Technical Top Dead Center According To Bruce Lancaster

[Bruce Lancaster]

Theoretical TDC issues that hurt my head have been pointed out to me by mathematically sophisticated HAMBers and by my little 1935 Ford engineering text. I only dimly understand this stuff, BUT...
Engines have offsets to control piston side thrust issues, noise, and things beyond mortal comprehension. Flatheads achieve the offset by having crank offset from cylinder centers slightly...some others do this at the piston pin.
The simple story is that piston goes UP differently than it goes DOWN!
An effect that I only dimly grasp is that the measurements detailed in my post are close but not entirely accurate.
Measuring the distance a ways down the hole will give UP and DOWN distances closer to equal and so more accurate BTC and ATC correlation at same distance!
I have charts of the movement curves, but they severely hurt my head.
The differences ar minor...the methods given ar more than sufficiently accurate and almost certainly closer than original punch marks on pulley.
I do not know the perfect distance down the hole...and I believe it will vary by stroke. Study of the charts continues.
The distance issue referred to means that using a bolt an inch or two down the hole (head OFF method) is more accurate than the head ON method, but the difference should be slight and not critical. Blown 49 has posted on the offset issues, and can go deeper than I can.
Both methods are entirely satisfactory, but be aware that they are not perfect.
You cannot gauge TDC accurately by purely visual methods, INCLUDING use of a dial indicator!
Long rod engines with offsets have 2-3 degrees of DWELL at TDC with no motion that can be consistently shown even with a high zoot dial indicator. TDC has to be calculated, though finding it with an indicator might be as accurate as factory pulley marks.
The interference method is bot better ans easier.
The limit on the head ON method is of course what you can fit through the plug hole and maneuver...which is to say you cannot get piston down much, maybe 1/4".

[choprodinc]

Screw Bo! Bruce KNOWS!

[Mr 42]

This is how i did it on my 59AB
http://www.jalopyjournal.com/forum/s...&highlight=tdc

Bruce helped me to :-)

[firingorder1]

Somewhere in the back of my mind is a 1964 HRM article where Garlits "revealed" his speed secrets by stripping his fueler. In the article he said he never measured TDC but did it by feel. I started trying it on my motorcycles and was surprised as to how accurate you could be.

[Bruce Lancaster]

In the fifties...the way TDC was measured in HRM tech articles and the Iskenderian book on valve timing was similar to the interference method...but harder!
A dial indicator was used, but not at TDC because that was recognized as not precise.
Piston position was measured part way down, towards the limit of the indicator travel. Then engine was turned over so piston was coming up the other side of TDC until it reached the same reading on the indicator. Using a fixed object (bolt through heavy strap) of unchanging length produces the same result without need to read the indicator.
Thump, thump, split the dif.

[Jimmy2car]

On my engine I took the pulley off, found a peice of 5" tubing, had it cut to 1/2" length, had it welded to the back side of the pulley. Thru Jeg's or Speedway, I found timing tape for a Pontiac that used a 5 1/4" diameter pulley. I installed the tape knowing that it would not be perfectly accurate, but is VERY close. Now I can use a light and see exactly what's going on with the distrbutor advance. I can't get a picture of it cuz I can't get the camera in there. This is on a 59 type engine.
Jim

[vilanar]

Bruce knows his stuff !!!
Once I read somewhere one neat trick for heads on timing as an alternative
for that zip tie method.
It went something like this.
Take an old spark plug and break the porcelain and weld a piece of small
tubing on it. Put piece of clear plastic tubing on it (10 inch or so)
Set engine to near TDC and screw your new apparatus in #1 cylinder
and make a bend on that tube and pour little amount of ATF in it.
Now when you turn engine that ATF moves up and down and when it moves
direction from upward to downward there is your TDC.
I believe you gearheads get the idea altough my explanation was what it was.

Ari

[Petejoe]

Thanks Bruce and Ryan for sharing. If it wasn't for Bruce and another good Friend here, there wouldnt be any running flatheads in Zoar Ohio and it would probably go for alot on the Hamb too. "Discarded 8th grader" cracks me up. Bruce you have single handedly resurrected alot of flatties not just because of your knowledge but because you are so willing to help us backyard mechanics here and on the Fordbarn. Hope to meet you someday. PJ

[firingorder1]

In an effort to be more accurate I have been using a degree wheel and dial indicator. I set the piston at approx. TDC. Then turn it back 30 deg. or so. Move it back to TDC watching the dial indicator. When the dial indicator stops moving, note the point on the degree wheel. Keep moving until the dial indicator starts moving again and note to point on the degree wheel. Divide the degrees by two and that gives me TDC. So far thats as accurate as I can get. The cam on my engine has no marks and the valve timing is set off of the inlet valve fully open. I guess it works. With two records at Bonneville and two at El Mirage I must be doing something right.

[Red Ryder]

You can't beat accurate information, and that's just one of the things I like about the H.A.M.B.

[J&JHotrods]

[QUOTE
Thump, thump, split the dif.[/QUOTE]

Nuff said. Good read too. Thanks Mr. Lancaster and Ryan and merry xmas to all.

[Adam F]

mmm, very interesting.
My understanding was that the brake was "a drag" and no longer required because of better fuels etc. I have discarded mine long ago- perhaps I should re-think that?

Revisting my inital advance is something that I want to do because I constantly have hard starting issues which I am sure are related to timing.

Thanks Bruce!

[Kevin Lee]

You just can't beat a description written by Bruce. You might occasionally be able to obtain the same information someplace else, but you'll find yourself searching for his posts for the great delivery.

[jambottle]

Bruce;It always seemed to me that the piston stop method was better than a dial guage because it removed all slack from any slack in the system(timing gears or chain,piston pins,bearings etc.)The dial guage not having any resistance?Is there a flaw in my thinking?

[telecustom]

Bruce; One of the best, I learned that early on..

[Gambino_Kustoms]

thanxs ryan & bruce!

[bloodyjack]

I used the ti-rap in the spark plug hole to mark TDC on my flattie and then just used a modern timing light that allows you do dial in the advance needed (it electronically delays the flash for the required advance) no need to break out the protractor!

[Bruce Lancaster]

Solid stop probably is better...another issue with dial indicator, at least very sensitive ones, is you get funny, tiny blips visible around TDC. Very minor up-down at random places that I suspect may just be oil film supporting things a tad high then leaking down!

My first try at Real Science involved a really nice WWII German surplus indicator and a degree wheel...I thought I had located TDC to the last micron, and happily marked it. Then I started playing with the thing, watching the wheel and found that top dead center was almost 3 degrees wide, and contained really minute minor blips as noted above...WTH??!? This was downright disturbing, and with absolutely no idea what I had seen I just moved on, almost not believing what I had measured with my own eyes.

Many years later, along came Smokey Yunicks ABSOLUTELY MANDATORY READING book of Power Secrets; this discussed the effects of long rods and offsets in creating dwell at TDC. Now I have the little 1935 Ford of England engineering text discussing the offsets, rod lengths, and effects on piston movement/displacement at given moments during the cycle...
Short summary...comparison between offset and "normal" engine: Offset one is running slightly behind the other on DOWN piston movement, slightly AHEAD on upstroke!! Many charts and formulae included. This stuff actually affects effective stroke, and of course rod length has its own role in creating the incomprehensible.
Makes you appreciate the amount of thought and subtlety in what looks to be a fairly simple lump of iron...
For real world thought and finding TDC, all effects are tiny and you can go ahead and ignore most of this stuff. Just consider it as taken care of for you by Henry and his merry band of slaves.

[MattStrube]

I tried all that fancy stuff with a new fangled timing light and all, but found that giving her all the advanceshe can handle, then backing off once I heard a pinging in high gear at low speed worked better for me. Good info here for more patient souls.

[Bruce Lancaster]

The early dial-back timing lights were a bit erratic and not to be trusted...I assume but don't know that recent ones are accurate??
Anyway, add timing marks for your engine's total range and just read the numbers off the iron with a normal light...no chance for electronic deviltry.