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Top Dead Center According To Bruce Lancaster

Discussion in 'The Hokey Ass Message Board' started by Ryan, Dec 21, 2010.

  1. J&JHotrods
    Joined: Oct 22, 2008
    Posts: 549

    J&JHotrods
    Member

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

    Nuff said. Good read too. Thanks Mr. Lancaster and Ryan and merry xmas to all.
     
  2. Adam F
    Joined: Jun 19, 2001
    Posts: 323

    Adam F
    Member

    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!
     
  3. Kevin Lee
    Joined: Nov 12, 2001
    Posts: 7,582

    Kevin Lee
    Super Moderator
    Staff Member

    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.
     
  4. jambottle
    Joined: Apr 11, 2003
    Posts: 564

    jambottle
    Member

    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?
     
  5. telecustom
    Joined: Feb 17, 2009
    Posts: 336

    telecustom
    Member
    from Langey, BC

    Bruce; One of the best, I learned that early on..
     
  6. Gambino_Kustoms
    Joined: Oct 14, 2005
    Posts: 6,561

    Gambino_Kustoms
    Alliance Vendor

    thanxs ryan & bruce!
     
  7. bloodyjack
    Joined: Aug 29, 2007
    Posts: 649

    bloodyjack
    Member

    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!
     
  8. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    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.
     
  9. MattStrube
    Joined: Nov 18, 2002
    Posts: 1,073

    MattStrube
    Member

    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.
     
  10. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    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.
     
  11. autobilly
    Joined: May 23, 2007
    Posts: 3,123

    autobilly
    Member

    It's guys like Bruce that inspired me to be formally trained and eventually become a fully qualified mechanic. I still aspire to his level of knowlege and skill.
     
  12. BStoltz
    Joined: Mar 30, 2009
    Posts: 123

    BStoltz
    Member

    Dam you learn something new every-time you get on here......
     
  13. bloodyjack
    Joined: Aug 29, 2007
    Posts: 649

    bloodyjack
    Member

    The new ones are very accurate I checked the one i got from schucks with a different engine with real advance marks and its exact.
    I Also used it to check when my Bubba dizzy centrifical advAnce was all in and it was exactly were Bubba claimed it would be.
     
  14. now I have to FIND AND READ all 15602 posts by Bruce .. see ya next August
     
  15. Oh ya i asked and 8th grader about math equip. Said there was a ap for that now ..
     
  16. slepe67
    Joined: Jan 22, 2008
    Posts: 1,146

    slepe67
    Member

    Bruce typed:

    "Find/verify TDC:
    Heads OFF: Bolt a strap across #1, bolt through center to stop piston a ways down the hole. Rotate one way, mark pulley at stopping point, rotate other way, mark. Very accurate TDC is 1/2 way between – mark it.
    Heads ON: Slide a large tie-wrap or any sort of lump on a wire through plug hole and against farthest point where it hits the wall. Have apprentice holditthere. Rotate as above… Bump, bump, divide. Repeat a couple times to be sure it is repeatable and your stop isn’t moving. Have apprentice flogged if inaccurate."

    Maybe I went to bed or got up too early with my feverish 2 year old...but fellas, I feel like an idiot...I can't get a mental image of the steps I highlighted in red.

    How far down is "a ways"?

    Are measurements (stopping point) pulled are from piston travel at full down and full up???

    Maybe I'm reading too much into this...
     
  17. jambottle
    Joined: Apr 11, 2003
    Posts: 564

    jambottle
    Member

    It is easier to explain in regards to an engine with the spark plug located over the piston.(eg;smallblock chev).I made my piston stop out of an old spark plug;knock the porslen out of it and then you have a hole right thru the plug.I used a tap to put a common thread thru the hole thru the plug.Then i threaded a bolt up into the bottom of the plug .The bolt should be long enough to leave a couple of inches sticking out the bottom of the plug.Now cut the head of the bolt and grind the bolt smooth and rounded so as not to mark the piston when you very carefully turn the No 1 piston up by hand on its FIRING STROKE.You will notice that the timing mark will be heading up to your factory top dead center pointer.The length of your stop bolt will determine how close the mark gets to the pointer before your piston hits the stop.Go very gental until the piston hits the stop.Then you can apply a LITTLE pressure to take up all the slack from the timing chain or gears ,piston pin and bearing clearances.Now you mark your front pulley where your factory pointer is pointing.i just mark it lightly with a file.Now turn the engine the other way and mark the pulley where the factory pointer is pointing on the pulley.Exactly half way between the two file marks is the real top dead center.Mark your pully with a good permanent mark and ignore the factory top dead center mark on your pulley unless it is correct(i did find one that was correct on a big block chev)If the heads are off it is easy to bolt a strap across the #1 cylinder and put a bolt thru a hole in the center of the strap,with a nut on each side of the strap to locate the bolt head sticking down into the cylinder to stop the piston before it reaches top dead center.(make sure the head of the bolt is smooth) And remember to go easy ,so as not to mark the piston.I hope this helps.
     
  18. slepe67
    Joined: Jan 22, 2008
    Posts: 1,146

    slepe67
    Member

    Jambottle, that summed it up for my simpleton brain. Thanks.
     
  19. GMC BUBBA
    Joined: Jun 15, 2006
    Posts: 3,420

    GMC BUBBA
    Member Emeritus

     

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  20. hydroshawn
    Joined: May 27, 2006
    Posts: 334

    hydroshawn
    Member
    from Tx,Ca

    you lost me at the eighth grader........;) thanks for the info. I'm working on mine now.
     
  21. blown49
    Joined: Jul 25, 2004
    Posts: 2,212

    blown49
    Member Emeritus

    Well Ibought the book Bruce referred to and ended up making a pretty accurate CAD drawing of the piston travel with the cylinder centerline (C) offset clockwise 3/16" to the centerline (A) of the crankshaft. The atached drawing shows the left set of cylinders at their 45 degree angle. Crank pulley is divided up into 30 degree segments. Drawing shows travel with a 4" crank. Note degrees of travel of crank is greater on downstroke than on upstroke by about 2 degrees. This leads me to believe the factory bump is 2 degrees before top dead center.

    Jim

    [​IMG]
     
    Last edited: Dec 24, 2010
  22. jambottle
    Joined: Apr 11, 2003
    Posts: 564

    jambottle
    Member

    Now i`m confused.Aren`t we only concerned with the compression stroke and the timing of the spark plug fireing and the degrees of advance of the spark plug fireing before top dead center on the compression stroke?I can not wrap my head around how what happens on the down stroke will affect our timing of the plug fireing.It does not make sence to me that there can be 2 degree difference on one compression stroke.We are turning the crank both ways to find top dead center on this positive stop method and i would think we have therefore averaged both up and down compression strokes?I must be missing something here? HELP BRUCE!
     
  23. With offset pins, rods, cylinder bores, etc; piston TDC may not necessarily be exactly the same as crankshaft TDC. It's really crankshaft TDC that we want to set the spark to, so cylinder pressure gives us the most advantageous push on the crank arm. In most cases we are splitting hairs and piston TDC is close enough. It is possible to find crankshaft TDC when the engine is torn down, but MUCH harder if it's assembled.
     

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