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Timing Protractor Accuracy

Discussion in 'Norton Commando Motorcycles (Classic)' started by Brooking 850, Aug 12, 2018.

  1. Brooking 850

    Brooking 850 VIP MEMBER

    Joined:
    Oct 3, 2011
    As my road bike is apart I would like to double check the outer primary cover protractor accuracy.
    With the head yet still to go on and the motor on the bench, I have set up my dial gauge and crank protractor and measured off TDC as zero then set 28* BTDC on the crank protractor.
    My dial gauge reads 0.2514" flat top piston down the bore (6.80mm) no barrel gasket and barrel recently machined flat, probably 0.004" to get it flat and true.
    I left the crank in this position and then mounted the inner primary cover, alternator rotor and nut.
    I then bolted up the outer cover and checked the riveted protractor, just a hair over 30*

    Has any one done this before and if so what measurements down the bore did they get at 28* BTDC with flat top standard pistons?
    Regards Mike
     
    Last edited: Aug 12, 2018
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  2. kommando

    kommando

    Joined:
    May 7, 2005
  3. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    Your 2+ degree error sounds rather high though I'm not saying the factory might not have had that level of imprecision on some occasions. If I might suggest an alternate method that will be more accurate...

    When the engine is together, use a degree wheel, a pointer and a piston stop to determine TDC. Then set the degree wheel to 0 and rotate the engine to any advance setting that is on the outer primary chanecase cover scale - say 30 degrees before TDC. Remove the degree wheel and install the outer primary cover and see where the rotor's timing mark aligns on the scale. Make whatever mark adjustment necessary - frankly I'd just make a note of the difference and use that from then on. I wouldn't bother trying to change the scale or the rotor's timing mark. IOW, if at 30 degrees before TDC the rotor mark pointed to 32, I'd just remember that the actual timing is 2 degrees retarded from the scale indication. Well, no I wouldn't remember it, I'd write it down somewhere! ;)

    When I checked my 850, there was no meaningful difference in actual/indicated - less than 1/2 of the width of the rotor mark.

    FWIW, a dial indicator is not precise when measuring TDC because there is several degrees of crankshaft rotation with no piston movement at TDC.

    One thing to consider re the Norton method of using the outer cover to hold the scale is that there is enough "slop" in the cover installation to change the alignment of the scale every time the cover is removed/installed. :(
     
    Nater_Potater and Time Warp like this.
  4. Brooking 850

    Brooking 850 VIP MEMBER

    Joined:
    Oct 3, 2011
    MMike , I rotated the engine through several rotations and back again to get the the precise TDC and wanted the measurement down the bore for my records at 28,29, 30,31 and 32 *'s and I split the difference of the non rotation at TDC
    I can still use a piston stop with the head off if necessary.
    I have kinda done the reverse of your suggestion, I have set the degree wheel at the above timing s 28, etc then measured down the bore at each degree mark . Then I have set up the inner cover with gasket and rotor on the keyway, and used the measurements down the bore to see where the outer cover marks are against the rotor mark, each time being 2* out.
    I wont know how accurate it is until perhaps on a dyno and change the timing to see if I lose/make power with minor adjustments, then check again with the factory timing marks.
    I was just wondering if anyone had carried this out themselves and what measurements they had down the bore with flat top pistons and standard length conrods?
    Regards Mike
     
  5. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    (IMHO) The only way to do TDC with the cylinder head off is the cylinder secured and a positive stop, the crankshaft turned both directions and split the difference on your degree wheel for TDC. (It also means you can check both pistons are at the same TDC degree)
    Even if the primary cover port scale (in the case of a Commando) is off any discrepancy can be accounted for ie 28° on the scale can be the same as 30°on your positive stop checked degree wheel.

    Edit.... a positive stop has been posted while I was typing.
     
  6. gtiller

    gtiller VIP MEMBER

    Joined:
    Nov 5, 2012
    Mike, you’re way more of a guru than i’ll ever be, but here is my technique for anyone who is newer to rebuilds.



    With no head on, I find a dial test indicator (DTI gauge) and a deck bridge works well, this one has magnets on so is good for iron barrels.

    This is my ES2 engine, but the same applies for the twin.

    869DE137-9E4F-4EA1-917B-79D18B70B14F.jpeg

    You can then turn the crank and measure the exact piston height at a given mark.

    C257DA31-2EE1-42DD-8BC7-87B50404BD63.jpeg

    I like to read height at say 40 degrees before top dead center and then rotating through bottom dead center make sure that the height at 40 degrees after top dead center is precisely the same.

    For me, it’s a fool proof way to make sure you can read off an exact top dead center on your degree wheel.



    On my 72 combat engine I found that the graduations on the primary cover were between two and three degrees out.

    However, on the 76 MK3 it was absolutely spot on!

    Shame it wasn’t the other way round, as the MK3 ignition timing degree scale is slotted, so you have a little wiggle room to make a correction.

    B6A4B635-658A-4DF3-B878-AEE5AF17A99D.jpeg

    Whereas the pre-mk3 is fixed, with no adjustment.

    0F27179A-27D7-49A1-AE07-40B341050311.jpeg
     
  7. Brooking 850

    Brooking 850 VIP MEMBER

    Joined:
    Oct 3, 2011
    Thanks guys, i am using a dial gauge with a strong magnet on top of the barrel and have an offset adjuster in one leg of the gauge mounting legs so I can get the dial gauge exactly vertical.
    As much as i was enquiring about the actual measurement down the bore for reference, i will do as many cycles of the crank both sides of TDC until I get a good result and measure off both pistons as well.
    Ill keep at it until Im happy with my actual setting on the factory protractor at 28* actual crank/piston setting .
    Regards Mike
     
  8. Holmeslice

    Holmeslice VIP MEMBER

    Joined:
    Sep 27, 2009
    If you affix a degree wheel and have found accurate TDC you can stake the alternator stator with with a chisel at your desired timing mark. Once you fit the primary cover the scale and your staked mark can be compared for accuracy.
     
  9. rvich

    rvich VIP MEMBER

    Joined:
    Jul 25, 2009
    In my experience the (stock) primary cover can be moved around on its dowels enough to change the timing alignment by a couple of degrees, so one needs to decide where they want the cover to be positioned before accuracy can be determined.

    Anybody else experience this or are mine just sloppy?
     
  10. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    If your looking for accuracy then forget the dial indicator on the piston.

    Use a piston stop, put a small cut in the stator potting for a stationary mark, hold the piston against the stop both BTC and ATC -mark both positions on the rotor, split the difference and make a mark on the rotor -this will be actual TDC. Then mount a degree wheel and go from there. Jim
     
  11. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    Jim, I agree with your concept but I know that I could not accurately determine that "split the difference" distance without using the degree wheel to start with.

    I've done the degree wheel/piston stop method to determine TDC on more engines than I can count and not only do I find it necessary for me to use the wheel, I use the largest wheel that can be mounted to the engine! :)
     
    Last edited: Aug 12, 2018
  12. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    I normally just use a pair of sharp dividers and swing a couple arcs from the marks on the rotor. It may not be perfect but with care it is accurate to less than a degree.

    And yes I do have a 14 inch degree wheel if I am looking for perfection. Jim
     
  13. Brooking 850

    Brooking 850 VIP MEMBER

    Joined:
    Oct 3, 2011
    Thanks Jim, will do on the solid stop , split the difference before BTDC and ATDC , be interesting to see how different the marks are from the dial gauge method .
    Will report back once Im done
    Jim original Q, how far down the bore is the flat top piston standard conrod at 28* no barrel gasket , 0.004" truing of the barrel top? +/-
    Regards Mike
     
  14. comnoz

    comnoz VIP MEMBER

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    Dec 10, 2008
    Oh, about the length of a nose hair...
     
  15. Brooking 850

    Brooking 850 VIP MEMBER

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    Oct 3, 2011
    Shite, as I get older that could mean the difference of 0.010" !!!
    Ill probably end up marking up the belt drive pulley and inner case with 28* marks as I can run the bike without the outer primary cover and not rely on the outer cover protractor.
    Its not like I will be checking it every other day.
    Regards Mike
     
  16. Brooking 850

    Brooking 850 VIP MEMBER

    Joined:
    Oct 3, 2011
    14 degrees difference so far between BTDC and ATDC using a mechanical stop and 8" timing disc with piston as close to TDC as visually possible
    Piston stop I made can be swapped to either piston and the difference on both pistons is 14*
    As it just turns out the stop can be adjusted to halt the piston at 28* BTDC (max thread length on the bolt bottomed out on the welded nut)so will leave the stop on and remount the inner case, rotor , stator and outer cover and check the protractor.
    Regards Mike
     
    Last edited: Aug 12, 2018
  17. Brooking 850

    Brooking 850 VIP MEMBER

    Joined:
    Oct 3, 2011
    It gets worse, the protractor is out 4* degrees using the mechanical stop to find TDC.
    I split the 14* difference to 7* set the disc on zero at this point , backed down the bore, set the piston stop at 28* to hold all firm.Double checked the timing disc for this measurement
    Mounted the inner case, rotor and stator and marked up the stator to the existing rotor mark.
    i then mounted the outer cover to find the protractor lined up on the stator/rotor mark and it is now indicated at 32*
    No slop in the dowels, flush all round to the inner cover.

    My original measurement of 6.80mm is the same from barrel top to piston top, so I was very close with the dial gauge.
    I can now make a mechanical stop the fits in the plug hole and know it will be accurate.
    Regards Mike
     
  18. Mr. Rick

    Mr. Rick VIP MEMBER

    Joined:
    Feb 6, 2009
    Using a degree wheel, I determined that my primary case indicator was off about 1/2 degree. Are you saying yours is off by 4 degrees?
     
  19. swooshdave

    swooshdave

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    Apr 15, 2009
    I should have paid attention in geometry class...
     
  20. Brooking 850

    Brooking 850 VIP MEMBER

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    Oct 3, 2011
    Mr Rick -yes.
    Im just about to recheck the whole setup just in case I missed something
    Regards Mike
     
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