Author Topic: Good timing  (Read 3555 times)

Online bsa-bill

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Re: Good timing
« Reply #30 on: 09.02. 2011 22:36 »
I can agree with all the above having found about 2 degrees variance.
A bit more accuracy can be had using a dial gauge ,when the heads off obviously, although I'm sure a method to use it with the head on could be found , the dial gauge does somehow seem to register much smaller movements of the piston than finger or eye can
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline Alan @Ncl

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Re: Good timing
« Reply #31 on: 09.02. 2011 23:14 »
What John and Bill  are saying makes sense if you use the device to try and directly detect TDC.  My little calclulator predicts that around TDC, 5 degrees is only about 0.1 mm (4 thou) and that takes a pretty high resolution instrument to detect (Bill will get close with his clock gauge, hence his 2 degrees which I reckon is about 1 thou).  

However, once you get further down the stroke, the piston stop gives much better resolution and in the critical region of 5/16 to 3/8 inch should be fine as a datum.  But for actual TDC, you can only use the 'bisection' principle which seems to imply a timing disc.  

Was also thinking about Nigel's point about his gauge and translating readings through the geometry.  I measured the angle of the plug hole as about 33 degrees to the vertical (how does this compare with what others think?). This means that the vertical travel is about 84% (cosine of the angle) of the distance measured by the instrument.  However, strictly speaking, this will only work accurately for a flat top piston.  If it is domed, the geometry will change a bit (although this may be a negligible secondary effect if we are only considering movements up in the 3/8 to 5/16 BTDC region).

Also reflecting Nigel's comments, I bet when some of us were poking 'sticks down holes' nearly 50 years ago, we would never have predicted our current level of interest in such things.

Alan

Offline kiwipom

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Re: Good timing
« Reply #32 on: 10.02. 2011 01:16 »
hi guys/Alan, yes the technical detail that we now delve into to find TDC, never used to be a problem in the old days as you say a stick down the hole worked well, obviously the interest in the exact is highlighted with age something that we would have never contemplated as a teenager,cheers
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Offline iansoady

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Re: Good timing
« Reply #33 on: 10.02. 2011 14:41 »
I can agree with all the above having found about 2 degrees variance.
A bit more accuracy can be had using a dial gauge ,when the heads off obviously, although I'm sure a method to use it with the head on could be found , the dial gauge does somehow seem to register much smaller movements of the piston than finger or eye can

I did mine using a home made extension for the dial gauge and with the magnetic base on the fins beside the plug hole (iron head). The clock gauge did respond to the slightest movement even around TDC, although I agree the only really accurate way is with a piston stop and degree wheel (John's method). However, given I'm not after the ultimate I think the clock gauge is accurate enough for my needs.
Ian.
1962 Golden Flash (arrived)
1955 Velo Viper/Venom (departed)
2004 Triumph Tiger 955i (staying)