TECH: Piston Stop for finding TDC.

[DrJ]

I wanted to verify my new dampener when I was assembling a SBC and needed a piston stop that wiuld be accurate, repeatedly.
So I made one.
I took a used-up sparkplug and brutalized it till the ceramic part gave up in a fragmented way and then tapped the metal part of the plug to recieve a 3/8" bolt. I screwed the bolt in tight and added some Loc-tite for good measure and then cut the hex head off the bolt and ground the end of the bolt into a half sphere.
It works like a charm, stopping the piston about 10º Before and after Top Dead Center.
Remember if you use a tool like this that it is stronger than the top of an aluminum piston so only turn the engine over by hand, not with the starter motor...

[36-3window]

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I wanted to verify my new dampener when I was assembling a SBC .

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that is a very good idea! i always check that while assembling the motor. chevy used several locations of the TDC mark on the dampener and with people mixing them with different timing chain covers/pointers,etc it is possible to get the timing marks off by quite a bit...i've seen it more than once

[studeboy]

DrJ that rocks!!! http://www.jalopyjournal.com/ubbthreads/images/graemlins/cool.gif And its cheap and I think thats what I like the best. Wanna make sure I have this right, the head of the bolt was the rounded correct? How long was the bolt you used? Thanks Eric

[Hot Rod To Hell]

just a tip, thread the bolt all the way through, and cut a screwdriver slot in the outside end. Then you have an "adjustable" piston stop. You will be able to check much more accurately if you can get it to where the piston is stopping within say 5 deg of TDC (or even less is possible with the adj. trick)

[C9]

Here's a slightly different version of the one Doc made.
This one from a cut down spark plug extender.
Coulda been made from an adapter for an air chuck to spark plug hole, but I'm fairly sure it was from an extender.
From the KD tool rack if memory serves.

The upper part of the adapter body - with it's thin hex was cut off and the inside drilled and tapped for 3/8-NF.
The bolt rounded as well.

I thought for a while a jam nut would be a good idea, but it doesn't seem to be needed.

You can find true TDC with this tool - or Docs - and a tape measure.
Rotate the engine by hand until it hits the stop.
Measure from pointer to the zero mark on the dampener.
Rotate the engine - again by hand - in the opposite direction and measure again.
If your zero mark is at true TDC you should come up with the same measurement.
If not, it won't take too much figuring to locate true TDC.

Doing it this way works well on an assembled and installed engine.
Most times, engines in the car make it a little tough to bolt on a degree wheel.

[fordiac]

in the HELP! section of advance auto parts and others, they sell a "spark plug anti-fouler" that would work for this. cheaper though, is do what was mentioned and smash the ceramic(the white part) out.

[TagMan]

Slightly different one I made back in the early 60's to find TDC on an old Indian M/C I had. After knocking out the porcelain, I inserted a nylon plug in the hole, then drilled a 1/4" hole through it and reamed it until a 1/4" rod was a slide fit. I set up a .001" long-travel dial indicator with the probe on the end of the 1/4" rod and turned the crank over slowly by hand, watching the indicator. I could get TDC exactly this way. Here's the set up I used:

http://home.rochester.rr.com/mrplates/DTCfndr.jpg

[C9]

From the simple to the sublime....
A very nice piece Tagman.

Kind of a mystery too.
That's one of those pieces someone might spot in your toolbox and wonder what it's for.

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cheaper though, is do what was mentioned and smash the ceramic(the white part) out

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Very true and I did start out to do that.
Maybe it was the brand of spark plug I was dealing with, but trying to knock the porcelain out ended up damaging the steel part.
I was patient and the hammer wasn't too big, but I gave it up and spent a couple bucks on the commercial piece.

[DrJ]

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...

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cheaper though, is do what was mentioned and smash the ceramic(the white part) out

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Very true and I did start out to do that.
Maybe it was the brand of spark plug I was dealing with, but trying to knock the porcelain out ended up damaging the steel part.

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Maybe my being a stone carver made it easy!
(Plus, I photoshopped the vice jaw scars off the metal in the pic...) http://www.jalopyjournal.com/ubbthreads/images/graemlins/cool.gif

On the length, it was a 4" bolt and the threaded part went flush to the outside end of the plug casing. There's now about 2-11/32" sticking out of the case, but that particular length isn't that important. I wanted it to work with flat top and dished pistons on a SBC but it would probably work better on a domed piston if it was shorter, but the parts are cheap so making another length one isn't that difficult, and in my "which safe place did I hide it this time?" mental situation, it may be faster to make one than to find the old one.
(I do this for a hobby. I put together several engines a decade, not several engines a week.) http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

[Bruce Lancaster]

When I made one of these years ago, the particular dead plug I picked up was made of something harder than the cheap 3/8 tap I had, so I just went inside and had a cup of coffee while Mr. Sparkplugshell spent an enjoyable 20 minutes in the gas flame atop the stove.

By the way, WHY you want to find TDC by the two-direction interference method, even if your heads are off: On engines with fairly long rods and offsets in piston or crank location (SBC or flathead Ford, for instance), TDC actually CANNOT be found perfectly when piston is actually at the top--there is a sort of dwell effect mebbe 3 degrees wide in which there is virtually no vertical movement! If you think you have located TDC visually, put on a degree wheel and checkitout.
I discovered this myself when I did my first complete build on a flathead. I had my first precision instrument --a Wehrmacht surplus dial indicator from a German antique shop--and figgered I was going to find my TDC within two tenths of a gnat's eyelash. I was flabbergasted to discover this dwell period in which only tiny amounts of wiggling (oil bubbles on the crank pin?)were visible on the indicator. I flatly didn't believe my own observation and had no knowledge of other techniques of finding TDC, so I just marked the middle of this range and slunk away, baffled.
I didn't begin to understand until many years later when I read a Smokey Yunick article on the blessings of long rods and dwell at TDC, and then learned about interference checking.

[Digger_Dave]

Another piston stop tool to check for TDC with the heads off.
A piece of flat bar stock (mines 3/8" x 2") that "bridges" the cylinder. Drilled and tapped for a 3/8" bolt in the center.Then on each end to match with cylinder head bolts. So far it has three "sets" of head bolt holes for different engines

I place a couple of nuts (larger than the bolts) on the bolts that hold the bar to the block. (to space the bar off the deck)

Then the bolt in the center is set to "stop depth." It has a lock nut to prevent any movement.