With respect - you are not reading the procedure. There is no oil to run down past the rings. The oil is in a container. It also takes account of the rotation of the crank when the piston is not moving at the top of its travel.|
|Okay, I'll bite on the hook. This took a few hours to respond, as I was writing and posting a few new web pages while analyzing the accuracy of various methods.|
You are all correct in various ways to find TDC. It is only a matter of how accurate you need to be for various uses. The thumb over the spark plug port will certainly get you close enough to install a distributor and put spark plug wires in correct sequence. The oil tube trick, with or without oil in the cylinder, is accurate enough for most regular maintenance applications. If you are going to be setting up special cam timing to half a degree you will certainly need a degree wheel and piston stop (or dial indicator).
So if you have enough interest to read half a dozen pages, for the bottom line on accuracy see here:
Cheers, -- Barney
Certain amount of truth in what you say; I was mixing up your way with the other, having read them both earlier. Nevertheless, your way is still dependent on perfect sealing of the combustion chamber. If you turn the engine4 slowly enough, no oil will ever move in the tube, since air will come in via the rings gaps, or leaky valve seats etc.
Further, step 5 is somewhat confusing, though basically correct.
"Continue rotating the engine. Bubbles will appear <<<in the jar>>> until the piston reaches the top of its travel."
I will concede Barney's nice calculations on this (without redoing them). However, since one reason for checking TDC is that you are trying to correct an engine that is not running right, valve or ring leakage is a somewhat probable situation.
And, no procedure that relies on valve positions is any better than crude approximation for determining TDC. But, if you know TDC, valve positions can be very useful for basic checking of cam, chain, valve problems.
Much better to just weld a stop in your stripped spark plug and use it as a stop. Mine is now about 50 years old.
|Can someone provide a drawing or a picture of the "stop welded in a spark plug". How long is the stop? How far do you screw the plug into the head? Maybe that doesn't matter if I understand the method described by Art's original post, you just split the difference between the two piston locations determined wherever the piston stops....?|
|I was going to ask a the same question about the length of the piston stop. FRM / Art / Barney.|
Would the following work and is this what you mean? - want to get it 100% clear. I know it's difficult to explain something to an amatuer when you've been doing it for years.
1. Get number 1 piston near TDC on compression stroke.
2. Insert piston stop
3. Turn crank pulley clockwise until top of piston touches stop. Mark pulley.
4. Remove stop.
5. Turn pulley clockwise so it continues past TDC and back down again below stop.
6. Reinsert stop.
7. Turn pulley anti-clockwise until top of piston touches stop. Mark pulley
8. The point midway between marks is TDC
|Phil, almost right. Don't remove the stop, just back the engine all the way round. I suppose you could remove it, but it might not seat quite the same position, and if the end is not quite hemispherical it would be different.|
I will try and find my stop and send a picture.
|Just to add to the confusion...Using the piston stop method, I don't think it should matter if #1 piston is on the compression or exhaust stroke in terms of marking the pulley as you're just determining when that piston is at the top of it's upward motion. It's later, when you go to use that mark on the crank pulley that you need to identifiy the top of the compression stroke rather than the top of the exhaust stroke. If I've got that wrong, somebody please correct me.|
|Not critical how long the stop is, about 1/4" or more below TDC is fine, but could be halfway down (so long as it doesn't run into the far side of the cylinder!). Technically, the further down the bore (to +/- 90deg) the piston stops the better for accuracy, but such length can give some flex in the stop, which negates the added accuracy. Also it is easier to split the two marks if they are reasonably close together. Mine sticks out of the plug body about 3/4". Nice rounded end. |
If you make the stop too short it won't work on deep dish pistons and/or low CR engines, then you need to make another one for those.
Phil, as Art says, don't remove the stop, turn the engine backwards for the second mark.
|The piston stop from Crane Cams does not have a shoulder. It is a long threaded rod (spark plug thread) with a blunt nose and small hole down the center. It can be adjusted to any depth, suitable for any engine, head or piston configuration. There is also a thin metal rod to poke through the center so you can "feel" approach of the piston before it hits the stop.|
Turn the engine near TDC, then turn it back about 20 degrees, and then screw the stop down until it touches the piston. This works on any engine regardless of head height or piston dish (or crown), only one tool required. If an engine has the smaller spark plug thread it requires a smaller threaded piston stop.
Since there is no shoulder on the piston stop, once you put it in place you have to leave it alone for the duration of the process. If you unscrewed it to let the piston pass you would never get it back to the same position again.
The 8-page instruction booklet for can timing (good reading) is here:
|Trouble with that is that if the stop is not locked in place with a nut or similar, the slop in the threads lets it flop about in an indeterminate manner, the error increasing as the protrusion does. |
I didn't go through the tedium of finding the price, but I fail to see the advantage over a free homemade tool.
|Dear TDC Gang,|
For us non-racing drivers, am I wrong in thinking that wear in the gears and timing chain would account for at least a degree, if not more?
|You can buy the Crane Cams piston stop for about $13. A usual with a lot of special tools, you either don't need it or you can make one yourself if you want to take the time to do it.|
It does not flop about in use. It threads nicely into the spark plug port. Whatever small clearance there is in the threads is taken up when the piston touches the stop. No matter how many times you bump and move the piston, the piston stop does not move unless you intentionally turn it in the threads.
I have one that I have been using for more than 10 years, and it works well. It is part of the Crane Cams Tune-A-Cam tool kit which includes lots of additional "toys", like the degree wheel, long travel dial indicator with mounting, and a whole set of super-soft valve springs that you can depress with one finger to measure amount of (required) over-travel before the valve head might hit something. An easy way to check internal clearances.
I'm not saying everyone should run our and buy one. The tool kit is the sort of thing that belongs in the local car club tool lending library for once in a life tine use by club members. I just happen to own one, and I often lend it to friends (usually with my right arm attached and my eyes to go with it).
|Here is my stop. the protrusion is 7/8" from a long reach plug. I think i used a 5/16 stud. Make sure to grind the tip round and smooth.
This thread was discussed between 03/03/2012 and 07/03/2012
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